,,! ,,telescope ,mc,graw-,hill ,book ,co4 ,inc4 ,,publi%]s ,,( ,,books ,,= ,coal ,age ,electric ,railway ,j\rnal ,electrical ,_w ,eng9e]+ ,news-,record ,am]ican ,ma*9i/ ,+5i]a ,int]nacional ,eng9e]+ ,m9+ ,j\rnal ,p[] ,*emical ,metallurgical ,eng9e]+ ,electrical ,m]*&is+ ,illu/ration3 ,galileo's ,telescopes4 7,frontispiece7 7,bull4 de la ,soc4 ,a/ron4 de ,france47 ,,! ,,telescope ,,by ,,l\is ,,bell1 ,,ph4,d4 ,,consult+ ,,eng9e]2 ,,fell[1 ,,am]ican ,,academy ,,( ,,arts ,,sci5ces2 ,,pa/-,,presid5t1 ,,! ,,illum9at+ ,,eng9e]+ ,,society2 ,,memb]1 ,,am]ican ,,a/ronomical ,,society ,,fir/ ,,edition ,,mcgraw-,,hill ,,book ,,company1 ,,inc4 ,,new ,,york3 #cgj ,,sev5? ,,av5ue ,,london3 #f #h ,,b\v]ie ,,/41 ,e4 ,c4 #d #aibb ,,copyright1 #aibb1 ,,by ,,! ,,mcgraw-,,hill ,,book ,,company1 ,,inc4 ,,! ,,maple ,,press ,,york ,,pa ,,preface ,?is book is writt5 = ! _m obs]v]s1 :o use telescopes = /udy or pleasure & desire m in=mation ab\t _! con/ruction & prop]ties4 ,n be+ a 8h&book0 9 two or m ?ick qu>tos1 x attempts nei!r exhau/ive te*nicalities nor popul> descriptions ( great obs]vatories & _! "w4 ,x deals prim>ily ) pr9ciples & _! application to su* in/rum5ts z are likely to come into ! possession1 or )9 rea*1 ( /ud5ts & o!rs = :om ! ,heav5s h a compell+ call4 ,mu* has be5 writt5 ( telescopes1 fir/ & la/1 b x is = ! mo/ "p scatt]$ ?r\gh pap]s 9 ?ree or f\r languages1 & q inaccessible to ! ord9>y read]4 ,= 8 b5efit ! ref]5ces are1 s f> z is practicable1 to ,engli% s\rces1 & dim5sions are giv51 regretfully1 9 ,engli% units4 ,c]ta9 bran*es ( ! subject are n "h discuss$ = lack ( space or because !re is rec5t lit]ature at h& to :i* ref]5ce c be made4 ,su* topics are telescopes notable *iefly = _! dim5sions1 & photographic app>atus on :i* special treatises are available4 ,cele/ial photography is a bran* ( a/ronomy :i* /&s on its own feet1 & al?\gh _m telescopes are successfully us$ = photography ?r\gh ! help ( color scre5s1 ! photographic telescope prop] & its use belongs to a field some:at ap>t1 requir+ a te*nique q its own4 ,x is _m ye>s1 h[ev]1 s9ce any book has dealt ) ! telescope itself1 ap>t f ! (t5 repeat$ acc\nts ( ! m>vels x discloses4 ,! pres5t volume conta9s nei!r pictures ( nebul nor speculations z to ! habitibility ( ! planets2 x m]ely attempts to br+ ! facts reg>d+ ! a/ronom]'s *ief in/rum5t ( rese>* some:]e )9 grasp & up to ! pres5t "t4 ,! au?or cordially ackn[l$ges 8 obligations to ! important a/ronomical j\rnals1 p>ticul>ly ! ,a/ro-physical ,j\rnal1 & ,popul> ,a/ronomy 9 ?is c\ntry2 ,! ,obs]vatory1 & ! publications ( ! ,royal ,a/ronomical ,society 9 ,engl&2 ! ,bullet9 de la ,socit ,a/ronomique de ,france2 & ! ,a/ronomis*e ,na*ri*t52 :i*1 ) a few o!r j\rnals & ! (ficial reports ( obs]vatories =m ! body ( a/ronomical k4 ,he also ackn[l$ges ! k9dness ( ! v>i\s publi%]s :o h ext5d$ ! c\rtesy ( illu/rations1 especially ,macmillan ,co4 & ! ,cl>5don ,press1 & above all r5d]s ?anks to ! _m fri5ds :o h cordially l5t a help+ h&--! ,director & /aff ( ! ,h>v>d ,obs]vatory1 ,dr4 ,george ,e4 ,hale1 ,c4 ,a4 ,r4 ,lund91 manag] ( ! ,alvan ,cl>k ,corporation1 ,j4 ,b4 ,mc,dowell1 successor ( ! ,bra%e> ,company1 ,j4 ,e4 ,b5nett1 ! ,am]ican repres5tative ( ,c>l ,zeiss1 ,j5a1 & n a few o!rs4 ,,l\is ,,bell4 ,,bo/on1 ,,mass41 ,febru>y1 #aibb4 ,,cont5ts ,,page ,,preface vii ,,*ap4 ,i4 ,,! ,,evolution ,,( ,,! ,,telescope #a ,,ii4 ,,! ,,mod]n ,,telescope #ca ,,iii4 ,,optical ,,glass ,,& ,,its ,,work+ #eg ,,iv4 ,,! ,,prop]ties ,,( ,,objectives ,,& ,,mirrors #gf ,v4 ,,m\nt+s #ih ,,vi4 ,,eye-,,pieces #acd ,,vii4 ,,h& ,,telescopes ,,& ,,b9ocul>s #aej ,,viii4 ,,accessories #afe ,,ix4 ,,! ,,te/+ ,,& ,,c>e ,,( ,,telescopes #bja ,x4 ,,sett+ ,,up ,,& ,,h\s+ ,,! ,,telescope #bbh ,,xi4 ,,see+ ,,& ,,magnification #bec ,,app5dix #bgi ,,index #bha ,,! ,,telescope ,,*apt] ,i ,,! ,,evolution ,,( ,,! ,,telescope ,9 ! cr$ul\s twaddle ( an essay on ! ,lo/ ,arts "o may g5]ally f9d ! telescope ascrib$ to f> antiquity4 ,9 place ( evid5ce !re is vague allusion ( classical times or wild flights ( fancy l "o :i* argu$ f ! ,scriptural /atem5t t ,satan took up ,*ri/ into a high m\nta9 & %[$ him all ! k+doms ( ! e>?1 t ! ,devil _h a telescope--bad optics & worse !ology4 ,9 po9t ( fact !re is n any indication t ei!r 9 classical times1 or 9 ! black ?\s& ye>s ( hopeless ignorance t foll[$ ! fall ( ,roman civilization1 0 !re any k ( optical in/rum5ts wor? m5tion+4 ,! peoples t t5d$ _! flocks by night 9 ! ,ea/ alone kept alive ! k ( a/ronomy1 & v gradually1 ) ! revival ( le>n+1 came ! _s ( exp]im5t t l$ to ! inv5tion ( aids to man's natural p[]s4 ,! l9eage ( ! telescope runs unmi/akably back to spectacles1 & ^! h an honorable hi/ory ext5d+ ov] m ?an six c5turies to ! e>ly & fruitful days ( ! ,r5aissance4 ,t _! orig9 0 9 ,italy ne> ! end ( ! ?irte5? c5tury admits ( little d\bt4 ,a ,flor5t9e manuscript lett] ( #abhi ref]s to 8,^? glasses !y call spectacles1 lately inv5t$1 to ! great advantage ( poor old m5 :5 _! sight gr[s weak10 & 9 #acje ,giordano da ,rivalto ref]s to !m z dat+ back ab\t tw5ty ye>s4 ,f9ally1 9 ! *ur* ( ,santa ,m>ia ,maggiore 9 ,flor5ce lay buri$ ,salv9o d',am>to degli ,armati1 7obiit #acag7 "u an epitaph1 n[ disappe>$1 ascrib+ to him ! inv5tion ( spectacles4 ,w4 ,b4 ,c>p5t]1 ,f4 ,r4 ,s41 /ates t ! inv5tor tri$ to keep ! valuable secret to himself1 b x 0 discov]$ & publi%$ be=e 8 dea?4 ,at all ev5ts ! discov]y mov$ swiftly4 ,by ! e>ly f\rte5? c5tury x _h spread to ! ,l[ ,c\ntries :]e x 0 de/9$ to lead to great results1 & pres5tly 0 common k ov] all civiliz$ ,europe4 ,x 0 ?ree hundr$ ye>s1 h[ev]1 betwe5 spectacles & ! comb9ation ( spectacle l5ses into a telescope1 a lapse ( "t :i* to "s inve/igators has seem$ altoge!r my/]i\s4 ,! oph?almological facts lead to a simple explanation4 ,! fir/ spectacles 7 = ! relief ( presbyopia1 ! common & lam5table affection ( advanc+ ye>s1 & = ?is purpose convex l5ses ( v mod]ate p[] suffic$1 nor 0 mat]ial v>iation 9 p[] necess>y4 ,glasses hav+ a uni=m focus ( a foot & a half or !reab\ts w\ld s]ve e practical purpose1 b w\ld be no mat]ial = telescopes4 ,myopia 0 little kn[n1 its acquir$ =m be+ r>e 9 a p]iod ( g5]al illit]acy1 & glasses = its correction1 especially z reg>ds its high] degrees1 probably came sl[ly & 7 9 v small dem&1 s t ! *ance ( an optical craftsman hav+ 9 h& ! ord9>y convex l5ses & ^? ( /rong negative curvature 0 altoge!r remote4 ,inde$ x 0 only 9 #aege t ,maurolycus publi%$ a cle> description ( myopia & hyp]metropia ) ! appropriate treatm5t by ! use ( concave & convex l5ses4 ,until bo? ( ^!1 9 q v>i\s p[]s1 7 available1 !re 0 small *ance ( hitt+ ^u an in/rum5t t requir$ _! use 9 a highly special comb9ation4 ,at all ev5ts !re is no def9ite trace ( ! discov]y ( telescopic vision until #afjh & ! inv5tor ( record is unque/ionably "o ,jan ,lipp]%ey1 a spectacle mak] ( ,middelburg 9 ,zeel&1 a native ( ,wesel4 ,on ,oct4 #b1 #afjh ! ,/ates-,g5]al took "u consid]ation a petition :i* _h be5 pres5t$ by ,lipp]%ey = a #cj-ye> pat5t to ! exclusive "r ( manufacture ( an in/rum5t = see+ at a di/ance1 or = a suitable p5sion1 "u ! condition t he %\ld make ! in/rum5t only = 8 c\ntry's s]vice4 ,! ,/ates ,g5]al prick$ up its e>s & promptly appo9t$ on ,oct4 #d a committee to te/ ! new in/rum5t f a t[] ( ,pr9ce ,maurice's palace1 allott+ #ijj flor9s = ! pur*ase ( ! inv5tion %\ld x prove good4 ,on ! #f? ! committee report$ favorably & ! ,assembly agre$ to give ,lipp]%ey #ijj flor9s = 8 in/rum5t1 b desir$ t x be arrang$ = use ) bo? eyes4 ,lipp]%ey !re=e pu%$ =w>d to ! b9ocul> =m & two mon?s lat]1 ,dec4 #i1 he ann\nc$ 8 success4 ,on ! #ae? ! new in/rum5t 0 exam9$ & pron\nc$ good1 & ! ,assembly ord]$ two m b9ocul>s1 ( rock cry/al1 at ! same price4 ,!y d5i$ a pat5t on ! gr\nd t ! inv5tion 0 kn[n to o!rs1 b paid ,lipp]%ey lib]ally z a sort ( reta9] to secure 8 exclusive s]vices to ! ,/ate4 ,9 fact ev5 ! ,fr5* ,ambassador1 wi%+ to obta9 an in/rum5t f him = 8 ,k+1 _h to secure ! necess>y au?orization f ! ,/ates-,g5]al4 ,illu/ration3 ,bull4 de la ,soc4 ,a/ron4 de ,france4 ,,fig4 #a4--;,jan ,lipp]%ey1 ,inv5tor ( ! ,telescope4 ,x is "h p]t95t to enquire :at mann] ( optic tube ,lipp]%ey %[$ to back up 8 petition1 & h[ x _h come to public k4 ,z ne>ly z we may "k ^! fir/ telescopes 7 ab\t a foot & a half long1 z not$ by ,huyg5s1 & probably an in* & a half or less 9 ap]ture1 be+ con/ruct$ ( an ord9>y convex l5s su* z 0 us$ 9 spectacles = ! ag$1 & ( a concave glass suitable = a bad case ( %ort sight$ness1 ! only k9d 9 t day likely to receive att5tion4 ,x probably magnifi$ no m ?an ?ree or f\r diamet]s & 0 mo/ likely 9 a sub/antial tube ( firmly roll$1 glu$1 & v>ni%$ pap]1 orig9ally )\t provision = focuss+1 s9ce ) an eye l5s ( r l[ p[] ! ne$ ( adju/m5t w\ld n be acute4 ,z to ! inv5tion be+ g5]ally kn[n1 ! only def9ite attempt to dispute priority 0 made by ,james ,metius ( ,alkma>1 :o1 le>n+ ( ,lipp]%ey's petition1 on ,oct4 #ag1 #afjh1 fil$ a simil> "o1 alleg+ t ?r\gh /udy & labor ext5d+ ov] a c\ple ( ye>s he1 hav+ accid5tally hit ^u ! idea1 _h s f> c>ri$ x out t 8 in/rum5t made di/ant objects z di/9ct z ! "o lately (f]$ to ! ,/ates by a citiz5 & spectacle mak] ( ,middelburg4 ,he app>5tly did n submit an in/rum5t1 0 politely told to p]fect 8 inv5tion be=e 8 petition 0 fur!r consid]$1 & !reaft] disappe>s f ! sc5e1 :atev] 8 m]its4 ,if he _h actually not$ telescopic vision he _h nei!r appreciat$ its enorm\s importance nor laid ! facts be=e o!rs :o might h done s4 ,! only o!r contempor>y = :om claims h be5 made is ,za*>ius ,jans51 also a spectacle mak] ( ,middelburg1 to :om ,pi]re ,borel1 on entirely second h& in=mation1 ascrib$ ! discov]y ( ! telescope4 ,b ,borel wrote ne>ly fifty ye>s lat]1 aft] all ! pr9cipals 7 dead1 & ! evid5ce he collect$ f ! prec>i\s memories ( v5]able witnesses is v conflict+ & po9ts to ab\t #afaj z ! date :5 ,jans5 0 mak+ telescopes--l _m o!r spectacle mak]s4#a #a ,!re is a v /rong probability t ,jans5 0 ! inv5tor ( ! comp\nd microscope ab\t ! beg9n+ ( ! sev5te5? c5tury4 ,borel also gave cr$5ce to a tale t ,metius1 seek+ ,jans51 /ray$ into ,lipp]%ey's %op & by 8 inquiries gave ! %rewd proprietor 8 fir/ h9t ( ! telescope1 b set ! date at #afaj4 ,a v>iation ( ?is tale ( ! my/]i\s /rang]1 due to ,hi]onymus ,sirturus1 conta9s ! int]e/+ intimation t he may h be5 ( sup]natural orig9--n fur!r specifi$4 ,!re are also ! reports1 common among ! ignorant or envi\s1 t ,lipp]%ey's discov]y 0 accid5tal1 ev5 p]haps made by 8 *ildr5 or appr5tice4 ,j h[ x actually 0 made we d n "k1 b !re is no reason to suppose t x 0 n 9 ! commonplace way ( exp]im5t+ ) & te/+ l5ses t he _h produc$1 p]haps ^? made to meet a vici\s case ( myopia 9 "o ( 8 patrons4 ,:5 ! discov]y 0 made is some:at cle>]4 ,pla9ly x ant$at$ ,oct4 #b1 & 9 ,lipp]%ey's petition is a def9ite /atem5t t an in/rum5t _h already be5 te/$ by "s1 at lea/1 ( ! memb]s ( ! ,/ates-,g5]al4 ,a some:at vague & gossipy note 9 ! ,m]cure ,franaise intimates t "o 0 pres5t$ to ,pr9ce ,maurice 8ab\t ,septemb] ( ! pa/ ye>0 7#afjh7 & t x 0 %[n to ! ,c\ncil ( ,/ate & to o!rs4 ,all[+ a reasonable "t betwe5 ,lipp]%ey's discov]y & ! actual production ( an example suitable = exhibition to ! au?orities1 x seems likely t ! inv5tion dates back c]ta9ly into ! summ] ( #afjh1 p]haps ev5 e>li]4 ,at all ev5ts !re is e indication t ! news ( x spread l wild-fire4 ,unless ,lipp]%ey 7 unusually c>eful 9 keep+ 8 secret1 & !re are traditions t he 0 n1 ! s5sational discov]y w\ld h be5 quickly kn[n 9 ! little t[n & e spectacle mak] ^: e>s x rea*$ w\ld h be5 busy ) x4 ,if ! dates giv5 by ,simon ,m>ius 9 8 ,mundus ,jovialis be correct1 a ,belgian ) an air ( my/]y & a glass ( :i* "o ( ! l5ses 0 crack$1 turn$ up at ! ,frank=t fair 9 ! autumn ( #afjh & at la/ all[$ ,fu*s1 a nobleman ( ,bimba*1 to look ?r\gh ! in/rum5t4 ,fu*s not$ t x magnifi$ 8sev]al0 times1 b fell out ) ! ,belgian ov] ! price1 & return+1 took up ! matt] ) ,m>ius1 fa?om$ ! con/ruction1 tri$ x ) glasses f spectacles1 attempt$ to get a convex l5s ( long] focus f a ,nuremburg mak]1 :o _h no suitable tools1 & ! foll[+ summ] got a fairly good glass f ,belgium :]e su* 7 already becom+ common4 ,) ?is ,m>ius ev5tually pick$ up ?ree satellites ( ,jupit]--! f\r? await$ ! arrival ( a sup]ior telescope f ,v5ice4 ,e>ly 9 #afji telescopes 8ab\t a foot long0 7 c]ta9ly = sale 9 ,p>is1 a ,fr5*man _h (f]$ "o 9 ,milan by ,may ( t ye>1 a c\ple ( mon?s lat] "o 0 9 use by ,h>riot 9 ,engl&1 an example _h rea*$ ,c>d9al ,borghese1 & specim5s are said to h rea*$ ,padua4 ,fig4 #b f ! 8,mundus ,jovialis10 %[s ,m>ius ) 8 8,p]spicilium10 ! fir/ publi%$ picture ( ! new in/rum5t4 ,e>ly 9 #afaj telescopes 7 be+ made 9 ,engl&1 b if ! few reports ( p]=mance1 ev5 at ?is date1 are tru/wor?y1 ! 8,dut* trunk0 ( t p]iod 0 ( v indiff]5t quality & p[]1 f> f be+ an a/ronomical in/rum5t4 ,illu/ration3 ,! ,obs]vatory4 ,,fig4 #b4--,simon ,m>ius & 8 ,telescope4 ,"o _c lay aside ?is prelim9>y phase ( ! evolution ( ! telescope )\t ref]5ce to ! alleg$ descriptions ( telescopic app>atus by ,rog] ,bacon1 7c4 #abgj71 ,giambatti/a della ,porta 7#aeeh71 & ,leon>d ,digges 7#aega71 details ( :i* may be f\nd 9 ,grant's ,hi/ory ( ,physical ,a/ronomy & _m o!r works4 ,( ^! ! fir/ on c>eful read+ conveys /rongly ! conviction t ! au?or _h a pretty cle> idea ( refraction f ! /&po9t ( visual angle1 yet )\t giv+ any evid5ce ( practical acqua9tance ) actual app>atus = do+ ! ?+s :i* he sugge/s4 ,giv5 a suitable supply ( l5ses1 x is reasonably c]ta9 t ,bacon 0 clev] en\gh to h devis$ bo? telescope & microscope1 b !re is no evid5ce t he did s1 al?\gh 8 manifold activities kept him con/antly 9 public view4 ,x does n seem unlikely1 h[ev]1 t 8 sugge/ions 9 manuscripts1 q available at ! "t1 may h l$ to ! contemporane\s inv5tion ( spectacles4 ,porta's comm5ts s\nd l an e*o ( ,bacon's1 plus a r muddl$ attempt to imag9e ! correspond+ app>atus4 ,kepl]1 c]ta9ly compet5t & famili> ) ! pr9ciples ( ! telescope1 f\nd 8 description entirely un9telligible4 ,porta1 h[ev]1 0 "o ( ! e>lie/ work]s on ! cam]a obscura & ^u ?is "s ( 8 cryptic /atem5ts may h borne4 ,some:at simil> is ! situation respect+ ,digges4 ,8 son makes ref]5ce to a ,ms4 ( ,rog] ,bacon z ! s\rce ( ! m>vels he describes4 ,! :ole acc\nt1 h[ev]1 /rongly sugge/s exp]im5ts ) ! cam]a obscura r ?an ) ! telescope4 ,! mo/ t c be said ) ref]5ce to any ( ! ?ree is t1 if he by any *ance fell ^u ! comb9ation ( l5ses t gave telescopic vision1 he fail$ to set d[n ! facts 9 any =m t c\ld be or 0 ( use to o!rs4 ,!re is no reason to believe t ! ,dut* discov]y1 important z x 01 _h gone beyond ! empirical obs]vation t a common convex spectacle l5s & a concave "o ( relatively l>ge curvature c\ld be plac$ 9 a tube1 convex ahead1 at su* a di/ance ap>t z to give a cle> enl>g$ image ( di/ant objects4 ,x rema9$ = ,galileo 7#aefd-afdg7 to grasp ! g5]al pr9ciples involv$ & to apply !m to a real in/rum5t ( rese>*4 ,x 0 9 ,may #afji t1 on a visit to ,v5ice1 he he>d reports t a ,belgian _h devis$ an in/rum5t :i* made di/ant objects seem ne>1 & ?is be+ quickly confirm$ by a lett] f ,p>is he awak5$ to ! importance ( ! issue &1 return+ to ,padua1 is said to h solv$ ! problem ! v night ( 8 arrival4 ,next day he procur$ a plano-convex & a plano-concave l5s1 fitt$ !m to a lead tube & f\nd t ! comb9ation magnifi$ ?ree diamet]s1 an obs]vation :i* indicates ab\t :at x 0 possible to obta9 f ! /ock ( ! contempor>y spectacle mak]4#b ,! relation betwe5 ! p[] & ! foci ( ! l5ses he evid5tly quickly fa?om$ = 8 next record$ trial rea*$ ab\t eight diamet]s4 #b ,! /atem5t by ,galileo t he 8fa%ion$0 ^! fir/ l5ses c h>dly be tak5 lit]ally if 8 v spe$y con/ruction is to be cr$it$4 ,) ?is in/rum5t he proce$$ to ,v5ice & dur+ a mon?'s /ay1 ,augu/1 #afji1 exhibit$ x to ! s5ators ( ! republic & ?rongs ( notables1 f9ally disclos+ ! secret ( its con/ruction & pres5t+ ! tube itself to ! ,doge sitt+ 9 full c\ncil4 ,?is p>ticul> telescope 0 ab\t tw5ty in*es long & "o & five eigh?s in*es 9 ap]ture1 %[+ pla9ly t ,galileo _h by ?is "t f\nd1 or m likely made1 an eye l5s ( %ort focus1 ab\t ?ree in*es1 q probably us+ a well poli%$ convex l5s ( ! ord9>y sort z objective4 ,illu/ration3 ,lodge 8,pione]s ( ,sci5ce40 ,,fig4 #c4--;,galileo4 ,lad5 ) honors he return$ to ,padua & settl$ d[n to ! h>d "w ( developm5t1 gr9d+ _m l5ses ) 8 own h&s & f9ally produc+ ! in/rum5t magnify+ "s #cb times1 ) :i* he began ! notable succession ( discov]ies t laid ! f\ndation ( obs]vational a/ronomy4 ,?is ) ano!r ( simil> dim5sions is /ill pres]v$ at ! ,galileo ,museum 9 ,flor5ce1 & is %[n 9 ! ,frontispiece4 ,! l>g] in/rum5t is =ty-n9e in*es long & an in* & ?ree qu>t]s ap]ture1 ! small] ab\t ?irty-sev5 in*es long & ( an in* & five-eigh?s ap]ture4 ,! tubes are ( pap]1 ! glasses /ill rema91 & ^! are 9 fact ! fir/ a/ronomical telescopes4 ,galileo made 9 ,padua1 & aft] 8 return to ,flor5ce 9 ! autumn ( #afaj1 _m telescopes :i* f\nd _! way ov] ,europe1 b q c]ta9ly none ( p[] equall+ or exce$+ ^!4 ,9 ?is connection ,john ,greaves1 lat] ,savilian ,pr(essor ( ,a/ronomy at ,ox=d1 writ+ f ,si5na 9 #afci1 says3 8,galileus nev] made b two good glasses1 & ^? 7 ( old ,v5ice glass40 ,9 ^! be/ telescopes1 h[ev]1 ! great ,flor5t9e _h cle>ly accompli%$ a mo/ workmanlike feat4 ,he _h br\ght ! focus ( 8 eye l5s d[n to t usual 9 mod]n op]a glasses1 & has pu%$ 8 p[] ab\t to ! limit = simple l5ses ?us comb9$4 ,! lack ( cle> & homog5e\s glass1 ! great difficulty ( =m+ true tools1 want ( suitable comm]cial abrasives1 impossibility ( buy+ %eet metals or tub+ 7except lead71 & default ( n[ famili> me?ods ( c5t]+ & te/+ l5ses1 made ! production ( respectably good in/rum5ts a task ! difficulty ( :i* x is h>d n[ to appreciate4 ,! s]vices ( ,galileo to ! art 7 ( su* pr(\nd importance1 t 8 =m ( in/rum5t may well be> 8 "n1 ev5 ?\gh 8 eyes 7 n ! fir/ t _h look$ ?r\gh x4 ,su*1 too1 0 ! judgm5t ( 8 contempor>ies1 & x 0 by ! act ( 8 colleagues 9 ! r5[n$ ,acaddemia dei ,l9cei1 ?r\gh ! le>n$ ,damiscianus1 t ! "n 8,telescope0 0 devis$ & has be5 h&$ d[n to u4 ,a s]i\s fault ( ! ,galilean telescope 0 its v small field ( view :5 ( any consid]able p[]4 ,galileo's l>ge/ in/rum5t _h a field ( b #gae1 less ?an "o qu>t] ! moon's diamet]4 ,! g5]al reason is pla9 if "o foll[s ! rays ?r\gh ! l5ses z 9 ,fig4 #d :]e ,,ab is ! di/ant object1 o ! objective1 e ! eye l5s1 ab ! real image 9 ! abs5ce ( e1 & ab ! virtual magnifi$ image due to e4 ,x w be at once se5 t ! axes ( ! p5cils ( rays f all p>ts ( ! object1 z %[n by ! heavy l9es1 act z if !y div]g$ f ! optical c5t] ( ! objective1 b div]g+ /ill m by refraction ?r\gh ! concave eye l5s e1 fall mo/ly outside ! pupil ( ! obs]v]'s eye4 ,9 fact ! field is approximately measur$ by ! angle subt5d$ by ! pupil f ! c5t] ( o4 ,to ! cr$it ( ! ,galilean =m may be set d[n ! conv5i5t erect image1 a %>p1 if small1 field some:at bett]$ by a p>tial comp5sation ( ! ab]rations ( ! objective by ! concave eye l5s1 & good illum9ation4 ,= a di/ant object ! l5ses 7 spac$ at ! diff]5ce ( _! focal l5g?s1 & ! magnify+ p[] 0 ! ratio ( ^!1 fo_/fe4 ,illu/ration3 ,,fig4 #d4--;,diagram ( ,galileo's ,telescope4 ,b ! difficulty ( obta9+ high p[] ) a fairly sizeable field 0 ultimately fatal & ! type n[ survives only 9 ! =m ( op]a & field glasses1 usually ( #b to #e p[]1 & 9 an occasional negative eye l5s = erect+ ! image 9 obs]vatory "w4 ,practically all ! mod]n in/rum5ts h a*romatic objectives & commonly a*romatic ocul>s4 ,illu/ration3 ,,fig4 #e4--;,diagram ( ,kepl]'s ,telescope4 ,! necess>y /ep =w>d 0 made by ,johann ,kepl] 7#aega-afcj71 ! immortal discov]] ( ! laws ( planet>y motion4 ,9 8 ,dioptrice 7#afaa7 he set =? ! a/ronomical telescope1 sub/antially1 save = ! *anges br\ght by a*romatism1 z x has be5 us$ ev] s9ce4 ,8 arrangem5t 0 t ( ,fig4 #e 9 :i* ! lett]s h ! same significance z 9 ,fig4 #d4 ,!re are "h ?ree /rik+ diff]5ces f ! ,galilean =m4 ,!re is a real image 9 ! front focus ( ! eye l5s e1 ! rays pass+ x are refract$ inw>ds in/ead ( outw>ds1 to ! great advantage ( ! field1 & any object plac$ 9 ! image plane w be magnifi$ toge!r ) ! image4 ,! fir/ two po9ts ,kepl] fully realiz$1 ! ?ird he probably did n1 ?\gh x is ! basis ( ! micromet]4 ,! l5ses o & e are obvi\sly spac$ at ! sum ( _! focal l5g?s1 & z be=e ! magnify+ p[] is ! ratio ( ^! l5g?s1 ! visible image be+ inv]t$4 ,kepl]1 s f> z kn[n1 did n actually use ! new telescope1 t honor fall+ ab\t half a doz5 ye>s lat]1 to ,*ri/oph] ,s*e9]1 a ,jesuit pr(essor ( ma!matics at ,+ol/adt1 be/ kn[n z a v e>ly & mo/ p]si/5t1 n to say v]bose1 obs]v] ( sun spots4 ,8 ,rosa ,urs9a 7#afcj7 indicates free use ( ,kepl]'s telescope = "s ye>s previ\sly1 9 j :at size & p[] is unc]ta94#c ,fontana ( ,naples also appe>s to h be5 e>ly 9 ! field4 #c ,s*e9] also devis$ a crude p>allactic m\nt :i* he us$ 9 8 sol> obs]vations1 probably ! fir/ ,european to grasp ! pr9ciple ( ! equatorial4 ,x 0 only ne> ! end ( ! c5tury t ,roem] foll[$ 8 example1 & bo? _h be5 anticipat$ by ,*9ese in/rum5ts ) sights4 ,b ! new in/rum5t despite its mu* l>g] field & f> great] possibilities ( p[]1 br\ght ) x "s v s]i\s problems4 ,) increas$ p[] came greatly aggravat$ tr\ble f sph]ical ab]ration & *romatic ab]ration z well1 & ! additive ab]rations ( ! eye l5s made matt]s /ill worse4 ,! e>li] ,kepl]ian in/rum5ts 7 probably r bad if ! draw+s ( ,fontana f #afbi to #afcf fairly repres5t !m4 ,if "o may judge f ! c\rse ( developm5ts1 ! fir/ great impulse to improvem5t came ) ! publication ( ,desc>tes' 7#aeif-afej7 /udy ( dioptrics 9 #afcg4 ,!re9 0 set =? mu* ( ! !ory ( sph]ical ab]ration & a/ronom]s promptly foll[$ ! clues1 practical & impractical1 ?us disclos$4 ,)\t go+ into ! !ory ( ab]rations ! fact ( importance to ! improvem5t ( ! e>ly telescope is t ! longitud9al sph]ical ab]ration ( any simple l5s is directly proportional to its ?ickness due to curvature4 ,h5ce1 o!r ?+s be+ equal1 ! long] ! focus = ! same ap]ture ! less ! sph]ical ab]ration bo? absolutely & relatively to ! image4 ,fur!r1 al?\gh ,desc>tes knew no?+ ( *romatic ab]ration1 & ! color$ fr+e ab\t objects se5 ?r\gh ! telescope mu/ !n h seem$ altoge!r my/]i\s1 x1 also1 0 greatly reliev$ by l5g!n+ ! focus4 ,= ! *romatic circle produc$ by a simple l5s ( giv5 diamet] has a radial wid? sub/antially irrespective ( ! focal l5g?4 ,b increas+ ! focal l5g? increases 9 exact proportion ! size ( ! image1 correspond+ly decreas+ ! relative effect ( ! *romatic error4 ,desc>tes also sugge/$ sev]al designs ( l5ses :i* w\ld be altoge!r free ( sph]ical ab]ration1 =m$ ) elliptical or hyp]bolic curvature1 & = "s "t fruitless ef=ts 7 made to realize ?is 9 practice4 ,x 0 9 fact to be ne> a c5tury be=e anyone successfully figur$ non-sph]ical surfaces4 ,x 0 sph]ical q z mu* z *romatic ab]ration t drove a/ronom]s to long telescopes4 ,mean:ile ! a/ronomical telescope fell into bett] h&s ?an ^? ( ,s*e9]4 ,! fir/ fully to grasp its possibilities 0 ,william ,gascoigne1 a gallant "y g5tleman ( ,middleton1 ,york%ire1 born ab\t #afbj 7"s say z e>ly z #afab7 & :o di$ fight+ on ! ,k+'s side at ,m>/on ,moor1 ,july #b1 #afdd4 ,to him came z e>ly z #afch ! inspiration ( utiliz+ ! real focus ( ! objective = e/abli%+ a telescopic sight4 ,illu/ration3 ,,fig4 #f4--;,diagram ( ,t]re/rial ,ocul>4 ,?is %ortly took ! =m ( a g5u9e micromet] consi/+ ( a pair ( p>allel blades 9 ! focus1 mov$ 9 opposite directions by a screw ( duplex pit*1 ) a scale = :ole revolutions1 & a head divid$ into #ajj p>ts = p>tial revolutions4 ,) ?is he obs]v$ mu* f #afch to #afdc1 measur$ ! diamet]s ( sun1 moon & planets ) a good degree ( precision1 & laid ! f\ndations ( mod]n micrometry4 ,he 0 equipp$ by #afci ) :at 0 !n call$ a l>ge telescope4 ,8 untimely dea?1 leav+ beh9d an unpubli%$ treatise on optics1 0 a grave loss to sci5ce1 ! m s9ce ! manuscript c\ld n be f\nd1 &1 swept away by ! /orms ( w>1 8 brilliant "w dropp$ out ( sight = above a score ( ye>s4 ,mean:ile ,de ,rheita 7#aeig-affj71 a ,capu*9 monk1 & an indu/ri\s & capable inve/igator1 _h be5 busy ) ! telescope1 & 9 #afde publi%$ at ,antw]p a some:at biz>re treatise1 d$icat$ to ,jesus ,*ri/1 & conta9+ n a little practical in=mation4 ,de ,rheita _h e>ly con/ruct$ b9ocul>s1 probably q indep5d5tly1 _h lately be5 dilig5tly exp]im5t+ ) ,desc>tes' hyp]bolic l5s1 x is ne$less to say )\t mu* success1 & 0 m$itat+ "w on a colossal scale--a glass to magnify #d1jjj times4 ,b 8 real contribution to optics 0 ! t]re/rial ocul>4 ,?is z he made x is %[n 9 ,fig4 #f :]e a b is ! image =m$ by ! objective 9 front ( ! eye l5s r1 s & t two equal l5ses sep>at$ by _! focal l5g?s & a b ! resultant re9v]t$ image4 ,?is =m rema9$ 9 common use until improv$ by ,doll& m ?an a c5tury lat]4 ,illu/ration3 ,,fig4 #g4--;,johannes ,hevelius4 ,a some:at e>li] =m ascrib$ to ,fa!r ,s*e9] _h m]g$ ! two l5ses =m+ ! inv]t+ sy/em ( ,fig4 #f1 into a s+le l5s us$ at its conjugate foci4 ,closely foll[+ ,de ,rheita came ,johannes ,hevelius 7#afaa-afhg7 ( ,danzig1 "o ( ! really important obs]v]s ( ! sev5te5? c5tury4 ,8 great treatise ,sel5ographia publi%$ 9 #afdg gives u ! fir/ sy/ematic /udy ( ! moon1 & a brief b illum9at+ acc\nt ( ! in/rum5ts ( ! "t & _! practical con/ruction4 ,at ?is "t ! ,galilean & ,kepl]ian =ms ( telescope 7 9 concurr5t use & ,hevelius gives directions = design+ & mak+ bo? ( !m4 ,app>5tly ! curr5t in/rum5ts 7 n g5]ally above five or six feet long & f ,hevelius' data w\ld give n above #cj diamet]s 9 ! ,galilean =m4 ,!re is m5tion1 h[ev]1 ( tubes up to #ab feet 9 l5g?1 & ( ! advantage 9 cle>ness & p[] ( ! long] focus plano-convex l5s4 ,pap] tubes1 evid5tly common1 are condemn$1 also ^? ( %eet iron on acc\nt ( _! weight1 & wood 0 to be pref]r$ = ! long] tubes4 ,evid5tly ,hevelius _h at ?is "t no notion ( ! effect ( ! plano-convex =m ( l5s z su* 9 less5+ ab]ration1 b he m5tions a curi\s =m ( telescope1 actually due to ,de ,rheita1 9 :i* ! objective is d\ble1 app>5tly ( two plano-convex l5ses1 ! weak] ahead1 & us$ ) a concave eye l5s4 ,if prop]ly proportion$ su* a d\blet w\ld h less ?an a qu>t] ! sph]ical ab]ration ( ! equival5t d\ble convex l5s4 ,hevelius also m5tions ! e>li] =m ( re-inv]t+ telescope above ref]r$ to1 & speaks r highly ( its p]=mance4 ,to judge f 8 num]\s draw+s ( ! moon made 9 #afdc & #afdd1 8 telescopes 7 mu* bett] ?an ^? ( ,s*e9] & ,fontana1 b /ill woefully lack+ 9 %>p def9ition4 ,nev]!less ! copp] plates ( ! ,sel5ographia1 repres5t+ e phase ( ! moon1 plac$ ! lun> details ) rem>kable accuracy & =m$ = m ?an a c5tury ! be/ lun> atlas available4 ,"o acquires an abid+ respect = ! pati5ce & skill ( ^! old a/ronom]s 9 see+ h[ mu* !y did ) means utt]ly inadequate4 ,"o may get a fair idea ( ! size1 appe>ance1 & m\nt+ ( telescopes 9 ?is e>ly day f ,fig4 #h1 :i* %[s a some:at advanc$ con/ruction cr$it$ by ,hevelius to a sugge/ion 9 ,desc>tes' ,dioptrica4 ,appe>ances indicate t ! tube 0 some:]e ab\t six feet long1 approximately two in*es 9 ap]ture1 & t x _h a draw tube = focuss+4 ,! (fset head ( ! m\nt to all[ obs]v+ ne> ! z5i? is wor? an extra glance4 ,incid5tally ,hevelius1 ) p]haps p>donable pride1 also expla9s ! 8,polemoscope10 a little inv5tion ( 8 own1 made1 he tells u1 9 #afcg4 ,x is no?+ else ?an ! fir/ p]iscope1 con/ruct$ z %[n 9 ,fig4 #i1 a tube c ) two "r angl$ bran*es1 a fairly long "o e = ! objective f1 a #de mirror at g1 ano!r at a1 & f9ally ! concave ocul> at b4 ,x 0 ( mode/ size1 ( tubes #a in* 9 diamet]1 ! long] tube be+ #bb in*es & ! upp] bran* #h in*es1 a size well suit$ = tr5* or p>apet4 ,illu/ration3 ,,fig4 #h4--;,a ,sev5? ,c5tury ,a/ronom] & 8 ,telescope4 ,ev5 9 ^! days ( 8 y\? ,hevelius _h le>n$ mu* ( practical optics z !n kn[n1 _h devis$ & 0 us+ v rational me?ods ( obs]v+ sun-spots by projection 9 a d>k5$ room1 & gives p]haps ! fir/ useful h9ts at te/+ telescopes by su* sol> obs]vations & on ! planets4 ,he 0 lat] to d mu* 9 ! developm5t & m\nt+ ( long telescopes & 9 obs]vation1 al?\gh1 :ile progressive 9 o!r respects1 he v curi\sly nev] seem$ to grasp ! importance ( telescopic sights & consi/5tly refus$ to use !m4 ,telescope con/ruction 0 n[ to fall into m skillful h&s4 ,%ortly aft] #afej ,*ri/ian ,huyg5s 7#afbi-afie71 & 8 accompli%$ bro!r ,con/ant9e awak5$ to a ke5 int]e/ 9 a/ronomy & devis$ new & excell5t me?ods ( =m+ accurate tools & ( gr9d+ & poli%+ l5ses4 ,illu/ration3 ,,fig4 #i4--,! fir/ ,p]iscope4 ,by #afee !y _h complet$ an in/rum5t ( #ab feet focus ) :i* ! /udy ( ,saturn 0 begun1 ,titan ! *ief satellite discov]$1 & ! r+ recogniz$4 ,pu%+ fur!r1 !y con/ruct$ a telescope ( #bc feet focal l5g? & #b in*es ap]ture1 ) :i* f\r ye>s lat] ,*ri/ian ,huyg5s f9ally solv$ ! my/]y ( ,saturn's r+4 ,evid5tly ?is glass1 :i* bore a p[] ( #ajj1 0 ( good def9+ quality1 z atte/$ by a sket* ( ,m>s late 9 #afie %[+ pla9ly ,syrtis ,major1 f obs]vation ( :i* ,huyg5s det]m9$ ! rotation p]iod to be ab\t #bd h\rs4 ,! ,huyg5s bro!rs 7 seem+ly ! fir/ fully to grasp ! advantage ( v long focus 9 cutt+ d[n ! ab]rations1 ! ap]ture be+ kept mod]ate4 ,_! usual proportions 7 ab\t z indicat$ above1 ! ap]ture be+ kept some:]e ne>ly z ! squ>e root ( ! focus 9 case ( ! l>g] glasses4 ,9 ! next two decades ! focal l5g? ( telescopes 0 pu%$ by all h&s to desp]ate extremes4 ,! ,huyg5s bro!rs ext5d$ !mselves to glasses up to #baj feet focus & built _m %ort] ones1 a fam\s example ( :i*1 ( #f in*es ap]ture & #abc feet focal l5g?1 pres5t$ to ! ,royal ,society1 is /ill 9 its possession4 ,auz\t produc$ ev5 long] telescopes1 & ,div9i & ,campani1 9 ,rome1 ( :om ! la/ nam$ made ,cass9i's telescopes = ! ,obs]vatory ( ,p>is1 7 n f> beh9d4 ,! ,engli% mak]s 7 simil>ly busy1 & ,hevelius 9 ,danzig 0 keep+ up ! record4 ,illu/ration3 ,,fig4 #aj4--;,*ri/ian ,huyg5s4 ,cle>ly ^! enorm\sly long telescopes c\ld n well be m\nt$ 9 tubes & ! us]s 7 driv5 to a]ial m\nt+s1 9 :i* ! objective 0 at ! upp] end ( a sp> or gird] & ! eye piece at ! l[]4 ,figure #aa %[s an actual con/ruction by ,hevelius = an objective ( #aej feet focal l5g?4 ,9 ?is case ! ma9 support 0 a ,t beam ( wood5 planks well brac$ toge!r4 ,additional /iffness 0 giv5 by light wood5 diaphragms at %ort int]vals ) ap]tures ( ab\t #h in*es next to ! objective1 & gradually increas+ d[nw>ds4 ,! :ole 0 l9$ up by equaliz+ tackle 9 ! v]tical plane1 & spread]s ) o!r tackle at ! jo9ts ( ! #dj;foot sections ( ! ma9 beam4 ,! ma/ :i* support$ ! :ole 0 ne>ly #ij feet high4 ,s unwieldly & inconv5i5t 7 ^! long affairs t1 q ap>t f _! usual optical imp]fections1 x is little wond] t !y l$ to no results comm5surate ) _! size4 ,9 fact ne>ly all ! productive "w 0 done ) telescopes f #bj to #ce feet long1 ) ap]tures r\ghly betwe5 #b & #c in*es4 ,illu/ration3 ,,fig4 #aa4--;,hevelius' #aej-;foot ,telescope4 ,dom9ique ,cass9i to be sure1 scrut9iz+ ,saturn 9 #afhd ) objectives by ,campani1 ( #ajj & #acf feet focus pick$ up ! satellites ,te?ys & ,dione1 b he _h previ\sly f\nd ,iapetus ) a #ag-;foot glass1 & ,rhea ) "o ( #cd feet4 ,! long] glasses above m5tion$ _h a]ial m\nts b ! small] ones 7 9 tubes support$ on a sort ( ladd] tripod4 ,a #bj-;foot objective1 p[] #ij1 gave ,cass9i ! division 9 ,saturn's r+4 ,a /ruggle 0 /ill be+ kept up = ! non-sph]ical curves urg$ by ,desc>tes4 ,x is q evid5t t ,huyg5s _h a g at !m1 & ,hevelius ?\ght at "o "t t he _h ma/]$ ! hyp]bolic figure1 b 8 publi%$ draw+s give no indication t he _h r$uc$ sph]ical ab]ration to any p]ceptible degree4 ,at ?is "t ! ma9 ?+ 0 to get good glass & give x true figure & poli%1 9 :i* ,huyg5s & ,campani excell$1 z ! "w on ,saturn witnesses4 ,^! 7 ! days ( ! dawn ( popul> a/ronomy & _m a g5tleman 0 ar\s$ to at lea/ a casual int]e/ 9 obs]v+ ! ,heav5s4 ,notes ,pepys 9 8 immortal ,di>y3 8,i f9d ,reeves !re1 x be+ a mighty f9e bright night1 & s ^u my leads1 ?\gh v sleepy1 till "o 9 ! morn+1 look+ on ! moon & ,jupit]1 ) ?is twelve foot glass1 & ano!r ( six foot1 t he ha? br\ght ) him to-night1 & ! sights mighty pleasant1 & "o ( ! glasses ,i w buy40 ,little poor ,pepys probably saw1 by reason ( 8 sev]e a/igmatism1 b a/ronomy 0 9 ! air ) ! impulse t comes to e sci5ce aft] a p]iod ( brilliant discov]y4 ,ano!r su* /imulus came ne> ! end ( ! eighte5? c5tury1 ) ! labors ( ,sir ,william ,h]s*el4 ,j at ?is juncture comes "o ( ! int]e/+ episodes ( telescopic hi/ory1 ! ineffectual & ab&on$ exp]im5ts on reflect+ in/rum5ts4 ,illu/ration3 ,,fig4 #ab4--;,gregory's ,diagram ( 8 ,telescope4 ,9 #affc ,james ,gregory 7#afch-afge7 a fam\s ,scotti% ma!matician1 publi%$ 8 ,optica ,promota1 9 :i* he describ$ ! r elegant con/ruction :i* be>s 8 "n1 a p]=at$ p>abolic mirror ) an elliptical mirror =w>d ( ! focus return+ an image to ! ocul> ?r\gh ! p]=ation4 ,x 0 conv5i5t 9 t x gave an erect image1 & x 0 s\nd !oretically1 &1 z ! future prov$1 practically1 b ! curves 7 q too mu* = ! contempor>y opticians4 ,figure #ab %[s ! diagrammatic con/ruction z publi%$4 ,! next ye> ,gregory />t$ ,reive1 a ,london optician1 d\btless ! same m5tion$ by ,pepys1 on ! con/ruction ( a #f foot telescope4 ,?is r ambiti\s ef=t fail$ ( mat]ial success ?r\gh ! inability ( ,reive to give ! ne$$ figures to ! mirrors1#d & ( x no?+ fur!r appe>s until ! +5i\s ,rob]t ,hooke 7#afce-agjc7 execut$ 9 #afgd a ,gregorian1 app>5tly )\t any notable results4 ,!re is a well def9$ tradition t ,gregory himself 0 us+ "o 9 #afge1 at ! "t ( 8 dea?1 b ! inv5tion !n dropp$ out ( sight4 #d ,he attempt$ to poli% !m on clo?1 :i* 9 itself 0 suffici5t to gu>antee failure4 ,no great] influ5ce on ! art att5d$ ! next attempt at a reflector1 by ,isaac ,newton 7#afdc-agbg74 ,?is 0 an e>ly outcome ( 8 notable discov]y ( ! disp]sion ( light by prisms1 :i* l$ him to despair ( improv+ refract+ telescopes & turn$ 8 m9d to reflectors4 ,unhappily 9 an exp]im5t to det]m9e :e!r refraction & disp]sion 7 proportional he committ$ ! s+ul> blund] ( rais+ ! refractive index ( a wat]-fill$ prism to equality ) glass by dissolv+ sug> ( lead 9 x4 ,)\t realiz+ ! impropriety ( ?us v>y+ two q unkn[n quantities at once 9 8 crucial exp]im5t1 he promptly jump$ to ! conclusion t refraction & disp]sion v>i$ 9 exact proportion 9 all sub/ances1 s t if two prisms or l5ses disp]s$ light to ! same ext5t !y mu/ also equally refract x4 ,x w\ld be int]e/+ to "k j h[ ! fact ( 8 bungl+ 0 pass$ along to po/]ity4 ,z a nave apologi/ once rem>k$1 x 0 n to be f\nd 9 8 8,optics40 ,b ,sir ,david ,brew/] & ,sir ,john ,h]s*el1 bo? /aun* admir]s ( ! great philosoph]1 /ate ! fact v positively4 ,if "o may haz>d a guess x crept out at ,cambridge & 0 pass$ along1 p]haps to ,sir ,william ,h]s*el1 via ! unpubli%$ hi/ory ( rese>* t is ri* 9 picturesque details ( ! m>e's ne/s ( sci5ce4 ,at all ev5ts a mi/ake ) a great "n beh9d x c>ries f>1 & ! result 0 to delay ! production ( ! a*romatic telescope by "s ?ree qu>t]s ( a c5tury4 ,turn+ f refractors he pres5t$ to ! ,royal ,society j aft] 8 election z ,fell[ 9 #afgb1 ! little six-in* model ( 8 device :i* 0 receiv$ ) acclamation & !n lay on ! %elf )\t mak+ ! slighte/ impression on ! art1 = full half a c5tury4 ,newton1 by dropp+ ! notion ( direct view ?r\gh ! tube1 hit ^u by f> ! simple/ way ( gett+ ! image outside x1 by a plane mirror a little inside focus & incl9$ at #de1 b injudici\sly ab&on$ ! p>abolic mirror ( 8 orig9al pap] on disp]sion4 ,8 inv5tion !re=e z actually made public 0 ( ! comb9ation ) a sph]ical concave mirror ( a plane mirror ( elliptical =m at #de1 a con/ruction :i* 9 lat] pap]s he def5d$ z fully adequate4#e #e ,9 ,fig #ac1 ,a is ! support ( ! tube & focuss+ screw1 ,b ! ma9 mirror1 an in* 9 diamet]1 ,,cd ! oblique mirror1 ,e ! pr9cipal focus1 ,f ! eye l5s1 & ,g ! memb] f :i* ! oblique mirror is c>ri$4 ,illu/ration3 ,,fig4 #ac4--,newton's ,model ( 8 ,reflector4 ,8 error 9 judgm5t d\btless came f lack ( practical a/ronomical exp]i5ce1 = he assum$ t ! :ole real tr\ble ) exi/+ telescopes 0 *romatic ab]ration1 :i* 9 fact worri$ ! obs]v] little m ?an ! faults due to o!r causes1 s9ce ! v l[ lum9osity t[>d ! ends ( ! spectrum enorm\sly less5s ! indi/9ctness due to disp]sion4 ,z a matt] ( fact ! long focus objective ( small ap]ture did v cr$itable "w1 & its errors w\ld n comp>e unfavorably ) ^? ( a sph]ical concave mirror ( ! wide ap]ture plann$ by ,newton4 ,_h he actually made "o ( 8 telescopes ( fair dim5sions & p[] ! def9ition w\ld infallibly h be5 wreck$ by ! ab]rations due to sph]ical figure4#f #f ,9 fact a 8f\r foot telescope ( ,mr4 ,newton's inv5tion0 br\ght be=e ! ,royal ,society two weeks aft] 8 orig9al pap]1 prov$ only fair 9 quality1 0 return$ some:at improv$ at ! next meet+1 & !n 0 ref]r$ to ,mr4 ,hooke to be p]fect$ z f> z might be1 aft] :i* no?+ m 0 he>d ( x4 ,illu/ration3 ,,fig4 #ad4--;,de ,b]c's sket* ( ,cassegra9's ,telescope4 ,x is q likely t appreciation ( ?is1 & ! grave d\bts ( bo? ,newton & ,huyg5s z to obta9+ a prop] p>abolic curve *eck$ fur!r developm5ts4 ,ab\t ! beg9n+ ( ! ye> #afgb ,m4 ,cassegra9 communicat$ to ,m4 de ,b]c a design = a reflect+ telescope1 :i* ev5tually f\nd its way into ! ,philosophical ,transactions ( ,may 9 t ye>1 aft] previ\s publication 9 ! ,j\rnal des ,savans4 ,figure #ad %[s de ,b]c's r\gh orig9al sket*4 ,x diff]$ f ,gregory's con/ruction 9 t ! latt]'s elliptical concave mirror plac$ outside ! ma9 focus1 0 replac$ by a convex mirror plac$ inside focus4 ,! image 0 !re=e inv]t$4 ,! inv5tor is ref]r$ to 9 hi/ories ( sci5ce z 8,cassegra91 a ,fr5*man40 ,he 0 9 fact ,sieur ,guillaume ,cassegra91 sculptor 9 ! s]vice ( ,l\is ,quatorze1 modell] & f\nd] ( _m /atues4 ,9 #afff he 0 paid #abjj livres = execut+ a bu/ ( ! ,k+ modell$ by ,b]t91 & lat] made _m replicas f ! antique = ! decoration ( ,8 ,maje/y's g>d5s at ,v]sailles4 ,he disappe>$ f ! royal records 9 #afhd & probably di$ )9 a ye> or two ( t date4 ,at ! p]iod "h conc]n$ he app>5tly1 l de ,b]c1 0 ( ,*>tres4 ,famili> ) work+ bronzes & ) ! art ( ! f\nd]1 he 0 a v likely p]son to h execut$ specula4 ,al?\gh !re is no c]ta9ty t he actually made a telescope1 a contempor>y ref]5ce 9 ! ,j\rnal des ,savans speaks ( 8 inv5tion z a 8petite lunette d'appro*e10 & "o does n usually sugge/ ! dim5sions ( a ?+ non-exi/5t4 ,h[ long he _h be5 work+ ^u x prior to ! p]iod ab\t ! beg9n+ ( #afgb :5 he disclos$ ! device to de ,b]c is unkn[n4 ,probably ,newton's inv5tion 0 ! e>li]1 b ! two 7 indep5d5t1 & x 0 some:at ung5]\s ( ,newton to criticise ,cassegra91 z he did1 = us+ sph]ical mirrors1 on ! /r5g? ( de ,b]c's v sup]ficial description1 :5 he himself consid]$ ! p>abolic ne$less4 ,h[ev]1 no?+ fur!r 0 done1 & ! devices ( ,gregory1 ,newton & ,cassegra9 w5t toge!r into ! disc>d = "s fifty ye>s4 ,^! e>ly exp]im5ts gave s+ul>ly little in=mation ab\t mat]ial = mirrors & me?ods ( work+ x1 s little t ^? :o foll[$1 ev5 up to ,"l ,rosse1 _h to "w ! problems out = !mselves4 ,we "k f 8 orig9al pap] t ,newton us$ bell-metal1 :it5$ by ! addition ( ars5ic1 foll[+ ! lore ( ! al*emi/s4 ,^! speculative wor?ies us$ to alloy copp] ) ars5ic1 ?9k+ t by giv+ x a :iti% ca/ !y _h rea*$ a sort ( half way po9t on ! road to silv]4 ,v silly at fir/ ?\ght1 b be=e ! days ( *emical analysis1 :5 ! ess5tial prop]ties ( ! metals 7 unkn[n1 ! way ( ! sci5tific exp]im5t] 0 h>d4 ,:at ! 8/eely matt]1 imploy$ 9 ,london0 ( :i* ,newton speaks 9 an e>ly pap] 01 we d n "k--v likely "o ( ! h>d alloys mu* ri*] 9 t9 ?an is ord9>y bell-metal4 ,nor d we "k to :at v>iety ( speculum metal ,huyg5s ref]s 9 8 correspond5ce ) ,newton4 ,z to me?ods ( work+ x ,newton only disclos$ 8 s*eme ( pit*-poli%+ "s ?irty ye>s aft] ?is p]iod1 :ile x is a matt] ( previ\s record1 t ,huyg5s _h be5 9 ! habit ( poli%+ 8 true tools on pit* f "s date unkn[n4 ,probably nei!r ( !m orig9at$ ! practice4 ,opticians are a peculi>ly secretive folk & %op me?ods are likely to be kept = a long "t be=e !y leak out or are r$iscov]$4 ,mod]n speculum metal is sub/antially a def9ite comp\nd ( f\r atoms copp] & "o t9 7,sn,cu#d71 practically #fh p] c5t copp] & #cb p] c5t t91 & is n[1 z x 0 9 all previ\s modifications1 a peculi>ly mean mat]ial to ca/ & "w4 ,?us exit ! reflector4 ,! long telescope cont9u$ to gr[ long] ) only sl[ improvem5t 9 quality1 b ! next decade 0 m>k$ by ! introduction ( ,huyg5s' eyepiece1 an imm5se improvem5t ov] ! s+le l5s :i* _h gone be=e1 & ) slight modifications 9 use today4 ,illu/ration3 ,,fig4 #ae4--;,diagram ( ,huyg5s' ,eyepiece4 ,?is is %[n 9 section 9 ,fig4 #ae4 ,x consi/s ( a field l5s ,a1 plano-convex1 & an eye l5s ,b ( "o-?ird ! focal l5g?1 ! two be+ plac$ at ! diff]5ce ( _! focal l5g?s ap>t ) 79 lat] days7 a /op half way betwe5 !m4 ,! eye piece is pu%$ inside ! ma9 focus until ! rays :i* fall on ! field l5s focus ?r\gh ! eye l5s4 ,! great ga9 f ,huyg5s' view-po9t 0 a v mu* enl>g$ cle> field--ab\t a f\r-fold increase--& 9 fact ! comb9ation is sub/antially a*romatic1 p>ticul>ly important n[ :5 high p[] ocul>s are ne$$4 ,/ill l>g] progress 0 made 9 giv+ ! objective a bett] =m ) respect to sph]ical ab]ration1 ! 8cross$0 l5s be+ r g5]ally adopt$4 ,?is =m is d\ble convex1 & if ( ord9>y glass1 ) ! re> radius six times ! front radius1 & gives ev5 bett] results ?an a plano-convex 9 its be/ position-plane side to ! re>4 ,objectives 7 rat$ on focal l5g? = ! gre5 rays1 t is1 ! bright c5tral "p ( ! spectrum1 ! violet rays ( c\rse fall+ %ort & ! r$ runn+ beyond4 ,to give cu/om>y dim5sions1 a telescope ( #c in*es ap]ture1 ) magnify+ p[] ( #ajj1 w\ld be ( ab\t #cj feet focus ) ! violet ne>ly #f in*es %ort & ! r$ a simil> am\nt long4 ,x is va/ cr$it to ! e>ly obs]v]s t ) su* sl5d] means !y did s mu*4 ,b 9 fact ! long telescope _h rea*$ a me*anical impasse1 s t ! la/ qu>t] ( ! sev5te5? c5tury & ! fir/ qu>t] ( ! next 7 m>k$ *iefly by ! developm5t ( a/ronomy ( position ) in/rum5ts ( mode/ dim5sions4 ,illu/ration3 ,,fig4 #af4--,! ,fir/ ,reflector4 ,john ,hadley1 #agbb4 ,9 due "t ! new ord] came & ) a/\nd+ sudd5ness4 ,j at ! end ( #agbb ,james ,bradley 7#afib-agfb7 measur$ ! diamet] ( ,v5us ) an objective ( #bab ft4 #c 94 focal l5g?2 ab\t ?ree mon?s lat] ,john ,hadley 7#afhb-agdd7 pres5t$ to ! ,royal ,society ! fir/ reflect+ telescope wor?y ! "n1 & ! old ord] practically end$4 ,john ,hadley %\ld 9 fact be reg>d$ z ! real inv5tor ( ! reflector 9 q ! same s5se t ,mr4 ,edison has be5 held1 de jure & de facto1 ! inv5tor ( ! inc&esc5t electric lamp4 ,actually ,hadley's case is ! /rong] ( ! two1 = ! only ?+s :i* c\ld h be5 cit$ aga9/ him 7 ab&on$ exp]im5ts fifty ye>s old4 ,moreov] he took successfully ! ess5tial /ep at :i* ,gregory & ,newton _h /umbl$ or turn$ back--p>aboliz+ 8 speculum4 ,! in/rum5t he pres5t$ 0 ( approximately #f in*es ap]ture & #fb in*es focal l5g?1 :i* he _h made & te/$ "s ?ree ye>s previ\sly2 on a sub/antial alt-azimu? m\nt ) sl[ motions4 ,he us$ ! ,newtonian oblique mirror & ! in/rum5t 0 provid$ ) bo? convex & concave eye l5ses1 ) magnifications up to ab\t #bcj4 ,! :ole arrangem5t is %[n 9 ,fig4 #af :i* is = ! mo/ "p self explanatory4 ,x is wor? not+ t ! speculum is position$ 9 ! wood5 tube by press+ x =w>d aga9/ ?ree equidi/ant /uds by ?ree correspond+ screws at ! re>1 t a slid] mov$ by a trav]s+ screw 9 a wide groove c>ries ! small mirror & ! ocul>1 t !re is a conv5i5t door = access to ! mirror1 & also a suitable f9d]4 ,! motion 9 altitude is obta9$ by a key w9d+ its cord aga9/ gravity4 ,t 9 azimu? is by a roll] support along a horizontal runway c>ri$ by an upright1 & is obta9$ by ! key ) a cord pull (f 9 "o direction1 & 9 ! o!r1 by spr+s )9 ! ma9 upright1 turn+ a po/ ( :i* ! head c>ries *eek pieces on :i* re/ ! trunnions ( ! tube4 ,a few mon?s lat] ?is telescope 0 c>efully te/$1 by ,bradley & ! ,rev4 ,j4 ,p\nd1 aga9/ ! ,huyg5s objective ( #abc feet focus possess$ by ! ,royal ,society1 & ) altoge!r satisfactory results4 ,hadley's reflector w\ld %[ ev]y?+ :i* c\ld be se5 by ! long in/rum5t1 be>+ z mu* p[] & ) equal def9ition1 ?\gh some:at less5$ light4 ,9 p>ticul> !y saw all five satellites ( ,saturn1 ,cass9i's division1 :i* ! inv5tor himself _h se5 ! previ\s ye> ev5 9 ! nor!rn edge ( ! r+ beyond ! planet1 & ! %ad[ ( ! r+ ^u ! ball4 ,! ca/+ ( ! l>ge speculum 0 f> f p]fect1 ) _m spots t fail$ to take poli%1 b ! figure mu/ h be5 r good4 ,a sph]ical mirror ( ^! dim5sions w\ld give an ab]ration blur some?+ l tw5ty times ! wid? ( ,cass9i's division1 & ! *ance ( see+ all five satellites ) x w\ld be negligibly small4 ,fur!r1 ,hadley pres5tly disclos$ to o!rs n only ! me?od he us$ 9 poli%+ & p>aboliz+ specula1 b 8 me?od ( te/+ = true figure by ! ab]rations disclos$ z he work$ ! figure away f ! sph]e--a s*eme frequ5tly us$ ev5 to ?is day4 ,! effect ( ,hadley's "w 0 pr(\nd4 ,"u 8 guidance o!rs began to produce well figur$ mirrors1 9 p>ticul> ,molyneux & ,hawksbee2 reflect+ telescopes became fairly common2 & 9 ! beg9n+ ( ! next decade ,james ,%ort1 7#agaj-agfh71 possess$ ( craftsman%ip t approa*$ wiz>dry1 n only fully ma/]$ ! art ( figur+ ! p>aboloid1 b at once took up ! ,gregorian con/ruction ) its ellipsoidal small mirror1 ) mu* success4 ,8 specula 7 ( great relative ap]ture1 ,f_/#d to ,f_/#f1 & f ! excell5t quality ( 8 metal "s ( !m h reta9$ _! f9e poli% & def9ition aft] m ?an a c5tury4 ,he is said to h gone ev5 up to #ab in*es 9 diamet]4 ,8 exact me?ods ( work+ di$ ) him4 ,ev5 8 tools he ord]$ to be de/roy$ be=e 8 dea?4 ,! ,cassegra9 reflector1 prop]ly hav+ a p>abolic l>ge mirror & a hyp]bolic small "o1 seems v r>ely to h be5 made 9 ! eighte5? c5tury1 ?\gh "o c]ta9ly came into ! h&s ( ,ramsd5 7#agce-ahjj74 ,few refractors = a/ronomical use 7 made aft] ! adv5t ( ! reflector1 :i* 01 & is1 h[ev]1 badly suit$ = ! purposes ( a portable spy-glass1 ow+ to tr\ble f /ray light4 ,! refractor !re=e p]man5tly held its own 9 ?is function1 despite its l5g? & uncorrect$ ab]rations4 ,relief 0 ne> at h&1 = h>dly _h ,%ort />t$ on 8 notable c>e] :5 ,*e/] ,moor ,hall1 ,esq4 7#agjd-agga7 a g5tleman ( ,essex1 design$ & caus$ to be con/ruct$ ! fir/ a*romatic telescope1 ) an objective ( cr[n & fl9t glass4 ,he is /at$ to h be5 /udy+ ! problem = sev]al ye>s1 l$ to x by ! errone\s belief 7%>$ by ,gregory long be=e7 t ! human eye 0 an example ( an a*romatic in/rum5t4 ,be ?is z x may ,hall _h 8 telescopes made by ,george ,ba/ ( ,london at lea/ z e>ly z #agcc1 & accord+ to ! be/ available evid5ce sev]al in/rum5ts 7 produc$1 "o ( !m ( above #b in*es ap]ture on a focal l5g? ( ab\t #bj in*es 7,f_/#h7 & fur!r1 subsequ5tly su* in/rum5ts 7 made & sold by ,ba/ & o!r opticians4 ,^! facts are cle> & yet1 ) k ( !m among ,london workm5 z well z among ,hall's fri5ds1 ! inv5tion made no impression1 until x 0 aga9 br\ght to light1 & pat5t$1 by ! celebrat$ ,john ,doll& 7#agjf-agfa7 9 ! ye> #ageh4 ,physical consid]ations give a clue to ?is s+ul> neglect4 ,! only glasses diff]+ mat]ially 9 disp]sion available 9 ,hall's day 7 ! ord9>y cr[n1 & su* fl9t z 0 9 use 9 ! glass cutt+ trade1--:at we w\ld n[ "k z a light fl9t1 & f> f homog5e\s at t4 ,illu/ration3 ,lodge 8,pione]s ( ,sci5ce40 ,,fig4 #ag4--;,john ,doll&4 ,out ( su* mat]ial x 0 practically v h>d 7z ! ,doll&s quickly f\nd7 to make a d\ble objective dec5tly free f sph]ical ab]ration1 especially = "o work+1 z ,hall q assur$ly did1 by rule ( ?umb4 ,) ! additional h&icap ( fl9t full ( faults x is altoge!r likely t ^! fir/ a*romatics1 :ile embody+ ! correct pr9ciples1 7 n good en\gh to make effective headway aga9/ ! *eap] & simpl] spy-glass ( ! "t4 ,doll&1 al?\gh 9 #agec he /rongly support$ ,newton's error 9 a ,royal ,society pap] aga9/ ,eul]'s belief 9 a*romatism1 %ift$ 8 view a c\ple ( ye>s lat] & aft] a consid]able p]iod ( skilful & well ord]$ exp]im5t+ publi%$ 8 discov]y ( a*romatism e>ly 9 #ageh1 = :i* a pat5t 0 grant$ him ,april #ai1 :ile 9 ! same ye> ! ,royal ,society honor$ him ) ! ,copley m$al4 ,f t "t until 8 dea?1 late 9 #agfa1 he & 8 son ,pet] ,doll& 7#agcj-ahbj7 7 actively produc+ a*romatic glasses4 ,! ,doll&s 7 admirable craftsm5 & _! e>ly product 0 probably consid]ably bett] ?an 7 ,hall's objectives b !y felt ! lack ( suitable fl9t & soon aft] ,john ,doll&'s dea?1 ab\t #agfe1 ! son s\ght relief 9 ! triple objective ( :i* an e>ly example is %[n 9 ,fig4 #ah1 & :i*1 ) "s modifications1 0 8 /&>d =m = _m ye>s4 ,illu/ration3 ,,fig4 #ah4--,pet] ,doll&'s ,triple ,objective4 ,o!r opticians began to make a*romatics1 &1 ,pet] ,doll& hav+ ?reat5$ action = infr+em5t1 a petition 0 br\ght by #ce opticians ( ,london 9 #agfd = ! annulm5t ( ,john ,doll&'s pat5t1 alleg+ t he 0 n ! orig9al inv5tor b _h k ( ,*e/] ,moor ,hall's prior "w4 ,9 ! li/ 0 ,george ,ba/1 :o 9 fact did make ,hall's objectives tw5ty five ye>s be=e ,doll&1 & also "o ,rob]t ,rew ( ,coldba? ,fields1 :o claim$ 9 #agee to h in=m$ ,doll& ( ! con/ruction ( ,hall's objective4 ,?is 0 j ! "t :5 ,doll& came to ! "r ab\t face on a*romatism1 & x may well be t f ,rew or else:]e he may h le>n$ t a duplex a*romatic l5s _h really be5 produc$4 ,b 8 ,royal ,society pap] %[s t 8 result came f hone/ inve/igations1 & at wor/ he is 9 ab\t ! position ( ,galileo a c5tury & a half be=e4 ,! petition app>5tly br\ght no action1 p]haps because ,pet] ,doll& next ye> su$ ,*ampneys1 "o ( ! sign]s1 & obta9$ judgm5t4 ,x 0 9 ?is case t ! judge 7,"l ,camd57 deliv]$ ! (t quot$ dictum3 8,x 0 n ! p]son :o lock$ up 8 inv5tion 9 8 scrutoire t ought to pr(it by a pat5t = su* inv5tion1 b he :o br\ght x =? = ! b5efit ( ! public4#g0 #g ,commonly1 b x appe>s errone\sly1 ascrib$ to ,"l ,mansfield4 ,?is 0 s\nd equity en\gh1 assum+ ! facts to be z /at$1 b :ile ,hall did n publi% ! inv5tion admitt$ly made by him1 x _h c]ta9ly become kn[n to _m4 ,*e/] ,moor ,hall 0 a sub/antial & respect$ lawy]1 a b5*] ( ! ,inn] ,temple1 & "o is incl9$ to ?9k t 8 alleg$ concealm5t 0 purely con/ructive1 9 8 fail+ to conte/ ,doll&'s claim4 ,_h he appe>$ at ! trial ) 8 fight+ blood up1 !re is e reason to believe t he c\ld h e/abli%$ a p]fectly good case ( public use q aside f 8 pro( ( te*nical priority4 ,h[ev]1 hav+ cle>ly lo/ 8 own claims ?r\gh la*es1 he n improbably 0 q cont5t to let ! tradesm5 fight x out among !mselves4 ,hall's telescopes 7 9 fact kn[n to be 9 exi/5ce z late z #ahbg4 ,z ! eighte5? c5tury drew t[>d its end+ ! reflect+ telescope1 *iefly 9 ! ,gregorian =m1 held ! field 9 a/ronomical "w1 ! old refractor ( _m draw tubes 0 ! spy-glass ( popul> use1 & ! newly introduc$ a*romatic 0 ! in/rum5t ( 8! exclusive trade40 ,no glass ( suitable quality = well correct$ objectives _h be5 produc$1 & t available 0 n to be _h 9 discs l>ge en\gh = s]i\s "w4 ,a #c-;in* objective 0 reckon$ r l>ge4 ,,*apt] ,,ii ,,! ,,mod]n ,,telescope ,! *ief l9k betwe5 ! old & ! new1 9 in/rum5tal z well z obs]vational a/ronomy1 0 ,sir ,william ,h]s*el 7#agch-ahbb74 ,9 ! fir/ place he c>ri$ ! figur+ ( 8 mirrors to a po9t n approa*$ by 8 pr$ecessors1 & second1 he taught by example ! imm5se value ( ap]ture 9 def9ition & grasp ( light4 ,8 life has nev] be5 adequately writt51 b ,miss ,cl]ke's 8,! ,h]s*els & ,mod]n ,a/ronomy0 is extremely well wor? ! read+ z a record ( a*ievem5t t knew n ! impossible4 ,illu/ration3 ,miss ,cl]ke's ,h]s*el ,mod]n ,a/ronomy 7,macmillan74 ,,fig4 #ai4--,sir ,william ,h]s*el4 ,he 0 ! son ( a capable b&-ma/] ( ,hanov]1 br\ght up z a musician1 9 a family ( exceptional musical abilities1 & 9 #ageg jump$ 8 milit>y responsibilities & emigrat$ to ,engl&1 to ! _w's great ga94 ,= ne>ly a decade he /ruggl$ upw>d 9 8 art1 tak+ mean:ile e opportunity = self education1 n only 9 ! !ory ( music b 9 ma!matics & ! languages1 & 9 #agfg we f9d him settl$ 9 fa%ionable ,ba?1 oboi/ 9 a fam\s or*e/ra1 & organi/ ( ! ,octagon ,*apel4 ,8 abilities br\ght him _m pupils1 & ultimately he became director ( ! or*e/ra 9 :i* he _h play$1 & ! musical dictator ( ! fam\s old resort4 ,9 #aggb came 8 inspiration 9 ! loan ( a #b-;foot ,gregorian reflector1 & a little casual />-gaz+ ) x4 ,x 0 ! op5+ ( ! k+dom ( ! skies1 & he s\ght to pur*ase a telescope ( 8 own 9 ,london1 only to f9d ! price too great = 8 means4 7,ev5 a #b-;foot1 ( #d in*es ap]ture1 by ,%ort 0 li/$ at five-&-?irty gu9eas47 ,!n aft] "s futile attempts at mak+ a pla9 refractor he settl$ d[n to h>d "w at ca/+ & poli%+ specula4 ,al?\gh possess$ ( great me*anical abilities ! difficult te*nique ( ! new art long baffl$ him1 & he ca/ & work$ "s #bjj small discs 9 ! production ( 8 fir/ successful telescopes1 to say no?+ ( a /ill great] numb] 9 l>g] sizes 9 8 imm$iately subsequ5t c>e]4 ,z "t w5t on he scor$ a l>g] proportion ( successes1 b at ! />t good figure seems to h be5 l>gely =tuit\s4 ,inside ( a c\ple ( ye>s1 h[ev]1 he _h ma/]$ some?+ ( ! art & turn$ out a #e-;foot in/rum5t :i* seems to h be5 ( excell5t quality1 foll[$ lat] by a #g-;foot 7ap]ture #f in*es7 ev5 bett]1 & !n by o!rs /ill bigg]4 ,! be/ ( ,h]s*el's specula mu/ h be5 ( exquisite figure4 ,8 #g-;foot 0 te/$ at ,gre5wi* aga9/ "o ( ,%ort's ( #i in*es ap]ture mu* to ! latt]'s disadvantage4 ,8 discov]y ) ! #g-;foot1 ( ! 8,georgium ,sidus0 7,uranus7 9 #agha won him imm$iate fame & recognition1 beside spurr+ him to great] ef=ts1 especially 9 ! direction ( l>g] ap]tures1 ( :i* he _h fully grasp$ ! importance4 ,9 #aghb he successfully complet$ a #ab-;in* speculum ( #bj feet focus1 foll[$ 9 #aghh by an #ah-;in* ( ! same l5g?4 ,! previ\s ye> he fir/ arrang$ 8 reflector z a 8front view0 telescope--! s-call$ ,h]s*elian4 ,up to ?is "t he1 except = a few ,gregorians1 _h us$ ,newton's oblique mirror4 ,! heavy loss ( light 7ar\nd #dj p] c5t7 9 ! second reflection mov$ him to tilt ! ma9 mirror s z to ?r[ ! focal po9t to ! edge ( ! ap]ture :]e "o c\ld look d[nw>d ^u ! image ?r\gh ! ocul> z %[n 9 ,fig4 #bj4 ,"h ,,ss is ! great speculum1 ,o ! ocul> & i ! image =m$ ne> ! rim ( ! tube4 ,9 itself ! tilt+ w\ld s]i\sly impair ! def9ition1 b ,h]s*el wisely built 8 telescopes ( mod]ate relative ap]ture 7,f_/#aj to ,f_/#bj71 s t ?is difficulty 0 consid]ably less5$1 :ile ! sav+ ( light1 am\nt+ to ne>ly a /ell> magnitude1 0 important4 ,mean:ile he 0 h>d at "w on 8 greate/ mirror1 ( #dh in*es cle> ap]ture & #dj feet focal l5g?1 ! fa!r ( ! great l9e ( mod]n telescopes4 ,x 0 f9i%$ 9 ! summ] ( #aghi4 ,! speculum 0 #di in*es 9 ov]-all diamet]1 #c in*es ?ick & weigh$ z ca/ #baah lbs4 ,! completion ( ?is in/rum5t1 :i* w\ld rank z l>ge ev5 today1 0 made notable by ! imm$iate discov]y ( two new satellites ( ,saturn1 ,enceladus & ,mimas4 ,x also prov$ ( v great value 9 sweep+ = nebul1 b its usefulness seems to h be5 mu* limit$ by ! flexure ( ! mirror "u its great weight1 & by its rapid t>ni%+4 ,x requir$ repoli%+1 :i* meant refigur+1 at lea/ e two ye>s1 a prodigi\s task4#h #h ,?is 0 probably due n only to unfavorable climate1 b to ! fact t ,h]s*el1 ) all 8 +5uity1 does n appe> to h ma/]$ ! ca/+ difficulty1 & 0 con/ra9$ to make 8 big speculum ( ,cu #ge p] c5t1 ,sn #be p] c5t1 a composition work+ r easily & tak+ beautiful1 b f> f p]man5t1 poli%4 ,he nev] seems to h us$ practically ! ,sn,cu#d =mula1 devis$ empirically by ,mudge 7,phil4 ,trans4 #fg1 #bih71 & 9 q g5]al use !reaft] up to ! pres5t "t4 ,illu/ration3 ,,fig4 #bj4--;,h]s*el's ,front ,view ,telescope4 ,x 0 us$ z a front view in/rum5t & 0 arrang$ z %[n 9 ,fig4 #ba4 ,obvi\sly ! front view =m has aga9/ x ! me*anical difficulty ( support+ ! obs]v] up to q ! full focal l5g? ( ! in/rum5t 9 air1 a difficulty va/ly increas$ 7 ! m\nt an equatorial "o1 s t = ! great mod]n reflectors ! ,cassegra9 =m1 look$ into axially upw>d1 & 9 l5g? only a ?ird or a qu>t] ( ! pr9cipal focus1 is almo/ univ]sal4 ,z soon z ! excell5t results obta9$ by ,h]s*el became g5]ally kn[n1 a l>ge dem& arose = 8 telescopes1 :i* he fill$ 9 s f> z he c\ld sp>e ! "t f 8 regul> "w1 & n ! lea/ ( 8 s]vices to sci5ce 0 ! di/ribution ( telescopes ( high quality & consequ5t /rong /imulus to g5]al int]e/ 9 a/ronomy4 ,two ( 8 in/rum5ts1 ( #d-;& #g-;feet focus respectively1 fell into ! wor?y h&s ( ,s*rt] at ,lili5?al & did /]l+ s]vice 9 mak+ 8 great sy/ematic /udy ( ! lun> surface4 ,at ! />t ev5 ,h]s*el's #g-;foot telescope br\ght #bjj gu9eas1 & ! funds ?us won he promptly turn$ to rese>*4 ,illu/ration3 ,miss ,cl]ke's ,h]s*el ,mod]n ,a/ronomy 7,macmillan74 ,,fig4 #ba4--;,h]s*el's ,=ty-foot ,telescope4 ,we sometimes ?9k ( ! late eighte5? c5tury z a "t ( lic5se unb\nd$ & ! high] life contemn$1 b ,h]s*el wak5$ a g5]al int]e/ 9 unappli$ sci5ce t has h>dly s9ce be5 equall$ & nev] surpass$4 ,try to picture social & (ficial ,wa%+ton ru%+ to d honor to "s a/ronom] :o by luck _h f\nd ! trans-,neptunian planet2 ! diplomatic corps cr[d+ 8 doors1 & 8 v way to ! ,naval ,obs]vatory block$ by ! lim\s9es ( ! curi\s & admir+1 & "s idea may be ga9$ ( :at really happ5$ to ! unassum+ music ma/] f ,ba? :o sudd5ly f\nd himself fam\s4 ,great z 7 ! advances made by ,h]s*el ! reflector 0 de/9$ to fall into disuse = _m ye>s4 ,! fact 0 t ! specula _h to be refigur$1 z 9 ! case ( ! great #dj-;foot telescope1 q too (t5 to meet ! requirem5ts ( ! ord9>y us]1 pr(essional or amateur4 ,only ^? capable ( do+ _! own figur+ c\ld keep _! in/rum5ts conv5i5tly 9 s]vice4 ,sir ,w4 ,h]s*el always _h relays ( specula at h& = 8 small] in/rum5ts1 & :5 8 di/+ui%$ son1 ,sir ,john ,f4 ,w4 ,h]s*el1 w5t on 8 fam\s obs]v+ exp$ition to ! ,cape ( ,good ,hope 9 #ahcd-ch he took along 8 poli%+ ma*9e & ?ree specula = 8 #bj-;foot telescope4 ,& he ne$$ !m inde$1 = a surface w\ld sometimes g bad ev5 9 a week1 & regul>ly became q useless 9 #b or #c mon?s4 ,mak]s :o us$ ! h>d] speculum metal1 v brittle & sc>cely to be t\*$ by a file1 f>$ bett]1 & "s small mirrors1 well c>$ =1 h held s]viceable poli% = _m ye>s4 ,_m ( ^! in/rum5ts ( ,h]s*el's "t1 too1 7 ( v admirable p]=mance4 ,"s ( ,h]s*el's own #g-;foot telescopes give evid5ce ( exquisite figure & he n only commonly us$ magnify+ p[]s up to "s #hj p] in* ( ap]ture1 a good /iff figure = a telescope old or new1 b w5t above #b1jjj1 ev5 ne>ly to #f1jjj on "o ( 8 #f-;in* mirrors )\t los+ ! r\ndness ( ! /> image4 8,empty magnification0 ( c\rse1 ga9+ no detail :atev]1 b evid5ce ( good workman%ip4 ,_m ye>s lat] ! ,rev4 ,w4 ,r4 ,dawes1 ! fam\s ,engli% obs]v]1 _h a #e-;in* ,gregorian1 commonly ref]r$ to z 8,! ,jewel10 on :i* he us$ #dcj diamet]s1 & pu%$ to #b1jjj on ,pol>is )\t di/ortion ( ! disc4 ,comp>+ x ) a #e-;foot 7approximately #d-;in* ap]ture7 refractor1 he reports ! ,gregorian some:at inf]ior 9 illum9at+ p[]2 8,b 9 %>pness ( def9ition1 smallness ( discs ( />s1 & h>dness ( outl9e ( planets x is sup]ior40 ,all ( :i* %[s t :ile me?ods & mat]ial may h improv$1 ! eld]s did n 9 ! lea/ lack skill4 ,! next /ep =w>d1 & a mom5t\s "o1 0 to be tak5 9 ! a*romatic refractor4 ,its g5]al pr9ciples 7 und]/ood1 b cle> & homog5e\s glass1 p>ticul>ly fl9t glass1 0 n to be _h 9 pieces ( any size4 8,optical glass10 z we und]/& ! t]m1 0 unkn[n4 ,x is a curi\s & dramatic fact t to a s+le man 0 due n only ! orig9 ( ! art b ! optical glass indu/ry ( ! _w4 ,if ! capacity = tak+ inf9ite pa9s be g5ius1 !n ! t]m rightfully belongs to ,pi]re ,l\is ,gu9&4 ,he 0 a ,swiss artisan liv+ 9 ! ,canton ( ,neu*atel ne> ,*aux-de-,fonds1 mak] ( bells = repeat]s1 & becom+ int]e/$ 9 con/ruct+ telescopes import$ "s fl9t glass f ,engl& & f\nd x bad4 ,he !reupon und]took ! task ( mak+ bett]1 & f #aghd kept /eadily at 8 exp]im5ts1 failure only spurr+ him on to r$\bl$ ef=ts4 ,all he c\ld e>n at 8 trade w5t into 8 furnaces1 until gradually he won success1 & 8 glass began to be he>d (2 = by #agii he 0 produc+ flawless discs ( fl9t z mu* z #f in*es 9 diamet]4 ,:at is m1 to ,gu9& is probably due ! production ( ! d5s]1 m highly refractive fl9ts1 especially valuable = a*romatic telescopes4 ,! mak+ ( optical glass has always be5 an art r ?an a sci5ce4 ,x is "o ?+ to "k ! exact composition ( a glass & q ano!r to "k 9 :at ord] & proportion ! +r$i5ts w5t into ! furnace1 to :at temp]ature !y 7 c>ri$1 & = h[ long1 & j h[ ! fus$ mass mu/ be treat$ to free ! products f bubbles & /ri4 ,ev5 today1 ?\gh mu* has be5 le>n$ by sci5tific inve/igation 9 ! pa/ few ye>s1 x is f> f easy to produce two consecutive melt+s ne> en\gh 9 refractive p[] to be treat$ z optically id5tical1 or to produce l>ge discs optically homog5e\s4 ,:at ,gu9& won by %e] exp]i5ce 0 invaluable4 ,he 0 p]suad$ 9 #ahje to move to ,muni* & ev5tually to jo9 =ces ) ,fraunh(]1 an association :i* made bo? ! ,g]man optical glass indu/ry & ! mod]n refractor4 ,he return$ to ,switz]l& 9 #ahad & cont9u$ to produce p]fect discs ( l>g] & l>g] dim5sions4 ,"o set ( #ab in*es work$ up by ,cau*oix 9 ,p>is furni%$ :at 0 = "s ye>s ! _w's l>ge/ refractor4 ,gu9aud di$ 9 #ahbd1 b 8 son ,h5ry1 mov+ to ,p>is1 br\ght 8 treasure ( practical k to ! glass works !re1 :]e x has be5 h&$ d[n1 9 effect f fa!r to son1 ga9+ /eadily by accretion1 ?r\gh successive firms to ! pres5t "o ( ,p>ra-,mantois4 ,bontemps1 "o ( ! e>ly pupils ( ,h5ry ,gu9&1 emigrat$ to ,engl& at ! ,revolution ( #ahdh & br\ght ! art to ! fam\s firm ( ,*ance 9 ,birm+ham4 ,mo/ ( its e>ly secrets h long be5 op51 b ! m9ute tea*+s ( exp]i5ce are a trem5d\sly valuable asset ev5 n[4 ,illu/ration3 ,,fig4 #bb4--;,dr4 ,joseph von ,fraunh(]1 ! ,fa!r ( ,a/rophysics4 ,to ,fraunh(]1 ! greate/ ma/] ( appli$ optics 9 ! n9ete5? c5tury1 is due ! a/ronomical telescope 9 sub/antially its pres5t =m4 ,n only did he become "u ,gu9&'s in/ruction extraord9>ily skillful 9 glass mak+ b he practically devis$ ! art ( work+ x ) ma!matical precision on an automatic ma*9e1 & ! sci5ce ( correctly design+ a*romatic objectives4 ,! =m :i* he orig9at$ 7,fig4 #bc7 0 ! fir/ 9 :i* ! ab]rations 7 treat$ ) adequate complet5ess1 &1 p>ticul>ly = small in/rum5ts1 is unexcell$ ev5 n[4 ,! curvatures "h %[n are extreme1 ! bett] to %[ _! relations4 ,! front radius ( ! cr[n is ab\t #b times long] ?an ! re> radius1 ! front ( ! fl9t is slightly flatt] ?an ! back ( ! cr[n1 & ! re> ( ! fl9t is only slightly convex4 ,fraunh(]'s workman%ip 0 ( ! utmo/ exactness & x is n putt+ ! case too /rongly to say t a fir/ class example ( ! ma/]'s craft1 9 good condition1 w\ld comp>e well 9 color-correction1 def9ition1 & field1 ) ! be/ mod]n in/rum5ts4 ,illu/ration3 ,,fig4 #bc4 ,! "w done by ! eld] ,/ruve at ,dorpat ) ,fraunh(]'s fir/ l>ge telescope 7#i4f in*es ap]ture & #agj in*es focal l5g?7 tells ! /ory ( its quality1 & ! ,knigsb]g heliomet]1 ! fir/ ( its class1 likewise1 :ile ev5 today "s ( 8 small] in/rum5ts are /ill do+ good s]vice4 ,x 0 he :o put 9 practice ! n[ g5]al conv5tion ( a relative ap]ture ( ab\t ,f_/#ae1 & /&>diz$ ! t]re/rial eyepiece into ! design q widely us$ today4 ,! improvem5ts s9ce 8 "t h be5 relatively slight1 due ma9ly to ! rec5t production ( v>ieties ( optical glass unkn[n a c5tury ago4 ,fraunh(] 0 born 9 ,/raub+1 ,bav>ia1 ,m>* #f1 #aghg4 ,self-educat$ l ,h]s*el1 he atta9$ to an extraord9>y comb9ation ( !oretical & practical k t w5t f> 9 lay+ ! f\ndations ( a/rophysics4 ,! fir/ mapp+ ( ! sol> spectrum1 ! inv5tion ( ! diffraction grat+ & its application to det]m9+ ! wave l5g? ( light1 ! fir/ exact inve/igation ( ! refraction & disp]sion ( glass & o!r sub/ances1 ! inv5tion ( ! objective prism1 & its use 9 /udy+ ! spectra ( />s & planets1 ! recognition ( ! correspond5ce ( ! sodium l9es to ! ,d l9es 9 ! sun1 & ! e>lie/ sugge/ion ( ! diffraction !ory ( resolution lat] work$ out by ,"l ,rayleigh & ,pr(essor ,abb1 make a long li/ ( notable a*ievem5ts4 ,to ^! may be add$ 8 p]fect+ ( ! a*romatic telescope1 ! equatorial m\nt+ & its clockwork drive1 ! improvem5t ( ! heliomet]1 ! inv5tion ( ! /age micromet]1 sev]al types ( ocul> micromet]s1 & ! automatic rul+ eng9e4 ,he di$ at ! height ( 8 creative p[]s ,june #g1 #ahbf1 & lies buri$ at ,muni* "u ! sublime ascription1 by none bett] e>n$1 ,approximavit ,sid]a4 ,f ,fraunh(]'s "t1 at ! h&s ( ,m]z 8 imm$iate successor1 ,cau*oix 9 ,france1 & ,tully 9 ,engl&1 ! a*romatic refractor /eadily won its way4 ,reflect+ telescopes1 despite ! s5sational "w ( ,"l ,rosse on 8 #f-;foot mirror ( #ec feet focus 7unequall$ 9 ap]ture until ! #f-;foot ( ! ,dom9ion ,obs]vatory sev5ty ye>s lat]71 & ! ev5 m successful in/rum5t ( ,mr4 ,lassell 7#d feet ap]ture1 #ci feet focus71 7 pass+ out ( use1 = ! reason already not$1 t repoli%+ meant refigur+ & ! us] _h to be at once a/ronom] & sup]latively skill$ optician4 ,^! l>ge specula1 too1 7 extremely prone to s]i\s flexure & c\ld h>dly h be5 us$ at all except = ! equilibrat+ lev]s devis$ by ,?omas ,grubb ab\t #ahcd1 & us$ effectively on ! ,rosse in/rum5t4 ,^! are 9 effect a gr\p ( upw>dly press+ c\nt]balanc$ planes di/ribut+ among !m ! d[nw>d compon5t ( ! mirror's weight s z to keep ! figure true 9 any position ( ! tube4 ,su* 0 ! situation 9 ! #ej's ( ! la/ c5tury1 :5 ! reflector 0 q unexpect$ly pu%$ to ! front z a practical in/rum5t by almo/ simultane\s activity 9 ,g]many & ,france4 ,! />t+ po9t 9 ea* 0 ,liebig's simple *emical me?od ( silv]+ glass1 :i* quickly & easily lays on a ?9 reflect+ film capable ( a beautiful poli%4 ,! honor ( te*nical priority 9 its application to silv]+ telescope specula work$ 9 glass belongs to ,dr4 ,k>l ,augu/ ,/e9heil 7#ahja-ahgj7 :o produc$ ab\t ! beg9n+ ( #ahef an in/rum5t ( #d-;in* ap]ture report$ to h giv5 ) a p[] ( #ajj a wond]fully good image4 ,! publication 0 m]ely f a news item 9 ! 8,allgeme9e ,zeitung0 ( ,augsburg1 ,m>* #bd1 #ahef1 s x is little wond] t ! inv5tion pass$ = a "t unnotic$4 ,e>ly ! next ye>1 ,feb4 #af1 #aheg1 work+ q indep5d5tly1 exactly ! same ?+ 0 br\ght be=e ! ,fr5* ,academy ( ,sci5ces by ano!r di/+ui%$ physici/1 ,jean ,b]n>d ,lon ,f\cault1 immortal = 8 pro( ( ! e>?'s rotation by ! p5dulum exp]im5t1 8 measurem5t ( ! velocity ( light1 & ! discov]y ( ! electrical eddy curr5ts t be> 8 "n4 ,illu/ration3 ,,fig4 #bd4--;,dr4 ,k>l ,augu/ ,/e9heil4 ,,fig4 #be4--;,jean ,b]n>d ,lon ,f\cault4 ,! ,inv5tors ( ! ,silv]-on-,glass ,reflector4 ,to ,f\cault1 *iefly1 ! _w owes ! developm5t ( ! mod]n silv]-on-glass reflector1 = n be+ a pr(essional optician he _h no hesitation 9 mak+ public 8 admirable me?ods ( work+ & te/+1 ! latt] n[ univ]sally employ$4 ,x is wor? not+ t 8 me?od ( figur+ 01 physically1 exactly :at ,jesse ,ramsd5 7#agce-ahjj7 _h po9t$ out 9 #aggi1 7,phil4 ,tr4 #aggi1 #dbg7 geometrically4 ,"o ( ,f\cault's v e>ly in/rum5ts m\nt$ equatorially by ,scrtan is %[n 9 ,fig4 #bf4 ,illu/ration3 ,,fig4 #bf4--;,e>ly ,f\cault ,reflector4 ,! imm$iate result ( ! admirable "w ( ,/e9heil & ,f\cault 0 ! ext5sive use ( ! new reflector1 & its rapid developm5t z a conv5i5t & practical in/rum5t1 especially 9 ,engl& 9 ! skillful h&s ( ,)1 ,br[n+1 & ,calv]4 ,n ! lea/ ( its advantages 0 its great sup]iority ov] ! old] type 9 light-grasp1 silv] be+ a bett] reflector ?an speculum metal 9 ! ratio ( v ne>ly #g to #e4 ,f ?is "t on bo? refractors & reflectors h be5 fully available to ! us] ( telescopes4 ,9 details ( con/ruction bo? h ga9$ some:at me*anically4 ,z we h se51 tubes 7 (t5 ( wood1 & n uncommonly ! m\nt+s also4 ,at ! pres5t "t metal "w ( e k9d be+ m readily available1 tubes & m\nt+s ( telescopes ( e size are q univ]sally ( metal1 save = ! tripod-legs ( ! portable in/rum5ts4 ,! tubes ( ! small] refractors1 say #c to #e in*es 9 ap]ture1 are g5]ally ( brass1 ?\gh 9 high grade in/rum5ts ?is is rapidly be+ replac$ by alum9um1 :i* saves consid]able weight4 ,tubes above #e or #f in*es are commonly ( /eel1 pa9t$ or lacqu]$4 ,! beautifully poli%$ brass ( ! small] tubes1 easily damag$ & objectionably %9y1 is giv+ way to a s]viceable matt f9i% 9 h>d lacqu]4 ,m\nt+s1 too1 are n[ m (t5 9 iron & /eel or alum9um ?an 9 brass1 ! fir/ nam$ q univ]sally 9 ! work+ p>ts1 = :i* ! alum9um is r s(t4 ,! typical mod]n refractor1 ev5 ( mode/ size1 is a good bit m ( a ma*9e ?an x looks at fir/ glance4 ,9 pr9ciple x is outl9$ 9 ,fig4 #e1 9 practice x is mu* m complex 9 detail & requires ! nice/ ( workman%ip4 ,9 fact if "o 7 to take completely ap>t a well-made small refractor1 includ+ its optical & me*anical p>ts "o w\ld reckon up "s #cj to #dj sep>ate pieces1 n c\nt+ screws1 all ( :i* mu/ be accurately fitt$ & assembl$ if ! in/rum5t is to "w prop]ly4 ,illu/ration3 ,,fig4 #bg4--,longitud9al ,section ( ,mod]n ,refractor4 ,fig4 #bg %[s su* an in/rum5t 9 section f end to end1 z "o w\ld f9d x c\ld he lay x op5 longitud9ally4 ,a is ! objective cap cov]+ ! objective ,b 9 its adju/able cell ,c1 :i* is squ>$ precisely to ! axis ( ! ma9 tube ,d4 ,look+ along ?is "o f9ds ! fir/ ( ! diaphragms1 ,e4 ,^! are commonly #c to #f 9 numb] spac$ ab\t equally d[n ! tube1 & are f> m important ?an !y look4 ,_! function is n to n>r[ ! beam ( light t rea*es ! ocul>1 b to trap light :i* might ent] ! tube obliquely & be reflect$ f its sides into ! ocul>1 fill+ x ) /ray gl>e4 ,no am\nt ( simple black5+ w answ] ! purpose1 = ev5 dead black pa9t su* z opticians use reflects at v oblique incid5ce q #aj to #bj p] c5t ( ! beam4 ,! importance ( bo? diaphragms & ?or\gh black5+ has be5 realiz$ = at lea/ a c5tury & a half1 & "o c h>dly lay too mu* /ress ^u ! matt]4 ,! diaphragms %\ld be s proportion$ t1 :5 look+ up ! tube f ! edge ( an ap]ture ( j ! size & position ( ! bigge/ l5s 9 ! l>ge/ eyepiece1 no "p ( ! edge ( ! objective is cut (f1 & no "p ( ! side ( ! tube is visible beyond ! ne>e/ diaphragm4 ,go+ fur!r d[n ! tube pa/ a diaphragm or two "o comes to ! clamp+ screws ,f4 ,^! s]ve to hold ! in/rum5t to its m\nt+4 ,!y may be set 9 sep>ate bases screw$ 9 place on ! inside ( ! tube1 or may be set 9 ! two ends ( a l5g?wise /rap ?us secur$4 ,!y are plac$ at ! balance po9t z ne>ly z may be1 g5]ally ne>] ! eye end ?an ! objective4 ,!n1 aft] "o or m diaphragms1 comes ! guide r+ ,g1 :i* /eadies ! ma9 draw tube ,h1 & ! rack ,i by :i* x is mov$ = ! focuss+ 9 turn+ ! mill$ head ( ! p9ion ,j4 ,! end r+ ,k ( ! ma9 tube furni%es ! o!r be>+ ( ,h1 & bo? ,g & ,k are commonly recess$ = accurately fitt$ clo? l9+ r+s ,l1 ,l1 to give ! draw tube ! necess>y smoo?ness ( motion4 ,= ! same reason ,i & ,j h to be cut & fitt$ ) ! utmo/ exactness s z to "w ev5ly & )\t backla%4 ,h is fitt$ at its out] end ) a slide r+ & tube ,m1 g5]ally aga9 clo? l9$ to /eady ! slid+ eyepiece tube ,n4 ,?is is t]m9at$ by ! spr+ coll> ,o1 9 :i* fits ! eyepiece ,p1 g5]ally ( ! two l5s =m2 & f9ally comes ! eyepiece cap ,q set at ! prop] di/ance f ! eye l5s & ) an ap]ture ( c>efully det]m9$ size4 ,"o ?us gets pretty well d[n 9 ! alphabet )\t go+ mu* into ! small] details ( con/ruction4 ,bo? objective m\nt & ocul> are some:at complex 9 fact1 & ! =m] is almo/ always made adju/able 9 in/rum5ts ( above #c or #d in*es ap]ture1 z %[n 9 ,fig4 #bh1 ! =m us$ by ,cooke1 ! fam\s mak] ( ,york1 ,engl&4 ,unless ! optical axis ( ! objective is true ) ! tube bad images result4 ,illu/ration3 ,,fig4 #bh4--;,adju/able ,cell = ,objective4 ,to ! upp] end ( ! tube is fitt$ a flang$ c\nt]-cell c1 to an outw>d flange f1 tapp$ = #c close pairs ( adju/+ screws z s#a1 s#aa spac$ at #abj ap>t4 ,! objective cell itself1 b1 is recess$ = ! objective :i* is held 9 place by an int]ior or ext]ior r+ d4 ,! two l5ses ( ! a*romatic objective are usually v slightly sep>at$ by spac]s1 ei!r t9y bits ( t9foil #abj ap>t1 or a v ?9 r+ ) its upp] edge cut d[n save at #c po9ts4 ,?is precaution is to insure t ! l5ses are q uni=mly support$ in/ead ( t\*+ at unc]ta9 po9ts1 & q usually ! pair z a :ole re/s bel[ on ?ree correspond+ spac]s4 ,( ea* pair ( adju/+ screws "o z #a 9 ! pair s#aa is ?read$ to pu% ! c\nt] cell out1 ! adjac5t "o1 #b1 to pull x 91 s t :5 adju/m5t is made ! objective is firmly held4 ,( ! l5ses t =m ! objective1 ! concave fl9t is commonly at ! re> & ! convex cr[n 9 front4 ,at ! eye end ! ocul> ord9>ily consi/s ( two l5ses ea* burni%$ into a brass screw r+1 a tube1 flange1 cap1 & diaphragm arrang$ z %[n 9 ,fig4 #bi4 ,!re are _m v>ieties ( ocul> z w pres5tly be %[n1 b ?is is a typical =m4 ,figure #cj %[s a complete mod]n refractor ( f\r in*es ap]ture on a portable equatorial /& ) sl[ motion 9 "r asc5sion & diagonal eye piece4 ,reflectors1 us$ 9 ?is c\ntry less ?an !y des]ve1 are1 :5 prop]ly m\nt$1 likewise possess$ ( _m p>ts4 ,! small] ones1 su* z are likely to come into ! read]'s h&s1 are almo/ always 9 ! ,newtonian =m1 ) a small oblique mirror to br+ ! image outside ! tube4 ,illu/ration3 ,,fig4 #bi4--,! ,eye-,piece & its ,fitt+s4 ,! ,gregorian =m has entirely vani%$4 ,its only special m]it 0 its erect image1 :i* gave x high value z a t]re/rial telescope be=e ! days ( a*romatics1 b f its con/ruction x 0 almo/ impossible to keep ! field f be+ flood$ ) /ray light1 & ! a*romatic soon displac$ x4 ,! ,cassegranian con/ruction on ! o!r h&1 %ort] & ) ab]rations mu* r$uc$1 has prov$ important = obta9+ long equival5t focus 9 a %ort m\nt1 & is almo/ univ]sally appli$ to l>ge reflectors1 = :i* a ,newtonian mirror is also g5]ally provid$4 ,figure #ca %[s 9 section a typical reflector ( ! ,newtonian =m4 ,"h ,a is ! ma9 tube1 fitt$ ne> its out] end ) a r+ ,b c>ry+ ! small elliptical mirror ,c1 :i* is set at #de to ! axis ( ! tube4 ,at ! bottom ( ! tube is ! p>abolic ma9 mirror ,d1 m\nt$ 9 its cell ,e4 ,j opposite ! #de small mirror is a hole 9 ! tube to :i* is fitt$ ! eye piece m\nt+ ,f1 c>ry+ ! eyepiece ,g1 fitt$ to a spr+ coll> ,h1 screw$ into a draw tube ,i1 slid+ 9 its m\nt+ & br\ght to focus by ! rack-&-p9ion ,j4 ,illu/ration3 ,,fig4 #cj4--,portable ,equatorial ,refractor 7,bra%e>74 ,at ,k1 ,k1 are two r+s fix$ to ! tube & be>+ smoo?ly aga9/ ! r+s ,l ,l rigidly fix$ to ! b> ,m c>ri$ by ! pol> axis ( ! m\nt4 ,! :ole tube c !re=e be rotat$ ab\t its axis s z to br+ ! eye piece into a conv5i5t position = obs]vation4 ,"o or m h&les1 ,n1 are provid$ = ?is purpose4 ,illu/ration3 ,,fig4 #ca4--,longitud9al ,section ( ,newtonian ,reflector4 ,brackets %[n 9 dott$ l9es at ,o1 ,o1 c>ry ! usual f9d]1 & a h+$ door ,p ne> ! l[] end ( ! tube enables "o to remove or replace ! close fitt+ metal cov] t protects ! ma9 mirror :5 n 9 use4 ,simil>ly a cov] is fitt$ to ! small mirror1 easily rea*$ f ! upp] end ( ! tube4 ,! proportions "h %[n are approximately ^? commonly f\nd 9 m$ium siz$ in/rum5ts1 say #g to #aj in*es ap]ture4 ,! focal ratio is some:]e ab\t ,f_/#f1 ! diagonal mirror is inside ( focus by ab\t ! diamet] ( ! ma9 mirror1 & its m9or axis is f to t diamet]4 ,illu/ration3 ,,fig4 #cb4--,reflector ) ,skeleton ,tube 7,bra%e>74 ,note t ! tube is n provid$ ) diaphragms4 ,x is m]ely black5$ z ?or\ghly z possible1 al?\gh /ray light is q z s]i\s "h z 9 a refractor4 ,"o c\ld fit diaphragms effectively only 9 a tube ( mu* l>g] diamet] ?an ! mirror1 :i* w\ld be inconv5i5t 9 _m ways4 ,a mu* bett] way ( deal+ ) ! difficulty is %[n 9 ,fig4 #cb 9 :i* ! tube is r$uc$ to a skeleton1 a con/ruction common 9 l>ge in/rum5ts4 ,no?+ is black] ?an a cle> op5+ into ! d>kness ( night1 & 9 addition !re c be no localiz$ air curr5ts1 :i* (t5 injure def9ition 9 an ord9>y tube4 ,illu/ration3 ,,fig4 #cc4 ,in/rum5ts by diff]5t mak]s v>y some:at 9 detail4 ,a good type ( mirror m\nt+ is t %[n 9 ,fig4 #cc1 & us$ = _m ye>s pa/ by ,br[n+1 "o ( ! fam\s ,engli% mak]s4 ,"h ! mirror ,a1 ! back ( :i* is made accurately plane1 is seat$ 9 its c\nt]-cell ,b1 ( :i* a wide annulus ,f1 ,f1 is also a good plane1 & is lightly held 9 place by a reta9+ r+4 ,?is c\nt] cell re/s 9 ! out] cell ,c on ?ree equidi/ant /uds regulat$ by ! conc5tric pu%-&-pull adju/+ screws ,d1 ,d1 ,e1 ,e4 ,! out] cell may be solid1 or a skeleton = lightness & bett] equalization ( temp]ature4 ,small specula may be well support$ on any flat surface sub/antial en\gh to be ?or\ghly rigid1 ) "o or m ?icknesses ( s(t1 ?ick1 smoo? clo? betwe51 be/ ( all ,brussels c>pet4 ,su* 0 ! common me?od ( support 9 in/rum5ts ( mod]ate dim5sions prior to ! day ( glass specula4 ,sir ,john ,h]s*el speaks ( ?us c>ry+ specula ( m ?an a hundr$-weight1 b some?+ ak9 to ,br[n+'s plan is g5]ally pref]able4 ,!re is also consid]able v>iety 9 ! means us$ = support+ ! small mirror c5trally 9 ! tube4 ,9 ! e>ly telescopes x 0 borne by a s+le /iff arm :i* 0 none too /iff & produc$ by diffraction a long diametral fl>+ ray 9 ! images ( bright />s4 ,a great improvem5t 0 introduc$ by ,br[n+ m ?an a half c5tury ago1 9 ! support %[n 9 ,fig4 #cd4 ,"h ! r+ ,a1 7,b1 ,fig4 #ca7 c>ries ?ree n>r[ /rips ( ?9 spr+ /eel1 ,b1 ext5d+ radially inw>d to a c5tral hub :i* c>ries ! mirror ,d1 on adju/+ screws ,e4 ,outside ! r+ ! t5sion screws ,c enable ! mirror to be accurately c5t]$ & held 9 place4 ,r>ely1 ! mirror is replac$ by a totally reflect+ "r angl$ prism :i* saves "s light1 b unless = small in/rum5ts is r heavy & h>d to obta9 ( ! requisite quality & precision ( figure4 ,a typical mod]n reflector by ,bra%e>1 ( #f in*es ap]ture1 is %[n 9 ,fig4 #ce1 complete ) circles & driv+ clock1 ! latt] conta9$ 9 ! holl[ iron pi]1 an arrangem5t usual 9 ,am]ican-made in/rum5ts4 ,illu/ration3 ,,fig4 #cd4--,support ( ,diagonal ,mirror 7,br[n+47 ,rec5t reflectors1 p>ticul>ly 9 ?is c\ntry1 h f\r support+ /rips in/ead ( ?ree1 :i* gives a little add$ /iffness1 & produces 9 /> images b f\r diffraction rays in/ead ( ! six produc$ by ! ?ree /rip arrangem5t1 ea* /rip giv+ a diametral ray4 ,9 "s con/ructions ! r+ ,a is arrang$ to c>ry ! eyepiece fitt+s1 plac$ at ! v end ( ! tube & arrang$ = rotat+ ab\t ! optical axis ( ! telescope4 ,?is all[s ! ocul> to be br\ght to any position )\t turn+ ! :ole tube4 ,9 small in/rum5ts a fix$ eyepiece c be us$ )\t mu* inconv5i5ce if locat$ on ! nor? side ( ! tube 79 mod]ate nor? latitudes74 ,reflectors are easily giv5 a mu* great] relative ap]ture ?an is practicable 9 a s+le a*romatic objective4 ,9 fact !y are usually giv5 ap]tures ( ,f_/#e to ,f_/#h & n[ & !n are pu%$ to or ev5 bel[ ,f_/#c4 ,su* mirrors h be5 successfully us$ = photography2#i & less frequ5tly = visual obs]vation1 m\nt$ 9 ! ,cassegranian =m1 :i* commonly increases ! virtual focal l5g? at lea/ ?ree or f\r times4 ,a telescope s arrang$1 ) an ap]ture ( a foot or m z 9 "s rec5t examples1 makes a v p[]ful & compact in/rum5t4 #i ,an ,f_/#c mirror ( #am ap]ture by ,zeiss 0 in/all$ 9 ! obs]vatory at ,b]g$orf 9 #aiaa1 & a simil> "o by ,s*a] is m\nt$ at ,c>re1 ne> ,g5eva4 ,illu/ration3 ,,fig4 #ce4--,small ,equatorially ,m\nt$ ,reflector4 ,?is is ! =m commonly adopt$ = ! l>ge reflectors ( rec5t con/ruction1 a type be+ ! #fj-;in* telescope ( ! ,m\nt ,wilson ,obs]vatory ( :i* ! prim>y focus is #be feet & ! ord9>y equival5t focus z a ,cassegranian #hj feet4 ,comp>atively few small reflectors h be5 made or us$ 9 ! ,unit$ ,/ates1 al?\gh ! climatic conditions "h are m favorable ?an 9 ,engl&1 :]e ! reflector orig9at$ & has be5 v fully develop$4 ,! explanation may lie 9 our small] numb] ( non-pr(essional active a/ronom]s :o are /eadily at obs]vational "w1 & c !re=e use reflectors to ! be/ advantage4 ,! relative advantages ( refractors & reflectors h long be5 a matt] ( acrimoni\s dispute4 ,9 fact1 m ( ! g5u9e odium !ologicum has gone into ! consid]ation ( ?is matt] ?an usually atta*es to diff]5ces 9 sci5tific op9ion4 ,a good _m misund]/&+s h be5 due to ! fact t until rec5tly few obs]v]s 7 practically famili> ) bo? in/rum5ts1 & ! pr(essional a/ronom] 0 a little incl9$ to look on ! reflector z fit only = amateurs4 ,! comp>ison is some:at cl>ifi$ at pres5t by ! fact t ! old speculum metal reflector has pass$ out ( use1 & ! case n[ /&s z betwe5 ! ord9>y refract+ telescope su* z has j be5 describ$1 & ! silv]-on-glass reflector discuss$ imm$iately !reaft]4 ,! facts 9 ! case are comp>atively simple4 ,( two telescopes hav+ ! same cle> ap]ture1 "o a reflector & ! o!r a refractor1 ea* assum$ to be ?or\ghly well figur$1 z x c be 9 fact today1 ! !oretical resolv+ p[] is ! same1 = ?is is det]m9$ m]ely by ! ap]ture1 s t ! only possible diff]5ce betwe5 ! two w\ld be 9 ! residual imp]fection 9 ! p]=mance ( ! refractor due to its n be+ p]fectly a*romatic4 ,?is diff]5ce is sub/antially a negligible "o = _m1 b n all1 purposes4 ,likewise1 ! g5]al def9ition ( ! pair1 assum+ fir/-class workman%ip1 w\ld be equal4 ,( ! two1 ! s+le surface ( ! mirror is some:at m difficult to figure ) ! necess>y precision ?an is any s+le surface ( ! refractor1 b reflectors c be1 & are1 giv5 s p]fect a p>abolic figure t ! image is 9 no wise inf]ior to t produc$ by ! be/ refractors1 & ! two types ( telescopes w /& "u favorable circum/ances ! same proportional magnify+ p[]s4 ,! mirror is mu* m s]i\sly affect$ by *anges ( temp]ature & by flexure ?an is ! objective1 s9ce 9 ! =m] case ! successive surfaces ( ! two l5ses 9 ! a*romatic comb9ation to a consid]able ext5t comp5sate ea* o!r's slight *anges ( curvature1 :i* act only by /ill slight] *anges ( refraction1 :ile ! mirror surface /&s alone & any *ange 9 curvature produces d\ble ! defect on ! reflect$ ray4 ,x is !re=e necess>y1 z we %all see pres5tly1 to take p>ticul> precautions 9 work+ ) a reflect+ telescope1 :i* is1 s to speak1 mat]ially m t5d] z reg>ds ext]nal conditions ?an ! refractor4 ,z reg>ds light-grasp1 ! p[] ( r5d]+ fa9t objects visible1 !re is m room = hone/ v>iety ( op9ion4 ,x 0 (t5 assum$ 9 e>li] days t a reflector 0 n mu* bright] ?an a refractor ( half ! ap]ture1 i4e41 ( "o qu>t] ! work+ area4 ,?is might h be5 true 9 ! case ( an old speculum metal reflector 9 bad condition1 b is c]ta9ly a libel on ! silv]-on-glass in/rum5t1 :i* ,f\cault on ! o!r h& claim$ to be1 ap]ture = ap]ture1 bright] ?an ! refractor4 ,su* a relation might 9 fact tempor>ily exi/1 b x is f> f typical4 ,! real relation dep5ds m]ely on ! light losses demon/rably occurr+ 9 ! two types ( telescopes4 ,^! are n[ q well kn[n4 ,! losses 9 a refractor are ^? due to absorption ( light 9 ! two l5ses1 plus ^? due to ! f\r free surfaces ( ^! l5ses4 ,! =m] item 9 objectives ( mod]ate size aggregates h>dly m ?an #b to #c p] c5t4 ,! latt]1 assum+ ! poli% to be q p]fect1 am\nt to #ah to #bj p] c5t ( ! incid5t light1 = ! glasses commonly us$4 ,! total light transmitt$ is !re=e n ov] #hj p] c5t ( ! :ole1 m (t5 some:at "u ?is figure4 ,= example1 a te/ by ,/e9heil ( "o ( ,fraunh(]'s refractors gave a transmission ( #gh p] c5t1 & o!r te/s %[ simil> results4 ,! relation betwe5 ! light transmitt$ by glass ( v>i\s ?ickness is v simple4 ,if unit ?ickness transmits m p] c5t ( ! incid5t light !n n units 9 ?ickness w pass mn p] c5t4 ,?us if "o half in* passes 4#ih1 two in*es w transmit 4#ihd1 or 4#ibb4 ,evid5tly ! bigg] ! objective ! great] ! absorptive loss4 ,if ! loss by reflection at a s+le surface leaves m p] c5t to be transmitt$ !n n surfaces w transmit mn4 ,& m be+ usually ab\t 4#ie1 ! f\r surfaces ( an objective let pass ne>ly 4#hae1 & ! ?ick] objective z a :ole transmits approximately #ge p] c5t4 ,z to ! reflector ! :ole relation h+es on ! coeffici5t ( reflection f a silv]$ surface1 "u ! circum/ances ( ! comp>ison4 ,9 ! case ( a reflect+ telescope z a :ole1 !re are commonly two reflections f silv] & if ! coeffici5t ( reflection is m !n ! total light reflect$ is m4 ,n[ ! reflectivity ( a silv]-on-glass film has be5 repeat$ly measur$4 7,*ant ,ap4 ,j4 #ba1 #baa7 f\nd values slightly 9 excess ( #ie p] c5t1 ,rayleigh 7,sci4 ,pap]s #b1 #d7 got #ic4i1 ,zeiss 7,l&olt u4 ,born/e91 ,tabell57 ab\t #ic4j = light ( av]age wave l5g?4 ,tak+ ! la/ nam$ value1 a d\ble reflection w\ld return sub/antially #hf4e p] c5t ( ! incid5t light4 ,no all[ance is "h made = any effect ( selective reflection1 s9ce = ! bright visual rays1 :i* alone we are consid]+1 !re is v slight selective effect4 ,9 ! photographic case x mu/ be tak5 into acc\nt1 & ! absorption 9 glass becomes a s]i\s factor 9 ! comp>ison1 am\nt+ = ! photographic rays to z mu* z #cj to #dj p] c5t 9 l>ge in/rum5ts4 ,n[ 9 comp>+ reflector & refractor "o mu/ subtract ! light /opp$ by ! small mirror & its supports1 commonly f #e to #g p] c5t4 ,"o is !re=e =c$ to ! conclusion t ) silv] coat+s fre% & v c>efully poli%$ reflector & refractor w %[ = equal ap]ture equal light grasp4 ,b z ?+s actually g ev5 fre% silv] films are q (t5 bel[ 4#ij 9 reflectivity & 9 g5]al t>ni% r rapidly1 s t 9 fact ! reflector falls bel[ ! refractor by j ab\t ! am\nt by :i* ! silv] films are out ( condition4 ,= example ,*ant 7loc4 cit47 f\nd aft] ?ree mon?s 8 reflectivity _h fall5 to 4#fi4 ,a mirror v badly t>ni%$ by fifte5 weeks ( exposure to dampness & du/1 uncov]$1 0 f\nd by ! writ] d[n to a scant 4#dj4 ,! l9e ( ,fig4 #cf %[s ! relative equival5t ap]tures ( refractors correspond+ to a #aj in* reflector at coeffici5ts ( reflection = a s+le silv]$ surface v>y+ f 4#ie to 4#ej at :i* po9t ! film w\ld be s evid5tly bad z to require imm$iate r5ewal4 ,! relation is obvi\sly l9e> :5 ! transmission ( ! objective is1 z "h1 assum$ con/ant4 ,! e/imates ( skill$ obs]v]s f actual comp>isons fall 9 well ) ! l9e1 %[+ reflectivities g5]ally ar\nd 4#hj to 4#he = well poli%$ films 9 good condition4 ,! long & %ort ( ! situation is t a silv]$ reflector det]iorates & at int]vals v>y+ f a few mon?s to a ye> or two dep5d+ on situation1 climate1 & usage1 requires repoli%+ or replacem5t ( ! film4 ,?is is a fussy job1 b quickly done if ev]y?+ goes well4 ,illu/ration3 ,,fig4 #cf4--,relative ,light-grasp ( ,reflector & ,refractor4 ,z to work+ field ! reflector z ord9>ily proportion$ is at a disadvantage *iefly because x works at ,f_/#e or ,f_/#f in/ead ( at ,f_/#ae4 ,at equal focal ratios !re is no sub/antial diff]5ce betwe5 reflector & refractor 9 ?is respect1 unless "o goes into special con/ructions1 z 9 photographic telescopes4 ,9 two items1 fir/ co/ & conv5i5ce 9 obs]v+1 ! reflector has ! advantage 9 ! mod]ate sizes4 ,r\ghly1 ! reflector simply m\nt$ co/s ab\t "o half to a qu>t] ! refractor ( equal light grasp & some:at less resolv+ p[]1 ! discrepancy gett+ bigg] 9 l>ge in/rum5ts 7#b feet ap]ture & upw>ds74 ,z to case ( obs]v+1 ! small refractor is a truly neck-wr++ in/rum5t = altitudes above #de or !reab\ts1 j ! situation 9 :i* ! equival5t reflector is mo/ conv5i5t4 ,9 consid]+ ! subject ( m\nts ^! relations w appe> m cle>ly4 ,practically ! man :o is obs]v+ r /eadily & c give 8 telescope a fix$ m\nt c make admirable use ( a reflector & w n f9d ! p]haps ye>ly or ev5 half ye>ly re-silv]+ at all burd5some aft] he has acquir$ ! knack--*iefly cleanl9ess & att5tion to detail4 ,if1 l _m really en?usia/ic amateurs1 he c get only an occasional ev5+ = obs]v+1 & f circum/ances has to use a portable m\nt set up on 8 lawn1 or ev5 ro(1 :5 =tune favors an ev5+'s "w1 he w f9d a refractor always 9 condition1 easy to set up1 & requir+ a m9imum ( "t to get into action4 ,! reflector is mu* ! m t5d] in/rum5t1 )1 h[ev]1 ! invaluable quality ( precise a*romatism1 to comp5sate = ! extra c>e x requires = its be/ p]=mance4 ,x suff]s m ?an ! refractor1 z a rule1 f scatt]$ light1 = imp]fect poli% ( ! film gives a field g5]ally pres5t+ a bright] backgr\nd ?an ! field ( a good objective4 ,aft] all ! pref]5ce dep5ds greatly on ! use to :i* ! telescope is to be put4 ,= a/rophysical "w 9 g5]al1 ,pr(essor ,george ,e4 ,hale1 ?an :om c]ta9ly no "o is bett] qualifi$ to judge1 emphatically endorses ! reflector4 ,mo/ l>ge obs]vatories are n[-a-days equipp$ ) bo? refractors & reflectors4 ,,*apt] ,,iii ,,optical ,,glass ,,& ,,its ,,work+ ,glass1 "o ( ! mo/ rem>kable & useful products ( man's devis+1 _h an orig9 n[ q lo/ 9 ! mi/s ( antiquity4 ,x dates back c]ta9ly ne> a ?\s& ye>s be=e ! ,*ri/ian era1 p]haps _m c5turies m4 ,respect+ its orig9 !re are only traditions ( ! place1 q probably ,syria1 & ( ! accid5tal melt+ toge!r ( s& & soda4 ,! product1 sodium silicate1 readily becomes a liquid1 i4e41 8wat]-glass10 b ! eld] ,pl9y1 :o tells ! /ory1 rec\nts ! lat] production ( a /able vitre\s body by ! addition ( a m9]al :i* 0 probably a magnesia lime/one4 ,?is comb9ation w\ld give a good p]man5t glass1 :e!r ! /ory is true or n1 & v long be=e ,pl9y's "t glass 0 made 9 great v>iety ( composition & color4 ,9 fact 9 default ( porcela9 glass 0 us$ 9 ,roman times relatively m ?an n[4 ,b )\t k ( optics !re 0 no ne$ = glass ( optical quality1 x 0 well into ! ,r5aissance be=e its manufacture _h rea*$ a po9t :]e any?+ ( ! sort c\ld be made available ev5 9 small pieces1 & x is b>ely ov] a c5tury s9ce glass-mak+ pass$ beyond ! crude/ empiricism4 ,glass is sub/antially a solid solution ( silica ) a v>iety ( metallic oxides1 *iefly ^? ( sodium1 potassium1 calcium & lead1 sometimes magnesium1 boron1 z9c1 b>ium & o!rs4 ,by itself silica is too refractory to "w easily1 ?\gh silica glass has "s v valuable prop]ties1 & ! alkal9e oxides 9 p>ticul> s]ve z ! fluxes 9 common use4 ,o!r oxides are add$ to obta9 v>i\s desir$ prop]ties1 & "s impurities may g ) !m4 ,! melt$ mixture is ?us a some:at complex solution conta9+ frequ5tly half a doz5 +r$i5ts4 ,ea* has its own natural melt+ & vaporiz+ po9t1 s t :ile ! bl5d rema9s fairly uni=m x may t5d to lose "s con/itu5t :ile molt51 or 9 cool+ to promote ! cry/allization ( ano!r1 if held too ne> its p>ticul> freez+ po9t4 ,"s comb9ations are m likely to give tr\ble f ?is cause ?an o!rs1 & :ile a v wide v>iety ( oxides c be co]c$ into solution ) silica1 a comp>atively limit$ numb] produce a homog5e\s & colorless glass useful = optical purposes4 ,_m mixtures entirely suitable = common comm]cial purposes are out ( ! "q = l5s mak+1 ?r\gh t5d5cy to surface det]ioration by wea!r+1 lack ( homog5e\s quality1 or objectionable coloration4 ,a v small am\nt ( iron 9 ! s& us$ at ! />t gives ! gre5 t+e famili> 9 *eap bottles1 :i* mat]ially decreases ! transp>5cy4 ,! bottle mak] (t5 adds oxide ( manganese to ! mixture1 :i* naturally ( itself gives ! glass a p9ki% t+e1 & s app>5tly :it5s x by comp5sat+ ! "o absorption by ano!r4 ,! result+ glass looks all "r on a casual glance1 b really cuts (f a v consid]able am\nt ( light4 ,a fur!r difficulty is t glass diff]s v mu* 9 its degree ( fluidity1 & its compon5ts sometimes seem to und]go mutual reactions t evolve p]si/5t f9e bubbles1 besides react+ ) ! fireclay ( ! melt+ pot & absorb+ impurities f x4 ,! molt5 glass is some:at visc\s & f> f homog5e\s4 ,its "* sugge/s ?ick syrup p\r$ into wat]1 & produc+ /reaks & eddies ( v>y+ d5sity4 ,imag9e su* a mixture sudd5ly froz51 & y h a good idea ( a common condition 9 glass1 transp>5t1 b full ( /ri4 ,^! are frequ5t en\gh 9 poor w9d[ glass1 & are almo/ impossible completely to get rid (1 especially 9 optical glass ( "s ( ! mo/ valuable v>ieties4 ,! great improvem5t introduc$ by ,gu9& 0 con/ant /irr+ ( ! molt5 mass ) a cyl9d] ( fire clay1 br++ bubbles to ! surface & keep+ ! mass ?or\ghly mix$ f its complete fusion until1 v sl[ly cool+1 x became too visc\s to /ir long]4 ,! f9e art ( ! process seems to be ! exact comb9ation ( temp]ature1 "t1 & /irr+1 suitable = ea* composition ( ! glass4 ,!re are1 too1 losses by volatilization dur+ melt+1 & ev5 aft]w>ds1 t mu/ be reckon$ ) 9 ! proportions ( ! v>i\s mat]ials put into ! melt+1 & 9 ! temp]atures rea*$ & ma9ta9$4 ,"o _c d$uce accurately ! p]c5tage mixture ( ! raw mat]ials f an analysis ( ! glass1 & x is notori\s t ! product ev5 ( ! be/ manufactur]s n infrequ5tly fails to run q true to type4 ,!re=e ! optical prop]ties ( ea* melt+ h c>efully to be asc]ta9$1 & ! product li/$ ei!r z a v slight v>iant f its /&>d type1 or z an odd lot1 useful1 b q special 9 prop]ties4 ,"s ( ^! odd melt+s 9 fact h optical peculi>ities ! regul> reproduction ( :i* w\ld be v desirable4 ,! purity ( ! mat]ials is ( ! utmo/ importance 9 produc+ high grade glass = optical or o!r purposes4 ,! silica is usually introduc$ 9 ! =m ( ! pure/ ( :ite s& c>ry+ only a few hundr$?s ( "o p] c5t ( impurities 9 ! way ( iron1 alum9a & alkali4 ,! ord9>y alkalis g 9 pref]ably z c>bonates1 :i* c be obta9$ ( great purity2 al?\gh 9 mo/ comm]cial glass ! soda is us$ 9 ! =m ( 8salt-cake10 crude sodium sulphate4 ,calcium1 magnesium1 & b>ium g5]ally ent] ! melt z c>bonates1 z9c & lead z oxides4 ,alum9a1 l iron1 is g5]ally an impurity d]iv$ f felsp> 9 ! s&1 b occasionally ent]s int5tionally z pure natural felsp>1 or z *emically prep>$ hydrate4 ,a few glasses conta9 a m9ute am\nt ( ars5ic1 g5]ally us$ 9 ! =m ( ars5i\s acid1 & /ill m r>ely o!r elem5ts ent]1 ord9>ily z oxides4 ,:atev] ! mat]ials1 !y are commonly r f9e gr\nd & v ?or\ghly mix$1 pref]ably by ma*9]y1 be=e go+ into ! furnaces4 ,glass furnaces are 9 ^! days commonly gas fir$1 & fall into two g5]al classes1 ^? 9 :i* ! *>ge is melt$ 9 a huge tank above :i* ! gas flames play1 & ^? 9 :i* ! *>ge is plac$ 9 crucibles or pots op5 or ne>ly clos$1 directly heat$ by ! gas4 ,9 ! tank furnaces ! production is sub/antially cont9u\s1 ! active melt+ tak+ place at "o end1 :]e ! mat]ials are introduc$1 :ile ! cle> molt5 glass fl[s to ! cool] end ( ! tank or to a cool] comp>tm5t1 :5ce x is )drawn = work+4 ,! ord9>y me?od ( mak+ optical glass is by a modification ( ! pot process1 ea* pot be+ fir$ sep>ately to p]mit bett] regulation ( ! temp]ature4 ,! pots !mselves are ( ! pure/ ( fire clay1 ( mod]ate capacity1 half a ton or s1 & ar*$ ov] to protect ! cont5ts f ! direct play ( ! gases1 leav+ a side op5+ suffici5t = *>g+ & /irr+4 ,! fundam5tal diff]5ce betwe5 ! mak+ ( optical glass & ! ord9>y comm]cial v>ieties lies 9 ! individual treatm5t ( ea* *>ge necess>y to secure uni=mity & regul>ity1 c>ri$ ev5 to ! ext5t ( cool+ ea* melt+ v sl[ly 9 its own pot1 :i* is f9ally brok5 up to recov] ! cont5ts4 ,! tank furnaces are "u heat week 9 & week out1 may hold sev]al hundr$ tons1 & on ?is acc\nt _c s readily be held to exactness ( composition & quality4 ,! optical glass works1 too1 is provid$ ) a p>ticul>ly effici5t set ( preheat+ & anneal+ kilns1 = ! heat treatm5t ( pots & glass mu/ be ( ! mo/ c>eful & ?or\gh k9d4 ,! production ( a melt+ ( optical glass beg9s ) a v gradual heat+ ( ! pot to a bright r$ heat 9 "o ( ! kilns4 ,x is !n transf]r$ to its furnace :i* has be5 br\ght to a simil> temp]ature1 seal$ 9 by slabs ( firebrick1 leav+ its m\? easy ( access1 & !n ! heat is pu%$ up to ne> ! melt+ temp]ature ( ! mixture 9 production1 :i* v>ies ov] a r wide range1 f a mod]ate :ite heat to ! utmo/ t a reg5]ative gas furnace c conv5i5tly produce4 ,aft] ! heat+ comes ! r c>eful process ( *>g+4 ,! mixture is add$ a portion at a "t1 s9ce ! fus$ mat]ial t5ds to foam1 & ! raw mat]ial z a solid is m bulky ?an ! fluid4 ,! *emical reactions z ! mass fuses are some:at complex4 ,9 _! simple/ =m !y repres5t ! =mation ( silicates4 ,at high temp]atures ! silica acts z a fairly /rong acid1 & decomposes ! fus$ c>bonates ( sodium & potassium ) evolution ( gas4 ,?is is ! rationale ( ! flux+ action ( su* alkal9e sub/ances ( r l[ melt+ po9t4 ,o!r mixtures act some:at analog\sly b 9 a fa%ion commonly too complex to foll[4 ,! f9al result is a ?ick solution1 & ! *ief conc]n ( ! optical glass mak] is to keep x homog5e\s1 free f bubbles1 & z ne>ly colorless z practicable4 ,to ! fir/ two ends ! temp]ature is pu%$ up to ga9 fluidity1 & frequ5tly sub/ances are add$ 7e4g41 ars5ic7 :i* by volatility or *emical effect t5d to =m l>ge bubbles f ! entra9$ gases1 capable ( cle>+ !mselves f ! fluid :]e f9e bubbles w\ld rema94 ,= ! same purpose is ! /irr+ process4 ,! /irr] is a h>d bak$ cyl9d] ( fire clay fa/5$ to an iron b>4 ,fir/ heat$ 9 ! m\? ( ! pot1 ! /irr] is plung$ 9 ! molt5 glass & giv5 a /eady rotat+ motion1 ! long b> be+ swivell$ & furni%$ ) a wood5 h&le = ! workman4 ,?is /irr+ is kept up pretty /eadily :ile ! heat is v sl[ly r$uc$ until ! mass is too ?ick to manage1 ! process tak+1 = v>i\s mixtures & conditions1 f ?ree or f\r h\rs to ! bett] "p ( a day4 ,illu/ration3 ,,fig4 #cg4--,te/+ ,optical ,glass 9 ! ,r\gh4 ,!n beg9s ! c>eful & t$i\s process ( cool+4 ,fairly rapid until ! mass is solid en\gh to prev5t ! =mation ( fre% /ri1 ! cool+ is cont9u$ m sl[ly1 9 ! furnace or aft] removal to ! anneal+ ov51 until ! crucible is cool en\gh = h&l+1 ! :ole process g5]ally tak+ a week or m4 ,!n ! real tr\ble beg9s4 ,! crucible is brok5 away & !re is f\nd a m or less crack$ mass ( glass1 sometimes badly brok5 up1 aga9 furni%+ a cle> lump weigh+ "s hundr$s ( p\nds4 ,?is glass is !n c>efully pick$ ov] & exam9$ = flaws1 /ri & o!r imp]fections4 ,^! c sometimes be *ipp$ away ) m or less break+ up ( ! mass4 ,! inspection ( ! glass 9 ! raw is facilitat$ by ! s*eme %[n 9 elevation ,fig4 #cg4 ,"h ,a is a tank ) p>allel sides ( plate glass4 ,9 x is plac$ ,b ! r\gh block ( glass1 & ! tank is !n fill$ ) a liquid :i* c be br\ght to ! same refractive p[] z ! glass1 z 9 ,newton's disa/r\s exp]im5t4 ,:5 equality is rea*$ =1 say1 yell[ light1 "o c see directly ?r\gh ! block1 ! rays no long] be+ refract$ at its surface1 & any int]ior /ri are readily se5 ev5 9 a mass a foot or m ?ick4 ,be=e add+ ! liquid a ray w\ld be skew$1 z ,c1 ,d1 ,e1 ,f1 aft]w>ds x w\ld g /raight ?r\gh2 ,c1 ,d1 ,g1 ,h4 ,! fraction t passes inspection may be f\nd to be f mu* less ?an a qu>t] to a half ( ! :ole4 ,?is good glass is !n ready = ! next op]ation1 =m+ & f9e anneal+4 ,! f9al =m to be rea*$ is a disc or block1 & ! *unks ( p]fect glass are heat$ 9 a kiln until pla/ic1 & !n m\ld$ into ! requir$ %apes1 sometimes concave or convex discs suitable = small l5ses4 ,!n ! blocks are transf]r$ to a kiln & all[$ to cool (f v gradually1 = sev]al days or weeks accord+ to ! size ( ! blocks & ! sev]ity ( ! requirem5ts !y mu/ meet4 ,9 ! highe/ class ( "w ! anneal+ ov5 has !rmo/atic control & close wat* is kept by ! pyromet]4 ,x is cle> t ! *ance ( gett+ a l>ge & p]fect *unk f ! crucible is f> small] ?an t ( gett+ fragm5ts ( a few p\nds1 s t ! production ( a p]fect disc = a l>ge objective requires bo? skill & luck4 ,little wond] !re=e t ! price ( discs = ! manufacture ( objectives increases sub/antially z ! cube ( ! diamet]4 ,! process ( optical glass mak+ z "h describ$ is ! cu/om>y "o1 us$ little *ang$ s9ce ! days ( ,gu9&4 ,! great advances ( ! la/ qu>t] c5tury h be5 9 ! production ( new v>ieties hav+ c]ta9 desirable qualities1 & 9 a bett] und]/&+ ( ! conditions t br+ a uni=m product ( high quality4 ,dur+ ! _w w> ! greatly increas$ dem& br\ght mo/ extraord9>y activity 9 ! manufacture1 & especially 9 ! sci5tific /udy ( ! problems involv$1 bo? "h & abroad4 ,! result has be5 a long /ep t[>d quantity production1 ! discov]y t modifications ( ! tank process c\ld s]ve to produce c]ta9 v>ieties ( optical glass ( at lea/ fair quality1 & great improvem5ts 9 ! precision & rapidity ( anneal+4 ,^! la/ are due to ! use ( ! electric furnace1 ! /udy ( ! /ra9s dur+ anneal+ "u pol>iz$ light1 & sci5tific pyrometry4 ,x is f\nd t cool+ c be mu* ha/5$ ov] c]ta9 ranges ( temp]ature1 & ! total "t requir$ v greatly %ort5$4 ,x has also be5 discov]$1 ?anks to captur$ in/rum5ts1 t "s ( ! glasses commonly reg>d$ z almo/ impossible to free f bubbles h 9 fact yield$ to improv$ me?ods ( treatm5t4 ,conv5tionally optical glass is ( two classes1 cr[n & fl9t4 ,orig9ally ! =m] 0 a simple comp\nd ( silica ) soda & pota%1 sometimes also lime or magnesia1 :ile ! latt] 0 ri* 9 lead oxide & ) less ( alkali4 ,! cr[n _h a l[ index ( refraction & small disp]sion1 ! fl9t a high index & /rong disp]sion4 ,cr[n glass 0 ! mat]ial ( g5]al use1 :ile ! fl9t glass 0 ! v>iety us$ 9 cut glass manufacture by reason ( its brilliancy due to ! qualities j not$4 ,illu/ration3 ,,fig4 #ch4--,! ,index ( ,refraction4 ,! refractive index is ! ratio betwe5 ! s9e ( ! angle ( incid5ce on a l5s surface & t ( ! angle ( refraction 9 pass+ ! surface4 ,fig4 #ch %[s ! relation ( ! incid5t & refract$ rays 9 pass+ f air into ! glass l5s surface ,l1 & ! s9es ( ! angles :i* det]m9e n1 ! conv5tional symbol = ! index ( refraction4 ,"h i is ! angle ( incid5ce & r ! angle ( refraction i4e4 n s_/s4 ,! indices ( refraction are usually giv5 = specific colors repres5t+ c]ta9 l9es 9 ! spectrum1 commonly ,a1 ! potassium l9e 9 ! extreme r$1 ,c ! r$ l9e due to hydrog51 ,d ! sodium l9e1 ,f ! blue hydrog5 l9e & ,g ! blue-violet l9e hydrog5 l9e1 & are di/+ui%$ z nc1 nd1 nf1 etc4 ,! /&>d disp]sion 7dn7 = visual rays is giv5 z betwe5 ,c & ,f1 :ile ! /&>d refractivity is tak5 = ,d1 9 ! bright yell[ "p ( ! spectrum4 7,note4 ,= ! conv5i5ce ( ^? :o are ru/y on _! trigonometry1 ,fig4 #ci %[s ! simpl] trigonometric functions ( an angle4 ,?us ! s9e ( ! angle ,a is1 num]ically1 ! l5g? ( ! radius divid$ into ! l5g? ( ! l9e dropp$ f ! end ( ! radius to ! horizontal base l9e1 i4e4 bc_/,ob1 ! tang5t is da_/,ob1 & ! cos9e ,oc_/,ob47 ,ord9>ily ! index ( refraction ( ! cr[n 0 tak5 z ab\t #c_/b1 t ( ! fl9t z ab\t #h_/e4 ,z "t has gone on & especially s9ce ! new glasses f ! ,j5a works 7 introduc$ ab\t #ce ye>s ago1 "o _c def9e cr[ns & fl9ts 9 any su* simple fa%ion1 = !re are cr[ns ( high index & fl9ts ( l[ disp]sion4 ,illu/ration3 ,,fig4#ci4--,! ,simple ,trigonometric ,functions ( an ,angle4 ,! foll[+ table gives ! optical data & *emical analyses ( a few typical optical glasses4 ,! li/ includes common cr[ns & fl9ts1 a typical b>yta cr[n & light fl9t1 & a telescope cr[n & fl9t = ! bett] a*romatization ( objectives1 z develop$ at ! ,j5a works4 ,! ?+ mo/ conspicu\s "h z di/+ui%+ cr[ns f fl9ts is t ! latt] h great] relative disp]sion 9 ! blue1 ! =m] 9 ! r$ end ( ! spectrum1 z %[n by ! bracket$ ratios4 ,?is z we %all see is ( s]i\s consequ5ce 9 mak+ a*romatic objectives4 ,9 g5]al1 too1 ! values ( = fl9ts are mu* l[] ?an = cr[ns1 & ! indices ( refraction !mselves commonly high]4 ,z we h j se51 glass comes to ! optician 9 blocks or discs1 = miscellane\s use ! =m]1 ?ree or f\r in*es squ>e & an in* ?9k1 m or less2 = telescope mak+ ! latt]4 ,! discs are commonly "s t5 p]c5t great] 9 diamet] ?an ! f9i%$ objective = :i* !y are int5d$1 & 9 ?ickness f #a_/h to #a_/aj ! diamet]4 ,!y are commonly well anneal$ & giv5 a prelim9>y poli% on bo? sides to facilitate close inspection4 ,,*>act]i/ics ,,( ,,optical ,,glasses ---------------------------------------- ----------------------- ,bracket$ numb]s are dn proportions ( dn ,glass nd ----- ------------------------ 7,f-,c7 ,d-,a ,f-,d ,g-,f ---- ---- ---- dn dn dn ---------------------------------------- --------------------- ,boro-silicate #a4ejfi 4#jjhac #fb4c 4#jjebi 4#jjefi 4#jjdeg cr[n 74#fea7 74#gja7 74#efb7 ,z9co-silicate #a4eagj 4#jjhei #fj4b 4#jjeee 4#jjfje 4#jjdhe 7h>d7 cr[n 74#fdf7 74#gjd7 74#efe7 ,d5se b>yta #a4ehii 4#jjigj #fj4h 4#jjfba 4#jjfhc 4#jjedf cr[n 74#fdj7 74#gjd7 74#efc7 ,b>yta light #a4egah 4#jaacc #ej4d 4#jjgjf 4#jjhjc 4#jjffj fl9t 74#fbc7 74#gji7 74#ehb7 ,common light #a4egaj 4#jacbg #dc4j 4#jjhai 4#jjidc 4#jjgia fl9t 74#fag7 74#gaj7 74#eif7 ,common d5se #a4faaf 4#jafch #cg4c 4#jjiie 4#jaagj 4#jjiia fl9t 74#fjg7 74#gad7 74#fjg7 ,v d5se #a4fdhi 4#jaiai #cc4h 4#jaaeb 4#jacgb 4#jaahj fl9t 74#fjj7 74#gad7 74#fae7 ,d5se/ fl9t #a4geda 4#jbgdc #bg4e 4#jafjg 4#jaigd 4#jagcj 74#ehe7 74#gbj7 74#fcj7 99,telescope #a4ebhe 4#jjhff #fa4j 4#jjeeg 4#jjfaj 4#jjdic cr[n 74#fdc7 74#gje7 74#egj7 99,telescope #a4ebhf 4#jajbe #ea4f 4#jjfed 4#jjgbc 4#jjeia fl9t 74#fch7 74#gje7 74#egf7 99 ,optical data close approximations only4 ---------------------------------------- -------------------------------- ,analysis ( glasses 9 p]c5tages ---------------------------------------- ------------------- ,si ,b#b ,k#b ,na#b ,,al#b;,z#b;,z#b;,fe#b,mn#b,sb#b ,o#b ,o#c,zn,o ,pb,o ,ba,o ,o ,o ,ca,o ,o#c ,o#e ,o#c ,o#c ,o#c ,o#c ,mg,o ---------------------------------------- ------------------- #gd4h #e4i -- -- -- #g4aaaa4c -- 4#ge -- 4#jf -- 4#jf #fe4d #b4eb4j -- #i4fae4j #e4j -- -- -- 4#d -- 4#a #cg4eae4j -- -- #da4j -- -- #e4j #a4e #ea4g -- #g4jaj4jbj4j #i4e #a4e -- -- 4#cj #ed4c #a4e --#cc4j -- #h4j #c4j -- -- 4#bj #ed4h -- --#cg4j -- #e4h 4#h 4#fj 4#d -- -- 4#gj -- -- 4#bj #dj4j -- --#eb4f -- #f4e 4#e -- -- 4#cj -- -- 4#ji #bi4c -- --#fg4e -- #c4j -- -- -- -- 4#bj -- 4#jd ------ #ee4b -- -- -- #bb4j #e4g #g4ee4i -- -- -- #c4g #ei4iab4g -- -- -- #e4a #c4e -- -- -- -- #b4g #af4a ---------------------------------------- -------------------- ,! fir/ /ep t[>d ! telescope is ! te/+ ( ^! discs ( glass1 fir/ = ! pres5ce or abs5ce ( /ri & o!r imp]fections2 second1 = ! p]fection ( ! anneal+4 ,! mak] has usually look$ out = all ! gross] imp]fections be=e ! discs left 8 works1 b a mu* clos] inspection is ne$$ 9 ord] to make ! be/ use ( ! glass4 ,illu/ration3 ,,fig4 #dj4--,te/+ ,glass = ,/ri4 ,bad /ri are ( c\rse se5 easily1 z !y w\ld be 9 a w9d[ pane1 b su* gross imp]fections are (t5 9 reality less damag+ ?an ! app>5tly slight] ones :i* mu/ be se>*$ =4 ,! simple/ te/ is to focus a good telescope on an artificial />1 remove ! eyepiece & br+ ! eye into its place4 ,:5 ! eye is 9 focus ! :ole ap]ture ( ! objective is uni=mly fill$ ) light1 & if ! disc to be te/$ be plac$ 9 front ( x1 any inequality 9 refraction w ann\nce itself by an inequality ( illum9ation4 ,a r\gh judgm5t z to ! s]i\sness ( ! defect may be =m$ f ! area affect$ & ! am\nt by :i* x affects ! local int5sity ( illum9ation4 ,fig4 #dj %[s ! arrangem5t = ! te/1 ,a be+ ! eye1 ,b ! objective & ,c ! disc4 ,! artificial /> is conv5i5tly made by sett+ a black bottle 9 ! sun a hundr$ feet or s away & gett+ ! reflection f its %\ld]4 ,illu/ration3 ,,fig4 #da4--,! ,mirror ,te/ = ,/ri4 ,a some:at m delicate te/1 v commonly us$1 is %[n 9 ,fig4 #da4 ,"h ,a is a truly sph]ical mirror silv]$ on ! front4 ,at ,b v close to its c5tre ( curvature is plac$ a lamp ) a scre5 9 front ( x p]=at$ ) a hole #a_/cb in* or s 9 diamet]4 ,! rays reflect$ f ! mirror come back q exactly ^u !mselves & :5 ! eye is plac$ at ,c1 _! reflect$ focus1 ! :ole mirror ,a is uni=mly light$ j z ! l5s 0 9 ,fig4 #dj1 ) ! incid5tal advantage t x is mu* easi] & *eap] to obta9 a sph]ical mirror = te/+ a sizeable disc ?an an objective ( simil> size & quality4 ,n[ plac+ ! disc ,d 9 front ( ! mirror1 ! light pass+ twice ?r\gh x %[s up ! slighte/ /ria or o!r imp]fection z a /reak or spot 9 ! field4 ,its place is obvi\s & c be at once m>k$ on ! glass1 b its exact position 9 ! sub/ance ( ! disc is n s obvi\s4 ,to det]m9e ?is1 :i* may indicate t ! fault c be gr\nd out 9 %ap+ ! l5s1 a modification ( ! fir/ te/ s]ves well1 z inde$ x does = ! g5]al exam9ation ( l>ge discs4 ,in/ead ( us+ a di/ant artificial /> & a telescope1 "o uses ! lamp & scre51 or ev5 a c&le flame t5 feet or m away & a cond5s+ l5s ( r %ort focus1 :i* may or may n be a*romatic1 s t ! eye w get into its focus conv5i5tly :ile ! l5s is held 9 ! h&4 ,fig4 #db %[s ! arrangem5t4 ,"h ,a is ! eye1 ,b ! cond5s+ l5s1 ,c ! disc & ,d ! s\rce ( light4 ,! cond5s+ l5s may be held on ei!r side ( ! disc z conv5i5ce sugge/s1 & ei!r disc or l5s may be mov$4 ,! op]ation is sub/antially ! exam9ation ( a l>ge disc piecemeal1 in/ead ( all at once by ! use ( a big objective or mirror4 ,illu/ration3 ,,fig4 #db4--,locat+ ,/ri 9 ! ,sub/ance ( a ,disc4 ,n[ :5 a /ria has be5 not$ m>k its location z to ! surface1 &1 mov+ ! eye a little1 look = p>allax ( ! fault ) respect to ! surface m>k4 ,if x appe>s to %ift try a m>k on ! opposite surface 9 ! same way4 ,comp>ison ( ! two inspections w %[ ab\t :]e ! fault lies ) respect to ! surfaces1 & !re=e :at is ! *ance ( work+ x out4 ,sometimes a look edgewise ( ! disc w help 9 ! diagnosis4 ,num]\s b>ely detectable /ri are usually worse ?an "o or two conspicu\s ones1 = ! latt] frequ5tly ?r[ ! light !y transmit s wide ( ! focus t x does n affect ! image1 :i* c\ld be greatly damag$ by slight blurs ( light t j miss focus4 ,giv5 a disc t passes well ! te/s = /ri & ! l ! next /ep is to exam9e ! p]fection ( ! anneal+1 :i* 9 its l>g] aspect is reveal$ by an exam9ation 9 pol>iz$ light4 ,illu/ration3 ,,fig4 #dc4--,te/+ a ,disc 9 ,pol>iz$ ,light4 ,= ?is purpose ! disc is set up aga9/ a frame plac$ on table or floor ) a good exposure to skylight beh9d x1 & incl9$ ab\t #ce f ! v]tical4 ,beh9d x is laid a flat %9y surface to s]ve z pol>iz]4 ,black enamel clo? smoo?ly laid1 a glass plate back$ ) black pa9t1 or ev5 a smoo? bo>d pa9t$ ) asphalt pa9t w answ] excell5tly4 ,!n hold+ a ,nicol prism be=e ! eye & look+ p]p5dicul> to ! face ( ! disc1 rotate ! prism on its axis4 ,fig4 #dc %[s ! arrangem5t1 ,a be+ ! eye1 ,b ! ,nicol1 ,c ! disc1 & ,d ! pol>iz] beh9d x4 ,if anneal+ has left no /ra9 ! only effect ( rotat+ ! ,nicol w be to *ange ! field f bright to d>k & back aga9 z if ! disc 7 n !re4 ,g5]ally a patt]n 9 ! =m ( a some:at hazy ,maltese cross w appe>1 ) its arms cross+ ! disc1 gr[+ d>k] & light] alt]nately z ! ,nicol is turn$4 ,if ! cross is /rongly m>k$ b symmetrical & well c5t]$ ! anneal+ is fair--bett] z ! cross is fa9t] & hazi]--altoge!r bad if colors %[ pla9ly or if ! cross is dec5t]$ or di/ort$4 ,! te/ is extremely s5sitive1 s t hold+ a f+] on ! surface ( ! disc may produce local /ra9 t w %[ z a fa9t cl\dy spot4 ,a disc free ( /ri & noticeable anneal+ /ra9s is usually1 b n inv>iably1 good1 = too frequ5t reheat+ 9 ! m\ld+ or anneal+ process occasionally leaves ! glass slightly alt]$1 ! effect ext5d+1 at wor/1 to ! cry/allization or devitrification to :i* ref]5ce has be5 made4 ,giv5 a good pair ( discs ! fir/ /ep t[>ds fa%ion+ !m into an objective is r\gh+ to ! approximate =m desir$4 ,z a guide to ! %ap+ ( ! necess>y curves1 templets mu/ be made f ! design$ curves ( ! objective z precisely z possible4 ,^! are laid out by /rik+ ! necess>y radii ) beam compass or pivot$ wire & scrib+ ! curve on ?9 /eel1 brass1 z9c or glass4 ,! two la/ are ! easi] to "w s9ce !y break closely to =m4 ,f ^! templets ! r\gh+ tools are turn$ up1 commonly f ca/ iron1 & ) ^!1 suppli$ ) c>borundum or ev5 s&1 & wat]1 ! discs1 be>+ aga9/ ! revolv+ tool1 are gr\nd to ! g5]al %ape requir$4 ,!y are !n secur$ to a sl[ly revolv+ table1 be>+ edgewise aga9/ a revolv+ gr9d/one1 & gr\nd truly circul> & ( ! prop] f9al diamet]4 ,at ?is po9t beg9s ! really c>eful "w ( f9e gr9d+1 :i* mu/ br+ ! l5s v close to its exact f9al %ape4 ,"h aga9 tools ( ca/ iron1 or sometimes brass1 are us$1 v precisely br\ght to %ape accord+ to ! templets4 ,!y are groov$ on ! face to facilitate ! ev5 di/ribution ( ! abrasive1 em]y or f9e c>borundum1 & ! "w is g5]ally done on a special gr9d+ ma*9e1 :i* moves ! tool ov] ! firmly support$ disc 9 a complicat$ s]ies ( /rokes imitat+ m or less closely ! /rokes f\nd to be mo/ effective 9 h& poli%+4 ,9 g5]al t]ms ! op]ator 9 h&work at ?is task supports ! disc on a firm v]tical po/1 by cem5t+ x to a suitable hold]1 & !n moves ! tool ov] x 9 a s]ies ( /raight or oval /rokes1 mean:ile walk+ ar\nd ! po/4 ,a skilful op]ator wat*es ! progress ( 8 "w1 v>ies ! l5g? & position ( 8 /rokes accord+ly1 &1 despite ! unavoidable we> on ! tool1 c bo? keep its figure true & imp>t a true figure to ! glass4 ,illu/ration3 ,,fig4 #dd4--;,dr4 ,drap]'s ,poli%+ ,ma*9e4 ,! poli%+ ma*9e1 ( :i* a type us$ by ,dr4 ,drap] is %[n 9 ,fig4 #dd1 produces a simil> motion1 ! disc sl[ly revolv+ & ! r small tool mov+ ov] x 9 oval /rokes kept (f ! c5t]4 ,m (t5 ! tool is ( approximately ! same diamet] z ! disc "u x4 ,! g5]al "* ( ! motion is evid5t f ! con/ruction4 ,! disc a is *uck$ by c c on ! b$1 turn$ by ! po/ d & worm :eel e4 ,?is is op]at$ f ! pulleys1 i1 g1 :i* drive ?r\gh k1 ! crank m1 adju/able 9 ?r[ by ! nuts n1 n1 & 9 position ( tool by ! clamps r1 r4 ,! motion may be consid]ably v>i$ by adju/m5t ( ! ma*9e1 always keep+ ! /roke f repeat+ on ! same "p ( ! disc1 by mak+ ! p]iod ( ! revolution & ( ! /roke incomm5surable s f> z may be4 ,ev5 9 spectacle gr9d+ ma*9es ! /roke may repeat only once 9 hundr$s ( times1 & ev5 ?is frequ5cy 9 a big objective w\ld1 if foll[$ 9 ! poli%+1 leave tool m>ks :i* c\ld be detect$ 9 ! f9al te/+4 ,9 ! f9e gr9d+1 especially ne> ! end ( ! process1 ! templets d n give suffici5t precision 9 te/+ ! curves1 & rec\rse is _h to ! sph]omet]1 by :i* measurem5ts d[n to ab\t #a_/ajjjjj in* c be consi/5tly made4 ,! next /age ( op]ations is poli%+1 :i* trans=ms ! grey transluc5cy ( ! f9e gr\nd l5s into ! cle> & brilliant surface :i* at la/ p]mits rigor\s optical te/s to be us$ = ! f9al f9i% ( ! l5s4 ,?is poli%+ is done g5]ally on ! f9e gr9d+ ma*9e b ) a v diff]5t tool & ) r\ge ( ! utmo/ f95ess4 ,! poli%+ tool is 9 any case gr\nd true & is !n fac$ ) a some:at yield+ mat]ial to c>ry ! *>ge ( r\ge4 ,*eap l5ses are commonly work$ on a clo? poli%]1 a texture simil> to billi>d clo? be+ suitable1 or sometimes on pap] work$ dry4 ,) c>e ei!r may produce a fairly good surface1 )1 h[ev]1 a t5d5cy to poli% out ! m9ute holl[s left by gr9d+ r ?an to cut a true surface cle> d[n to _! bottoms4 ,h5ce clo? or pap] is likely to leave microscopic inequalities app>5tly poli%$1 & ?is may be suffici5t to scatt] ov] ! field a v p]ceptible am\nt ( light :i* %\ld g to =m+ ! image4 ,all fir/ class objectives & mirrors are 9 fact poli%$ on optician's pit*4 ,?is is n ! ord9>y pit* ( comm]ce b a sub/ance ( v>i\s composition1 sometimes an asphaltic comp\nd1 aga9 on a base ( t>1 or ( res9 br\ght to ! "r consi/5cy by turp5t9e4 ,:atev] ! exact composition1 ! fundam5tal prop]ty is t ! mat]ial1 app>5tly fairly h>d & ev5 brittle :5 cold1 is actually some:at pla/ic to cont9u$ pressure4 ,seal+ wax has some?+ ( ?is quality1 = a /ick :i* may readily be brok5 w yet b5d "u its own weight if support$ at ! ends4 ,if ! f9e gr9d+ process has be5 prop]ly c>ri$ out ! l5s has receiv$ its correct =m z ne>ly z gauges & ! sph]omet] c det]m9e x4 ,! next /ep is to poli% ! surface z brilliantly & ev5ly z possible4 ,to ?is end advantage is tak5 ( ! pla/ic quality already m5tion$1 t ! glass may =m its own tool4 ,! base ( ! tool may be any?+ conv5i5t1 metal1 glass or ev5 wood4 ,its work+ surface is made z ne>ly ( ! "r curvature z practicable & x is !n coat$ ) w>m pit* to a ?ickness ( an eigh? ( an in* m or less1 ei!r cont9u\sly or 9 squ>es1 & :ile /ill slightly w>m ! tool is plac$ aga9/ ! f9e gr\nd disc1 ! exact %ape ( :i* x takes4 ,:5 cold ! pit* surface c easily be cut out into squ>es or symmetrically pitt$ ) a suitable tool1 at once facilitat+ ! di/ribution ( ! r\ge & wat] t s]ves = poli%+1 & p]mitt+ delicate adju/m5t ( ! work+ curvature 9 a way ab\t to be describ$4 ,fig4 #de %[s ! squ>$ surface ( ! tool z x w\ld be us$ = poli%+ a plane or v slightly convex or concave surface4 ,suppli$ ) ! ?9 abrasive pa/e1 x is all[$ to settle1 cold1 into its f9al contact ) ! glass1 & !n ! process ( poli%+ by h& or ma*9e is />t$4 ,! action ( ! tool mu/ be uni=m to avoid *ang+ ! %ape ( ! l5s4 ,x c be regulat$ z x 0 9 ! gr9d+1 by v>y+ ! l5g? & "* ( ! /roke1 b ev5 m delicately by v>y+ ! ext5t ( surface cov]$ by ! pit* actually work+ on ! glass4 ,illu/ration3 ,,fig4 #de4--,tool = ,flat ,surface4 ,illu/ration3 ,,fig4 #df4--,tool = ,concave ,surface4 ,?is is done by *annel+ or bor+ away pit* ne> ! rim or c5t] ( ! tool z ! case may be4 ,fig4 #df %[s a tool :i* has be5 ?us treat$ s t ! squ>es are progressively small] ne> ! p]iph]y4 ,su* a spac+ t5ds to produce a concave surface f a flat tool or to increase ! concavity f a curv$ "o4 ,trimm+ d[n ! squ>es t[>ds ! c5tre produces ! opposite result4 ,broadly1 ! pr9ciple is t ! tool cuts ! m 9 ! areas :]e ! contact surfaces are ! great]4 ,?is is n :olly by reason ( great] abrad+ surface1 b also because :]e ! contact is great] 9 area ! pit* settles less1 f ! dim9i%$ pressure1 ?us increas+ ! effective contact4 ,cle>ly ! effect ( trimm+ away is correlat$ ) ! =m & l5g? ( /roke1 & ! temp] ( ! pit*1 & 9 fact x requires ! wisdom ( ! s]p5t to comb9e ^! v>i\s factors s z to produce ! p]fectly uni=m & regul> action requir$ 9 poli%+4 ,n[ & !n1 at brief int]vals1 ! op]ation is /opp$ to supply r\ge & to avoid *ang+ ! conditions by ! heat ( friction4 ,especially mu/ heat+ be look$ out = 9 h& poli%+ ( l5ses :i* is (t5 done ) ! glass upp]mo/ = easi] inspection ( ! "w4 ,poli%+1 if ! f9e gr9d+ has be5 judici\sly done is1 = mod]ate siz$ surfaces1 a matt] ( only a few h\rs4 ,x proce$s q sl[ly at fir/ :ile ! hills are be+ gr\nd d[n & !n r sudd5ly comes up brilliantly z ! poli%] rea*es ! bottoms ( ! valleys4 ,l>ge l5ses & mirrors may require _m days4 ,n[ beg9s ! f9al & extraord9>ily delicate process ( figur+4 ,! l5s or mirror has its appo9t$ =m z ne>ly z ! mo/ precise me*anical me?ods c tell--say d[n to "o or two hundr$-?\s&?s ( an in*4 ,f ! optical /&po9t ! result may be ?or\ghly bad1 = an error ( a few million?s ( an in* may be s]i\s 9 ! f9al p]=mance4 ,! p]iph]y may be by su* an am\nt long] or %ort] 9 radius ?an x %\ld be1 or !re may be an int]m$iate zone t has gone a/ray4 ,9 case ( a mirror ! orig9al poli%+ is g5]ally int5d$ to leave a sph]ical surface :i* mu/ be conv]t$ into a p>aboloidal "o by a *ange 9 curvature totall+ only a few hundr$-?\s&?s ( an in* & s]i\sly affect$ by mu* small] v>iations4 ,! figur+ is done 9 a fa%ion v simil> to ! poli%+4 ,! fir/ /ep is to f9d out by optical te/s su* z are describ$ 9 ,*apt] ,,ix ! location ( ! errors exi/+ aft] ! poli%+1 & once f\nd1 !y mu/ be elim9at$ by pati5t & cauti\s "w on ! surface4 ,e optical exp]t has 8 own favorite me?ods ( work+ out ! figure4 ,if !re is a holl[ zone ! :ole surface mu/ be work$ d[n to its level by repoli%+2 if1 on ! o!r h&1 !re is an annul> hump1 "o may repoli% ) /roke & tool-face adapt$ to cut x d[n1 or "o may cauti\sly poli% x out until x m]ges ) ! g5]al level4 ,poli%+ is commonly done ) tools ( approximately ! size ( ! "w1 b 9 figur+ !re is great diff]5ce ( practice1 "s exp]t work]s dep5d+ entirely on manipulation ( a full siz$ tool1 o!rs work+ locally ) small poli%]s1 ev5 ) ! ball ( ! ?umb1 9 remov+ slight ab]rations4 ,9 small "w :]e ! glass c be dep5d$ on = homog5eity & ! tools are easily kept true ! =m] me?od is ! usual "o1 b 9 big objectives ! latt] is (t5 easi] & may successfully rea* faults o!rwise v difficult to elim9ate4 ,among well kn[n mak]s ( telescopes ! ,cl>ks & _! equally skill$ successors ! ,lund9s1 fa!r & son1 develop$ ! art ( local ret\*+ to a po9t little %ort ( wiz>dry2 ! late ,dr4 ,bra%e> dep5d$ almo/ entirely on ! adroitly us$ poli%+ ma*9e2 ,sir ,h[>d ,grubb uses local correction 9 c]ta9 cases1 & 9 g5]al ! cauti\sly modifi$ poli%]2 :ile "s ( ! ,cont95tal exp]ts are report$ to h develop$ ! local me?od v ?or\ghly4 ,! tru? probably is t ! p>ticul> error 9 h& %\ld det]m9e ! me?od ( attack & t its success dep5ds entirely on ! skill ( ! op]ator4 ,z to ! p]fection ( ! objectives figur$ 9 ei!r way1 no sy/ematic diff]5ce due to ! me?od ( figur+ c be detect$ by ! mo/ delicate te/s4 ,9 any case ! figur+ op]ation is long & t$i\s1 especially 9 l>ge "w :]e problems ( support+ to avoid flexure arise1 :]e temp]ature effects on tool & glass involve long delays betwe5 te/s & correction1 & :]e 9 ! la/ resort non-sph]ical surfaces mu/ (t5 be resort$ to 9 br++ ! image to its f9al p]fection4 ,! f9al te/ ( goodness is p]=mance1 a clean r\nd image )\t a trace ( sph]ical or zonal ab]ration & ! color correction ! be/ ! glasses w all[4 ,con/ant & rigor\s te/+ mu/ be appli$ all ?r\gh ! process ( figur+1 & ! result seems to dep5d on a comb9ation ( exp]i5ce1 intuition & tactual exp]tness r>ely unit$ 9 any "o p]son4 ,sir ,h[>d ,grubb1 9 a pap] to be comm5d$ to anyone int]e/$ 9 objectives1 once =cibly said3 8,i may safely say t ,i h nev] f9i%$ any objective ov] #aj in*es diamet]1 9 ! work+ ( :i* ,i did n meet ) "s new exp]i5ce1 "s new set ( conditions :i* ,i _h n met be=e1 & :i* _h !n to be met by special & newly devis$ arrangem5ts40 ,! mak+ ( reflect+ telescopes is n mu* easi] s9ce al?\gh only "o surface has to be work$1 t "o has to be figur$ ) extraord9>y c>e1 flexure has to be gu>d$ aga9/ at e /age ( ! work+1 & aft]w>ds1 temp]ature *ange is a busy foe1 :ile te/+ = correct figure1 ! surface be+ non-sph]ical1 is consid]ably m tr\blesome4 ,an exp]t c make a good mirror ) f> less actual labor ?an an objective ( simil> ap]ture1 b :5 "o reads ,dr4 ,h5ry ,drap]'s /atem5t t 9 spite ( kn[+ at fir/ h& ! me?ods & gr9d+ ma*9es ( ,"l ,rosse & ,mr4 ,lassell1 he gr\nd ov] a hundr$ mirrors1 & sp5t ?ree ye>s ( "t1 be=e he c\ld get a correct figure ) reasonable facility1 "o c]ta9ly ga9s a high respect = ! skill acquir$4 ,?is *apt] is necess>ily sket*y & n 9 ! lea/ int5d$ to give ! read] a complete acc\nt ( te*nical glass manufacture1 f> less ( ! intricate & almo/ incommunicable art ( mak+ objectives & mirrors4 ,x may h[ev] lead to a bett] und]/&+ ( ! diff]5ce betwe5 ! optical glass indu/ry & ! fabrication ( comm]cial glass1 & lead ! read] to a full] realization ( h[ f9e a "w ( art is a f9i%$ objective or mirror z comp>$ ) ! crude ef=ts ( ! e>ly mak]s or ! ha/y bungl+ ( too _m ( _! successors4 ,= fur!r details on mak+1 prop]ties & work+ ( optical glass see3 ,,hove/adt3 8,j5a] ,glas40 ,,ros5ha93 8,glass ,manufacture40 ,,sir ,,h[>d ,,grubb3 8,telescopic ,objectives & ,mirrors3 ,_! ,prep>ation & ,te/+40 ,nature #cd1 #he4 ,,dr4 ,,h5ry ,,drap]3 8,on ! ,con/ruction ( a ,silv]$ ,glass ,telescope40 7,smi?sonian ,contributions to ,k1 ,vol4 #cd47 ,g4 ,w4 ,,rit*ey3 ,on ! ,mod]n ,reflect+ ,telescope & ! ,mak+ & ,te/+ ( ,optical ,mirrors4 7,smi?sonian ,contributions to ,k1 ,vol4 #cd47 ,,"l ,,rayleigh3 ,poli%+ ( ,glass ,surfaces4 7,proc4 ,opt4 ,conv5tion1 #aije1 p4 #gc47 ,,bottone3 8,l5s ,mak+ = ,amateurs40 ,,*apt] ,,iv ,,! ,,prop]ties ,,( ,,objectives ,,& ,,mirrors ,! pa? ( ! rays ?r\gh an ord9>y telescope has be5 %[n 9 ,fig4 #e4 ,9 pr9ciple all ! rays f a po9t 9 ! di/ant object %\ld unite precisely 9 a correspond+ po9t 9 ! image :i* is view$ by ! eyepiece4 ,practically x takes v c>eful design & con/ruction ( ! objective to make !m meet 9 su* ord]ly fa%ion ev5 ov] an angul> space ( a s+le degree1 & ! wid] ! view requir$ ! m difficult ! con/ruction4 ,we h spok5 9 ! acc\nt ( ! e>ly work]s ( _! /ruggles to avoid *romatic & sph]ical ab]rations1 & x is *iefly ^! t /ill1 9 less measure1 worry _! successors4 ,illu/ration3 ,,fig4 #dg4--;,*romatic ,ab]ration ( ,convex ,l5s4 ,! fir/ nam$ is due to ! fact t a prism does n b5d light ( all colors equally1 b spreads !m out into a spectrum2 r$ refract$ ! lea/1 violet ! mo/4 ,s9ce a l5s may be reg>d$ z an assemblage ( prisms1 ( small angle ne> ! c5tre & great] ne> ! edge1 x mu/ on ! :ole & all ov] b5d ! blue & violet rays to meet on ! axis ne>] ! re> surface ?an ! correspond+ r$ rays1 z %[n 9 ,fig4 #dg4 ,"h ! incid5t ray a is split up by ! prismatic effect ( ! l5s1 ! r$ com+ to a focus at r1 ! violet at v4 ,"o c readily see ?is *romatic ab]ration by cov]+ up mo/ ( a common read+ glass ) 8 h& & look+ ?r\gh ! edge portion at a bright light1 :i* w be spread out into a color$ b&4 ,if ! l5s is concave ! violet rays w /ill be ! m b5t1 b n[ outw>ds1 z %[n 9 ,fig4 #dh4 ,! incid5t ray a is split up & ! violet is b5t t[>d v1 proce$+ z if com+ /raight f a virtual focus v 9 front ( ! l5s1 & ne>] x ?an ! correspond+ r$ focus r4 ,evid5tly if we c\ld comb9e a convex l5s1 b5d+ ! violet inw>d too mu*1 ) a concave "o1 b5d+ x outw>d too mu*1 ! two opposite v>iations might comp5sate ea* o!r s t r$ & violet w\ld come to ! same focus--:i* is ! pr9ciple ( ! a*romatic objective4 ,illu/ration3 ,,fig4 #dh4--;,*romatic ,ab]ration ( ,concave ,l5s4 ,if ! refractive p[]s ( ! l5ses 7 exactly proportional to _! disp]sive p[]s1 z ,newton errone\sly ?\ght1 x is evid5t t ! concave l5s w\ld pit* all ! rays outw>ds to an am\nt :i* w\ld annul bo? ! *romatic v>iation & ! total refraction ( ! convex l5s1 leav+ ! pair )\t p[] to br+ any?+ to a focus4 ,=tunately fl9t glass z comp>$ ) cr[n glass has ne>ly d\ble ! disp]sion betwe5 r$ & violet1 & only ab\t #bj great] refractive p[] = ! int]m$iate yell[ ray4 ,h5ce1 ! prismatic disp]sive effect be+ proportional to ! total curvature ( ! l5s1 ! *romatic ab]ration ( a cr[n glass l5s w be cur$ by a concave fl9t l5s ( ab\t half ! total curvature1 &1 ! refractions be+ ab\t z #e to #f1 ( ! total p[]4 ,s9ce ! 8p[]0 ( any l5s is ! reciprocal ( its focal l5g?1 a cr[n glass convex l5s ( focal l5g? #c1 & a concave fl9t l5s ( focal l5g? #e 7negative7 w =m an approximately a*romatic comb9ation4 ,! p[] ( ! comb9ation w be ! algebraic sum ( ! p[]s ( ! compon5ts s t ! focal l5g? ( ! pair w be ab\t #e_/b t ( ! cr[n l5s ) :i* we />t$4 ,to be m precise ! condition ( a*romatism is n n #j :]e is ! reciprocal ( a radius & n1 or n1 is ! diff]5ce 9 refractive index betwe5 ! rays *os5 to be br\ght to exact focus toge!r1 z ! r$ & ! blue or violet4 ,?is conv5tional equation simply /ates t ! sum ( ! reciprocals ( ! radii ( ! cr[n l5s multipli$ by ! disp]sion ( ! cr[n1 mu/ equal ! correspond+ quantity = ! fl9t l5s if ! two total disp]sions are to annul ea* o!r1 leav+ ! comb9ation a*romatic4 ,:atev] glass is us$ ! p[] ( a l5s made ( x is ,p7 #a_/;f7 7n - #a7 s t x w be se5 t1 o!r ?+s be+ equal1 a glass ( high index ( refraction t5ds to give mod]ate curves 9 an objective4 ,also1 ref]r+ to ! condition ( a*romatism1 ! great] ! diff]5ce 9 disp]sion betwe5 ! two glasses ! less curvatures w be requir$ = a giv5 focal l5g?1 a condition advantage\s = v>i\s reasons4 ,! det]m9ation ( a*romatism = any pair ( glasses & focal l5g? is greatly facilitat$ by employ+ ! auxili>y quantity :i* is tabulat$ 9 all li/s ( optical glass z a %ort cut to a some:at less manageable algebraic expression4 ,us+ ?is we c figure a*romatism = unity focal l5g? at once1 ,p _/7-7 ,p _/7-7 7n,d-#a7_/n be+ ! p[]s ( ! lead+ & foll[+ l5ses respectively4 ,! comb9$ l5s w br+ ! rays ( ! two *os5 colors1 z r$ & blue1 to focus at ! same po9t on ! axis4 ,x does n necess>ily give to ! r$ & blue images ( an object ! same exact size4 ,failure 9 ?is respect is kn[n z *romatic diff]5ce ( magnification1 b ! fault is small & may g5]ally be neglect$ 9 telescope objectives4 ,we h n[ se5 h[ an objective may be made a*romatic & ( det]m9ate focal l5g?1 b ! solution is 9 t]ms ( ! sums ( ! respective curvatures ( ! cr[n & fl9t l5ses1 & gives no in=mation ab\t ! radii ( ! individual surfaces4 ,! relation betwe5 ^! is all-important 9 ! f9al p]=mance4 ,illu/ration3 ,,fig4 #di4--,sph]ical ,ab]ration ( ,convex ,l5s4 ,= 9 a convex l5s ) sph]ical surfaces ! rays /rik+ ne> ! edge1 ( :atev] color1 are pit*$ inw>ds too mu* comp>$ ) rays /rik+ ! m mod]ate curvatures ne> ! axis1 z %[n 9 ,fig4 #di4 ,! ray a b ?us comes to a focus %ort] ?an ! ray a b4 ,?is con/itutes ! fault ( sph]ical ab]ration1 :i* ! old a/ronom]s1 foll[+ ! sugge/ions ( ,desc>tes1 tri$ ineffectually to cure by =m+ l5ses ) non-sph]ical surfaces4 ,illu/ration3 ,,fig4 #ej4--,sph]ical ,ab]ration ( ,concave ,l5s4 ,fig4 #ej sugge/s ! rem$y1 = ! out] ray a is pit*$ out t[>d b z if x came f a focal po9t c1 :ile ! ray ne>] ! c5t] a is mu* less b5t t[>d b z if x came f c4 ,! sph]ical ab]rations ( a concave l5s !re=e1 be+ opposite to ^? ( a convex l5s1 ! two mu/1 at lea/ to a c]ta9 ext5t1 comp5sate ea* o!r z :5 comb9$ 9 an a*romatic objective4 ,s 9 fact !y d1 &1 if ! curves t g to make up ! total curvatures ( ! two are prop]ly *os51 ! total sph]ical ab]ration c be made negligibly small1 at lea/ on & ne> ! axis4 ,tak+ into acc\nt ?is condition1 !re=e1 at once gives u a clue to ! di/ribution ( ! total curvatures & h5ce to ! radii ( ! two l5ses4 ,sph]ical ab]ration1 h[ev]1 involves n only ! curvatures b ! indices ( refraction1 s t exact correction dep5ds 9 "p on ! *oice ( glasses :]e) to obta9 a*romatization4 ,9 am\nt sph]ical ab]ration v>ies ) ! squ>e ( ! ap]ture & inv]sely ) ! cube ( ! focal l5g? i4e4 ) a_/f4 ,x is reckon$ z :51 z 9 ,fig4 #di1 ! rim rays come to ! %ort] focus1 z-1 :5 !y come to ! long] focus4 ,9 any ev5t1 s9ce ! sph]ical ab]ration ( a l5s may be v>i$ 9 above ! ratio ( #d3a1 = ! same total p[]1 m]ely by *ang+ ! ratio ( ! radii1 x is evid5t t ! two l5ses be+ fairly correct 9 total curvature might be giv5 consid]able v>iations 9 curvature & /ill mutually annul ! axial sph]ical ab]ration4 ,su* is 9 fact ! case1 s t to get det]m9ate =ms = ! l5ses "o mu/ introduce "s fur!r condition or make "s assumption t w p9 d[n ! sep>ate curvatures to "s def9ite relations4 ,! requirem5t may be entirely arbitr>y1 b 9 work+ out ! !ory ( objectives has usually be5 *os5 to give ! l5s "s real or hypo!tical additional advantage4 ,! commone/ arbitr>y requirem5t is t ! cr[n glass l5s %all be equiconvex1 m]ely to avoid mak+ an extra tool4 ,?is fixes "o pair ( radii1 & ! fl9t l5s is !n giv5 ! requir$ comp5sat+ ab]ration *oos+ ! easie/ =m to make4 ,?is results 9 ! objective ( ,fig4 #ea4 ,illu/ration3 ,,fig4 #ea4--,objectives ) ,equiconvex ,cr[n4 ,probably n9e t5?s ( all objectives are ( ?is g5]al =m1 equiconvex cr[n & ne>ly or q plano-concave fl9t4 ,! inside radii may be ! same1 9 :i* case ! l5ses %\ld be cem5t$1 or !y may diff] slightly 9 ei!r direction z a1 ,fig4 #ea ) ! front ( ! fl9t less curv$ ?an ! re> ( ! cr[n1 & b :]e ! fl9t has ! %>p] curve4 ,! result+ l5s if ord9>y glasses are *os5 gives excell5t correction ( ! sph]ical ab]ration on ! axis1 b n mu* away f x1 yield+ a r n>r[ %>p field4 ,only a few exceptional comb9ations ( glasses relieve ?is situation mat]ially4 ,! id5tity ( ! inn] radii s t ! surfaces c be cem5t$ is kn[n hi/orically z ,clairault's condition1 & s9ce x fixes two curvatures at id5tity some:at limits ! *oice ( glasses1 :ile to get prop] corrections dem&s q wide v>iations 9 ! contact radii = comp>atively small v>iations 9 ! optical con/ants ( ! glass4 ,:5 two adjac5t curves are id5tical !y %\ld be cem5t$1 o!rwise rays reflect$ f say ! ?ird surface ( ,fig4 #ea w be reflect$ aga9 f ! second surface1 & pass+ ?r\gh ! re> l5s 9 almo/ ! pa? ( ! orig9al ray w come to ne>ly ! same focus1 produc+ a tr\blesome 8gho/40 ,h5ce ! curvatures ( ! second & ?ird surfaces :5 n cem5t$ are v>i$ "o way or ! o!r by two or ?ree p] c5t1 en\gh to ?r[ ! twice reflect$ rays f> out ( focus4 ,9 ?is case1 z 9 mo/ o!rs1 ! analytical expression = ! fundam5tal curvature to be det]m9$ turns up 9 ! =m ( a quadratic equation1 s t ! result takes ! =m a b & !re are two sets ( radii t meet ! requirem5ts4 ,( ^! ! "o pres5t+ ! g5tl] curves is ord9>ily *os54 ,fig4 #eb a & c %[s ! two cem5t$ =ms1 ?us relat$1 = a common pair ( cr[n & fl9t glasses1 bo? cleanly correct$ = *romatic & axial sph]ical ab]ration4 ,ne>ly a c5tury ago ,sir ,john ,h]s*el propos$ ano!r def9+ condition1 t ! sph]ical ab]ration %\ld be remov$ bo? = p>allel incid5t rays & = ^? proce$+ f a ne>] po9t on ! axis1 say t5 or m times ! focal l5g? 9 front ( ! objective4 ,?is condition _h little practical value 9 itself1 & its *ief m]it 0 t x approximat$ "o t became ( real importance if ! second po9t 7 tak5 f> en\gh away4 ,illu/ration3 ,,fig4 #eb4--;,alli$ ,=ms ( ,cem5t$ ,objectives4 ,a little lat] ,gauss sugge/$ t ! sph]ical ab]ration %\ld be annull$ = two diff]5t colors1 mu* z ! *romatic ab]ration is treat$4 ,&1 be+ a ma!matical wiz>d1 he succe$$ 9 work+ out ! v intricate !ory1 :i* result$ 9 an objective approximately ( ! =m %[n 9 ,fig4 #ec4 ,x does n give a wide field b is valuable = spectroscopic "w1 :]e ke5 def9ition 9 all colors is ess5tial4 ,tr\blesome to compute1 & difficult to m\nt & c5t]1 ! type has n be5 mu* us$1 ?\gh !re are f9e examples ( ab\t #i in*es ap]ture at ,pr9ceton1 ,utre*t1 & ,cop5hag51 & a few small] ones else:]e1 *iefly = spectroscopic use4 ,x 0 ,fraunh(] :o f\nd & appli$ ! det]m9+ condition ( ! highe/ practical value = mo/ purposes4 ,?is condition 0 abs5ce ( coma1 ! comet %ap$ blur g5]ally se5 9 ! out] portions ( a wide field4 ,x is due to ! fact t p>allel oblique rays pass+ ?r\gh ! opposite rims ( ! l5s & ?r\gh po9ts ne> its c5t] d n commonly come to ! same focus1 & x practically is ak9 to a sph]ical ab]ration = oblique rays :i* greatly r$uces ! ext5t ( ! %>p field4 ,x is reckon$ :5 ! blur po9ts outw>ds1-:5 x po9ts inw>ds1 & is directly proportional to ! tang5t ( ! obliquity & ! squ>e ( ! ap]ture1 & inv]sely to ! squ>e ( ! focal l5g? i4e4 x v>ies ) atan7u7_/f4 ,illu/ration3 ,,fig4 #ec4--;,gaussian ,objective4 ,j h[ ,fraunh(] solv$ ! problem is q unkn[n1 b solve x he did1 & v completely1 z he indicates 9 "o ( 8 lat] pap]s 9 :i* he speaks ( 8 objective z r$uc+ all ! ab]rations to a m9imum1 & z ,seidel prov$ #cj ye>s lat] 9 ! analysis ( "o ( ,fraunh(]'s objectives4 ,v probably he work$ by trac+ axial & oblique rays ?r\gh ! objective =m by trigonometrical computation1 ?us f9d+ 8 way to a /&>d =m = ! glasses he us$4#aj #aj ,m rec5tly 8 condition proves to be q ! exact equival5t ( ,abb's s9e condition :i* /ates t ! s9e ( ! angle made ) ! optical axis by a ray ent]+ ! objective f a giv5 axial po9t %all be> a uni=m ratio to ! s9e ( ! correspond+ angle ( em]g5ce1 :atev] ! po9t ( incid5ce4 ,= p>allel rays along ! axis ?is r$uces to ! requirem5t t ! s9es ( ! angles ( em]g5ce %all be proportional to ! respective di/ances ( ! incid5t rays f ! axis4 ,fraunh(]'s objective1 ( :i* ,fig4 #ed;a is an example work$ by mod]n =mul = ! s9e condition1 gives v exact corrections ov] a field ( #b-c :5 ! glasses are suitably *os5 & h5ce is invaluable = any "w requir+ a wide angle ( view4 ,) c]ta9 comb9ations ( glasses ! coma-free condition may be comb9$ successfully ) ,clairault's1 al?\gh ord9>ily ! coma-free =m falls betwe5 ! two =ms cle> ( sph]ical ab]ration1 z 9 ,fig4 #eb1 b1 :i* has its oblique rays well comp5sat$ b reta9s s]i\s axial faults4 ,illu/ration3 ,,fig4 #ed4--,! ,fraunh(] ,types4 ,fraunh(]'s objective has = all advantage\s comb9ations ( glasses ! front radius ( ! fl9t long] ?an ! re> radius ( ! cr[n h5ce ! two mu/ be sep>at$ by spac]s at ! edge1 :i* 9 small l5ses 9 simple cells is slightly inconv5i5t4 ,h[ev]1 ! common attempt to simplify m\nt+ by mak+ ! front fl9t radius ! %ort] almo/ inv>iably violates ! s9e condition & r$uces ! %>p field1 =tunately n a v s]i\s matt] = mo/ a/ronomical "w4 ,! only mat]ial objection to ! ,fraunh(] type is ! /rong curvature ( ! re> radius ( ! cr[n :i* gives a =m some:at susceptible to flexure 9 l>ge objectives4 ,?is is met 9 ! fl9t-ahead =m1 develop$ ess5tially by ,/e9heil1 & us$ 9 mo/ ( ! objectives ( 8 fam\s firm4 ,fig4 #ed;b %[s ! fl9t-ahead objective correspond+ to ,fig4 #ed;a4 ,obvi\sly its curves are me*anically r resi/ant to flexure4#aa #aa ,x is int]e/+ to note t 9 comput+ ,fig4 #ed;a = ! s9e condition1 ! o!r root ( ! quadratic gave r\ghly ! ,gaussian =m ( ,fig4 #ec4 ,illu/ration3 ,,fig4 #ee4--;,cl>k ,objective4 ,me*anical consid]ations are n unimportant 9 l>ge objectives1 & ,fig4 #ee1 a highly useful =m introduc$ by ! ,cl>ks & us$ 9 rec5t ye>s = all _! big l5ses1 is a case 9 po9t4 ,"h !re is an int]val ( ab\t ! proportion %[n betwe5 ! cr[n & fl9t compon5ts4 ,?is secures effective v5tilation all[+ ! l5ses to come quickly to _! /eady temp]ature1 & enables ! inn] surfaces to be clean$ readily & fre$ ( moi/ure4 ,optically x less5s ! deviation f ! s9e condition o!rwise practically insep>able f ! equiconvex cr[n1 r$uces ! *romatic diff]5ce ( sph]ical ab]ration1 & gives an easy way ( controll+ ! color correction by slightly v>y+ ! sep>ation ( ! l5ses4 ,"o fur!r special case is wor? not+1 t ( annull+ ! sph]ical ab]ration = rays pass+ ?r\gh ! l5s 9 bo? directions4 ,by prop] *oice ( glass & curvatures ?is c be accompli%$ to a close approximation & ! result+ =m is %[n 9 ,fig4 #ef4 ,! front ( ! cr[n is notably flat & ! re> ( ! fl9t conspicu\sly curv$1 ! %ape 9 fact be+ int]m$iate betwe5 ,figs4 #eb;b & #eb;c4 ,! type is useful 9 read+ telescopes & ! l1 & = "s spectroscopic applications4 ,illu/ration3 ,,fig4 #ef4--;,correct$ 9 ,bo? ,directions4 ,!re are two well kn[n =ms ( ab]ration n yet consid]$2 a/igmatism & curvature ( field4 ,! =m] is due to ! fact t :5 ! pa? ( ! rays is away f ! axis1 z f an ext5d$ object1 ^? com+ f a l9e radial to ! axis1 & ^? f a l9e tang5t to a circle ab\t ! axis1 d n come to ! same focus4 ,! net result is t ! axial & tang5tial elem5ts are br\ght to focus 9 two coaxial surfaces t\*+ at ! axis & dep>t+ m & m widely f ea* o!r z !y dep>t f x4 ,bo? surfaces h consid]able curvature1 t = tang5tial l9es be+ ! %>p]4 ,x is possible by suitable *oice ( glasses & _! curvatures to br+ bo? image surfaces toge!r into an approximate plane = a mod]ate angul> space ab\t ! axis )\t s]i\sly damag+ ! corrections = *romatic & sph]ical ab]ration4 ,to d ?is g5]ally requires at lea/ ?ree l5ses1 & photographic objectives ?us design$ 7ana/igmats7 may give a sub/antially flat field ov] a total angle ( #ej to #fj ) corrections p]fect f ! ord9>y photographic /&po9t4 ,if "o dem&s ! rigor\s precision ( corrections call$ = 9 a/ronomical "w1 ! possible angle is v mu* r$uc$4 ,few a/rographic l5ses cov] m ?an a #aj or #ae field1 & ! wid] ! relative ap]ture ! h>d] x is to get an ana/igmatically flat field free ( mat]ial errors4 ,a/igmatism is r>ely noticeable 9 ord9>y telescopes1 b is sometimes conspicu\s 9 eyepieces4 ,curvature ( field results f ! t5d5cy ( oblique rays 9 objectives1 o!rwise well correct$1 to come to %ort] focus ?an axial rays1 f _! m consid]able refraction result+ f greatly increas$ angles ( incid5ce4 ,?is applies to bo? ! a/igmatic image surfaces1 :i* are concave t[>d ! objective 9 all ord9>y cases4 ,=tunately bo? ^! faults are negligible ne> ! axis4 ,!y are bo? proportional to tanu_/f :]e u is ! obliquity to ! axis & f ! focal l5g?2 turn up ) s]i\s effect 9 wide angl$ l5ses su* z are us$ 9 photography1 b may g5]ally be =gott5 9 telescopes ( ! ord9>y ,f ratios1 l ,f_/#ab to ,f_/#af4 ,s also "o may commonly =get a gr\p ( residual ab]rations ( high] ord]s1 b bel[ ab\t ,f_/#h look out = tr\ble4 ,objectives ( wid] ap]ture require a v c>eful *oice ( special glasses or ! sub-division ( ! curvatures by ! use ( ?ree or m l5ses in/ead ( two4 ,fig4 #eg %[s a cem5t$ triplet ( ,/e9heil's design1 ) a cr[n l5s betwe5 two fl9ts4 ,su* triplets are made up to ab\t #d in*es diamet] & ( ap]tures rang+ f ,f_/#d to ,f_/#e4 ,illu/ration3 ,,fig4 #eg4--,/e9heil ,triple ,objective4 ,illu/ration3 ,,fig4 #eh4--,tolles ,quadruple ,objective4 ,9 cases ( dem& = extreme relative ap]ture1 objectives compos$ ( f\r cem5t$ elem5ts h n[ & !n be5 produc$4 ,an example is %[n 9 ,fig4 #eh1 a f\r-"p objective ( #a in* ap]ture made by ,tolles ye>s ago = a small h& telescope4 ,its p]=mance1 al?\gh x work$ at ,f_/#d1 0 report$ to be excell5t ev5 up to #ge diamet]s4 ,! ma9 difficulty ) ^! objectives ( high ap]ture is ! relatively great curvature ( field due to %ort focal l5g? :i* prev5ts full utilization ( ! improv$ corrections (f ! axis4 ,di/ortion is simil>ly due to ! fact t magnification is n q ! same = rays pass+ at diff]5t di/ances f ! axis4 ,x v>ies 9 g5]al ) ! cube ( ! di/ance f ! axis1 & is usually negligible save 9 photographic telescopes1 ord9>y visual fields be+ too small to %[ x conspicu\sly4 ,di/ortion is mo/ readily avoid$ by adopt+ ! =m ( a symmetrical d\blet ( at lea/ f\r l5ses z 9 common photographic use4 ,no simple a*romatic pair gives a field :olly free ( di/ortion & also ( ! ord9>y ab]rations1 except v ne> ! axis1 & 9 measur+ plates tak5 ) su* simple objectives corrections = di/ortion are g5]ally requir$4 ,at times x becomes necess>y to dep>t some:at f ! objective =m :i* !oretically gives ! lea/ ab]rations 9 ord] to meet "s specific requirem5t4 ,luckily "o may modify ! ratios ( ! curves v p]ceptibly )\t s]i\s results4 ,! ab]rations produc$ come on gradually & n by jumps4 ,illu/ration3 ,,fig4 #ei4--8;,b5t0 ,objective4 ,a case 9 po9t is t ( ! s-call$ 8b5t0 objective 9 :i* ! curvatures are all *ang$ symmetrically1 z if "o _h put 8 f+]s on ! p]iph]y & 8 ?umbs on ! c5tre ( ! :ole affair1 & _h sprung x noticeably "o way or ! o!r4 ,! corrections 9 g5]al are slightly det]iorat$ b ! field may be 9 effect mat]ially flatt5$ & improv$4 ,an extreme case is ! photographic l&scape l5s4 ,figure #ei is an actual example f a telescope :]e l[ p[] & v l>ge angul> view 7 requir$4 ,! objective 0 fir/ design$ f c>efully *os5 glass to meet accurately ! s9e condition4 ,ev5 s ! field1 :i* cov]$ an app>5t angle ( fully #dj1 fell (f s]i\sly at ! edge4 ,be>+ 9 m9d ! re/ ( ! sy/em1 ! objective 0 !n 8b5t0 into ! =m giv5 by ! dott$ l9es1 & ! telescope !n %[$ beautiful def9ition cle> to ! p]iph]y ( ! field1 )\t any visible loss 9 ! c5tre4 ,?is spuri\s flatt5+ _c be pu%$ f> )\t gett+ into tr\ble = x does n cure ! a/igmatic diff]5ce ( focus1 b x is sometimes v useful4 ,practically curvature ( field is an out/&+ error t _c be rem$i$ 9 objectives requir$ to /& high magnify+ p[]s1 except by go+ to ! ana/igmatic =ms simil> to ^? us$ 9 photography4#ab #ab ,! curvature ( ! image is ! ?+ :i* sets a limit to %ort5+ ! relative focus1 z already not$1 = ! a/igmatic image surfaces z we h se51 fall rapidly ap>t away f ! axis1 & bo? curvatures are consid]able4 ,! tang5tial is ! great]1 correspond+ r\ghly to a radius notably less ?an ! focal l5g?1 :ile ! radial fits a radius ( less ?an ?is l5g? ) all ord9>y glasses1 giv5 =ms correct+ ! ord9>y ab]rations4 ,! curves are concave t[>ds ! objective except 9 8ana/igmats0 & "s objectives hav+ bad ab]rations o!rwise4 ,_! approximate curvatures assum+ a semiangul> ap]ture = an a*romatic objective n ov] say #e1 h be5 %[n to be1 to focus unity r #a 7#a_/7-77_/n - _/n771 & t #c #a_/7-77_/n - _/n7 r & t be+ ! respective reciprocals ( ! radii4 ,! surfaces are really some:at egg %ap$ r ?an sph]ical z "o dep>ts f ! axis4 ,aside f curvature ! *ief residual error 9 objectives is imp]fection ( a*romatism4 ,?is arises f ! fact t cr[n & fl9t glasses d n disp]se ! v>i\s colors q 9 ! same ratio4 ,! cr[n gives slightly disproportionate importance to ! r$ end ( ! spectrum1 ! fl9t to ! violet end--! s-call$ 8irrationality ( disp]sion40 ,h5ce if a pair ( l5ses mat* up accurately = two *os5 colors l ^? repres5t$ by ! ,c & ,f l9es1 !y w fail ( mutual comp5sation else:]e4 ,figure #fj %[s ! situation4 ,"h ! spectra f cr[n & fl9t glasses are br\ght to exactly ! same ext5t betwe5 ! ,c & ,f l9es1 :i* s]ve z l&m>ks4 ,cle>ly if two prisms or l5ses are ?us adju/$ to ! same refractions = ,c & ,f1 ! light pass+ ?r\gh ! comb9ation w /ill be slightly color$ 9 virtue ( ! diff]5ces else:]e 9 ! spectrum4 ,^! residual color diff]5ces produce :at is kn[n z ! 8second>y spectrum40 ,:at ?is does 9 ! case ( an a*romatic l5s may be cle>ly se5 f ! figure2 ,c & ,f hav+ exactly ! same refractions 9 ! fl9t & cr[n1 come to ! same focus4 ,= ,d1 ! yell[ l9e ( sodium1 ! fl9t l5s refracts a %ade ! less1 h5ce is n q p[]ful en\gh to balance ! cr[n1 :i* !re=e br+s ,d to a focus a little %ort] ?an ,c & ,f4 ,on ! o!r h& = ,a & ,g1 ! fl9t refracts a bit m ?an ! cr[n1 ov]balances x & br+s ^! r$ & violet rays to a focus a little long] ?an ! jo9t ,c & ,f focus4 ,illu/ration3 ,,fig4 #fj4--;,irrationality ( ,disp]sion4 ,! diff]5ce = ,d is q small1 r\ghly ab\t #a_/bjjj ( ! focal l5g?1 :ile ! r$ runs long by ne>ly ?ree times t am\nt1 ! violet by ab\t f\r4 ,t[>ds ! ,h l9e ! diff]5ce increases rapidly & 9 l>ge telescopes ! actual range ( focus = ! v>i\s colors am\nts to sev]al in*es4 ,?is difficulty _c be avoid$ by any *oice among ord9>y pairs ( glasses1 :i* are ne>ly alike 9 ! matt] ( second>y spectrum4 ,9 ! latt] "p ( ! la/ c5tury det]m9$ ef=ts 7 made to produce glasses t w\ld give m ne>ly an equal run ( disp]sion1 at fir/ by ,engli% exp]im5t]s1 & !n ) f9al success by ,s*ott & ,abb at ,j5a4 ,bo? cr[n & fl9t _h to be q abnormal 9 composition1 especially ! latt]1 & ! pair 7 ( v ne>ly ! same refractive index & ) small diff]5ce 9 ! quantity :i* we h se5 det]m9es ! g5]al am\nt ( curvature4 ,moreov] x prov$ to be extremely h>d to get ! cr[n q homog5e\s & x is li/$ by ,s*ott ) ! res]vation t x is n free f bubbles & /ri4 ,nev]!less ! new glasses r$uce ! second>y spectrum greatly1 to ab\t ( its ord9>y value1 9 ! av]age4 ,x is difficult to get rid ( ! sph]ical ab]ration1 h[ev]1 f ! %>p curves requir$ & ! small diff]5ce betwe5 ! glasses1 & x seems to be impracticable on ?is acc\nt to g to great] ap]ture ?an ab\t ,f_/#bj4 ,figure #fa %[s ! deeply curv$ =m necess>y ev5 at half ! relative ap]ture usable ) common glasses4 ,at ,f_/#bj ! second>y spectrum f ! latt] is n conspicu\s & ,roe 7,pop4 ,a/4 #ah1 #aic71 te/+ side by side a small ,/e9heil ( ! new glasses1 & a ,cl>k ( ! old1 ( almo/ id5tical size & focal ratio1 f\nd no diff]5ce 9 _! practical p]=mance4 ,ano!r attack on ! same problem 0 m successfully made by ,h4 ,d4 ,taylor4 ,realiz+ ! difficulty f\nd ) a d\blet objective ( ev5 ! be/ mat*$ ( ! new glasses1 he adopt$ ! plan ( gett+ a fl9t ( exactly ! "r disp]sion by av]ag+ ! disp]sions ( a prop]ly select$ pair ( fl9ts =m$ into l5ses ( ! appropriate relative curvatures4 ,illu/ration3 ,,fig4 #fa4--;,apo*romatic ,d\blet4 ,illu/ration3 ,,fig4 #fb4--;,apo*romatic ,triplet4 ,! result+ =m ( objective is made1 especially1 by ,cooke ( ,york1 & also by ,cont95tal mak]s1 & c>ries ! "n ( 8photo-visual0 s9ce ! exactness ( corrections is c>ri$ well into ! violet1 s t "o c see & photograph at ! same focus4 ,! residual *romatic error is v small1 n above #a_/h to #a_/aj ! ord9>y second>y spectrum4 ,by ?is con/ruction x is practicable to increase ! ap]ture to ,f_/#ab or ,f_/#aj :ile /ill reta9+ mod]ate curvatures & r$uc+ ! residual sph]ical ab]ration4 ,!re are a r\nd doz5 triplet =ms possible1 ( :i* ! be/1 adopt$ by ,taylor1 is %[n 9 ,fig4 #fb4 ,x has ! duplex fl9t ahead--fir/ a b>yta light fl9t1 !n a borosilicate fl9t1 & to ! re> a special light cr[n4 ,! two latt] glasses h be5 "u "s suspicion z to p]man5ce1 b ! difficulty has ( late ye>s be5 report$ z rem$i$4 ,be t z x may1 nei!r d\blets nor triplets ) r$uc$ second>y spectrum h come into any l>ge use = a/ronomical purposes4 ,_! increas$ co/ is consid]able1#ac _! ap]ture ev5 9 ! triplet1 r small = a/rophotography1 & _! a*romatism is /ill lack+ ! p]fection rea*$ by a mirror4 #ac ,! d\blet co/s ab\t "o & a half times1 & ! triplet m ?an twice ! price ( an ord9>y a*romatic ( ! same ap]ture4 ,! matt] ( a*romatism is fur!r complicat$ by ! fact t objectives are usually ov]-a*romatiz$ to comp5sate = ! *romatic errors 9 ! eyepiece1 & especially 9 ! eye4 ,z a g5]al rule an out/&+ error 9 any "p ( an optical sy/em c be m or less p]fectly balanc$ by an opposite error any:]e else 9 ! sy/em--! p>ticul> po9t *os5 be+ a matt] ( conv5i5ce ) respect to o!r corrections4 ,! eye be+ q uncorrect$ = color ! image produc$ ev5 by a reflector is likely to %[ a color$ fr+e if at all bright1 ! m conspicu\s z ! relative ap]ture ( ! pupil increases4 ,= l[ p[] eyepieces ! em]g+ ray may q fill a wide pupil & !n ! *romatic error is tr\blesome4 ,h5ce x has be5 ! cu/om ( skill$ opticians1 f ! "t ( ,fraunh(]1 :o probably />t$ ! practice1 to ov]do ! correction ( ! objective j a little to balance ! fault ( ! eye4 ,:at actually happ5s is %[n 9 ,fig4 #fc1 :i* gives ! results ( a*romatization z practic$ by "s ( ! _w's adepts4 ,! %orte/ focus is 9 ! yell[ gre51 n f> f ! l9e ,d4 ,! longe/ is 9 ! violet1 & ,f1 in/ead ( co9cid+ 9 focus ) ,c z x is conv5tionally suppos$ to d1 actually co9cides ) ! deep & fa9t r$ ne> ! l9e m>k$ ,b4 ,h5ce ! visible effect is to l5g!n ! focus = blue en\gh to make up = ! t5d5cy ( ! eye 9 ! o!r direction4 ,! result+ image !n %\ld %[ no conspicu\s rim ( r$ or blue4 ,! actual adju/m5t ( ! color correction is almo/ :olly a matt] ( skill$ judgm5t b ,fig4 #fc %[s t ( ! great mak]s to be q uni=m4 ,! smalle/ ov]correction is f\nd 9 ! ,grubb objective1 ! l>ge/ 9 ! ,fraunh(]4 ,! diff]5ces seem to be due ma9ly to individual v>iations ( op9ion z to :at diamet] ( pupil %\ld be tak5 z typical = ! eye4 ,! common practice is to get ! be/ possible adju/m5t = a fairly high p[]1 correspond+ to a beam h>dly #a_/fd in* 9 diamet] ent]+ ! pupil4 ,9 any case ! bigg] ! p5cil ( rays utiliz$ by ! eye1 i4e41 ! l[] ! p[]1 ! m ov]correction mu/ be provid$1 s t telescopes int5d$1 l comet seek]s1 = regul> use ) l[ p[]s mu/ be design$ accord+ly1 ei!r z respects objective or ocul>4 ,illu/ration3 ,,fig4 #fc4--;,a*romatization ,curves by ,v>i\s ,mak]s4 #a4 ,fraunh(] #b4 ,cl>k #c4 ,/e9heil #d4 ,ha/+s-,bra%e> #e4 ,grubb ,! diff]5ces conc]n$ 9 ?is *romatic correction = p[] are by no means negligible 9 obs]v+1 & an objective actually con=m+ to ! ,c to ,f correction assum$ 9 tables ( optical glass w\ld produce a decid$ly unpleasant impression :5 us$ ) v>i\s p[]s on bright objects4 ,& ! values = impli$ 9 ! actual color correction are n immat]ial 9 comput+ ! be/ =m = a propos$ objective4 #a is f ,franunh(]'s own h&s1 ! in/rum5t ( #i4f in*es ap]ture & #agj in*es focus 9 ! ,b]l9 ,obs]vatory4 #b ,! ,cl>k refractor ( ! ,l[ell ,obs]vatory1 #bd in*es ap]ture & #chf in*es focal l5g?4 ,?is is ( ! usual ,cl>k =m1 cr[n ahead1 ) l5ses sep>at$ by ab\t ( _! diamet]4 #c is a ,/e9heil refractor at ,potsdam ( #e4c in*es ap]ture1 & #he in*es focus4 #d is f ! f9e equatorial at ,johns ,hopk9s ,univ]sity1 design$ by ,pr(essor ,ha/+s & execut$ by ,bra%e>4 ,! objective 0 design$ ) special ref]5ce to m9imiz+ ! sph]ical ab]ration n only = "o *os5 wave l5g? b = all o!rs1 has ! fl9t l5s ahead1 ap]ture #i4d in*es1 focal l5g? #adb in*es1 & ! l5ses sep>at$ by in* 9 ! f9al adju/m5t ( ! corrections4 #e is f ! ,potsdam equatorial by ,grubb1 #h4e in*es ap]ture #abd in*es focus4 ,! great simil>ity ( ! color curves is evid5t at a glance1 ! diff]5ces due to v>iations 9 ! glass be+ on ! :ole mu* less significant ?an ^? result+ f ! adju/m5t = p[]4 ,really v little c be done to ! color correction )\t go+ to ! new special glasses1 ! use ( :i* involves o!r difficulties1 & leaves ! matt] ( adju/m5t = p[] q 9 ! air1 to be br\ght d[n by special eye pieces4 ,n[ & !n a melt+ ( glass has a run ( disp]sion some:at m favorable ?an usual1 b !re is small *ance ( gett+ l>ge discs ( special *>act]i/ics1 & ! mak] has to take 8 *ance1 m9ute diff]5ces 9 *romatic quality be+ f> less important ?an uni=mity & good anneal+4 ,reg>d+ ! ab]rations ( mirrors some?+ has be5 said 9 ,*ap4 ,i1 b x may be well "h to %[ ! practical side ( ! matt] by a few simple illu/rations4 ,figure #fd %[s ! simple/ =m ( concave mirror--a sph]ical surface1 9 ?is in/ance ( #ij ap]ture1 ! bett] to %[ its prop]ties4 ,if light proce$$ radially outw>d f ,c1 ! c5t] ( curvature ( ! surface1 evid5tly any ray w\ld /rike ! surface p]p5dicul>ly z at a & w\ld be turn$ squ>ely back ^u itself1 pass+ aga9 ?r\gh ! c5t] ( curvature z indicat$ 9 ! figure4 ,a ray1 h[ev]1 proce$+ p>allel to ! axis & /rik+ ! surface z at bb w be deflect$ by twice ! angle ( incid5ce z is ! case ) all reflect$ rays4 ,b ?is angle is measur$ by ! radius ,cb f ! c5t] ( curvature & ! reflect$ ray makes an angle ,cb,f ) ! radius1 equal to ,f,cb4 ,= po9ts v ne> ! axis b,f1 !re=e1 equals ,,fc1 & sub/antially also equals c,f4 ,?us rays ne> ! axis & p>allel to x meet at ,f ! focus half1 way f c to ,c4 ,! equival5t focal l5g? ( a sph]ical concave mirror ( small ap]ture is !re=e half its radius ( curvature4 ,illu/ration3 ,,fig4 #fd4--,reflection f ,concave ,sph]ical ,mirror4 ,b obvi\sly = l>ge angles ( incid5ce ^! conv5i5t equalities d n hold4 ,z ! upp] half ( ! figure %[s1 ! ray p>allel to ! axis & incid5t on ! mirror #de away at e is turn$ /raight d[n1 = x falls ^u a surface incl9$ to x by #de & is !re=e deflect$ by #ij1 cutt+ ! axis f> inside ! nom9al focus1 at d4 ,foll[+ up o!r rays ne>] ! axis x appe>s t !re is no long] a focal po9t b a cusp-l focal surface1 kn[n to geometrical optics z a cau/ic & p]mitt+ no well def9$ image4 ,a p>aboloidal reflect+ surface z 9 ,fig4 #fe has ! prop]ty ( br++ to a s+le po9t focus all rays p>allel to its axis :ile q fail+ ( unit+ rays proce$+ f any po9t on its axis1 s9ce its curvature is *ang+ all ! way out f v]tex to p]iph]y4 ,"h ! p>allel rays a1 a1 a1 a meet+ ! surface are reflect$ to ! focus ,f1 :ile 9 a p]fectly symmetrical way ! prolongation ( ^! rays a1 a1 a1 a if incid5t on ! convex surface ( ! p>aboloid w\ld be reflect$ 9 ,r1 ,r1 ,r ,r j z if !y proce$$ f ! same focus ,f4 ,! diff]5ce betwe5 ! sph]ical & p>abolic curves is well %[n 9 ,fig4 #ff4 ,"h are sections ( ! =m]1 & 9 dott$ l9es ( ! latt]4 ,! diff]5ce po9ts ! moral4 ,! p>abola falls away t[>d ! p]iph]y & h5ce pu%es outw>d ! m>g9al rays4 ,b x is ( relatively %>p] curvature ne> ! c5t] & pulls 9 ! c5tral to meet ! m>g9al portion4 ,9 ! actual con/ruction ( p>abolic mirrors "o always />ts ) a sph]e :i* is easy to te/ = precision ( figure at its c5t] ( curvature4 ,!n ! surface may be modifi$ into a p>aboloid less5+ ! curvature t[>ds ! p]iph]y1 or by increas+ ! curvature t[>d ! c5t] />t+ 9 ?is case ) a sph]e ( a bit long] radius z 9 ,fig4 #ff;a4 ,illu/ration3 ,,fig4 #fe4--,reflection f ,p>aboloid4 ,illu/ration3 ,,fig4 #ff;a4 ,,fig4 #ff;b4 ,v>iation ( ,p>aboloid f ,sph]e4 ,practice diff]s 9 ?is respect1 ei!r process lead+ to ! same result4 ,9 any case ! dep>ture f ! sph]ical curve is v slight--a few hundr$ ?\s&?s or at mo/ t5 ?\s&?s ( an in* dep5d+ on ! size & relative focus ( ! mirror4 ,yet ?is small v>iation makes all ! diff]5ce betwe5 admirable & hopelessly bad def9ition4 ,h[ev] ! "w is done x is guid$ by frequ5t te/+1 until ! p]=mance %[s t a truly p>abolic figure has be5 rea*$4 ,its atta9m5t is a matt] ( skill$ judgm5t & exp]i5ce4 ,! weak po9t ( ! p>abolic mirror is 9 deal+ ) rays com+ 9 p>allel b oblique to ! axis4 ,figure #fg %[s ! situation pla9ly en\gh4 ,! reflect$ rays a1 a no long] meet 9 a po9t at ! focus ,f b inside ! focus = p>allel rays1 at f =m+ a surface ( ab]ration4 ,! practical effect is t ! image rapidly det]iorates z ! /> passes away f ! axis1 tak+ on an oval "* t sugge/s a bad case ( a/igmatism ) s]i\s complications f coma1 :i* 9 fact is sub/antially ! case4 ,illu/ration3 ,,fig4 #fg4--;,ab]ration ( ,p>abolic ,mirror4 ,ev5 :5 ! angul> ap]ture is v small ! focal surface is nev]!less a sph]e ( radius equal to "o half ! focal l5g?1 & ! ab]rations (f ! axis increase approximately z ! squ>e ( ! relative ap]ture1 & directly z ! angul> di/ance f ! axis4 ,! ev5 tol]ably %>p field ( ! mirror is !re=e g5]ally small1 r>ely ov] #cj ( arc z mirrors are cu/om>ily proportion$4 ,at ! relative ap]ture usual ) refractors1 say ,f_/#ae1 ! %>p fields ( ! two are q comp>able 9 ext5t4 ,! mo/ effective help = ! usual ab]rations#ad ( ! mirror is ! adoption ( ! ,cassegra9 =m1 by all odds ! mo/ conv5i5t = l>ge in/rum5ts1 ) a hyp]boloid second>y mirror4 #ad ,a v useful treatm5t ( ! ab]rations ( p>abolic mirrors by ,poor is 9 ,ap4 ,j4 #g1 #aad4 ,9 ?is is giv5 a table ( ! maximum dim5sion ( a /> disc (f ! axis 9 reflectors ( v>i\s ap]tures4 ,?is table cond5ses to ! closely approximate =mula a lld_/f :]e a is ! ab]rational diamet] ( ! /> disc1 9 seconds ( arc1 d ! di/ance f ! axis 9 m9utes ( arc1 f ! d5om9ator ( ! ,f ratio 7,f_/#h c47 & #aa1 a con/ant4 ,obvi\sly ! sep>at+ p[] ( a telescope 7see ,*ap4 ,x7 be+ sub/antially #d4ef_/;,d :]e ,d is ! diamet] ( objective or mirror 9 in*es1 ! sep>at+ p[] w be impair$ :5 a #d4ef_/;,d4 ,9 ! photographic case ! critical quantity is n #d4ef_/;,d1 b ! maximum image diamet] tol]able = ! purpose 9 h&4 mirror is ! adoption ( ! ,cassegra9 =m1 by all odds ! mo/ conv5i5t = l>ge in/rum5ts1 ) a hyp]boloidal second>y mirror4 ,! hyp]boloid is a curve ( v int]e/+ optical prop]ties4 ,j z a sph]ical mirror returns aga9 rays proce$+ f its c5t] ( curvature )\t ab]ration1 & ! p>aboloid s5ds f its focus a p>allel axial beam free ( ab]ration1 or returns su* a beam to an exact focus aga91 s a hyp]boloid1 ,fig4 #fh1 s5ds out a div]g5t beam free f ab]ration or br+s x1 return+1 to an exact focus4 ,su* a beam a1 a1 a1 9 fact behaves z if x came f & return$ to a virtual conjugate focus ,f on ! o!r side ( ! hyp]bolic surface4 ,& if ! convex side be reflect+1 conv]g+ rays ,r1 ,r1 ,r1 fall+ ^u x at ,p1 ,p1 ,p1 z if head$ = ! virtual focus ,f1 w actually be reflect$ to ,f1 n[ a real focus4 ,?is surface be+ convex its ab]rations (f ! axis are ( opposite sign to ^? due to a concave surface1 & c 9 "p at lea/1 be made to comp5sate ! ab]rations ( a p>abolic ma9 mirror4 ,! rationale ( ! op]ation appe>s f comp>ison ( ,figs4 #fg & #fh4 ,illu/ration3 ,,fig4 #fh4--,reflection f ,hyp]boloid4 ,9 ! =m] ! oblique rays a1 a are pit*$ too %>ply d[n4 ,:5 reflect$ f ! convex surface ( ,fig4 #fh z a conv]g+ beam along ,r1 ,r1 ,r1 !y c nev]!less1 if ! hyp]bola be prop]ly proportion$1 be br\ght d[n to focus at ,f conjugate to ,f1 _! approximate mutual po9t ( conv]g5ce4 ,evid5tly1 h[ev]1 ?is comp5sation _c be complete ov] a wide angle1 :5 ,f spreads into a surface1 & ! net result is t :ile ! total ab]rations are mat]ially r$uc$ !re is "s residual coma toge!r ) "s increase ( curvature ( field1 & di/ortion4 ,"h j z 9 ! p>aboliz+ ( ! l>ge speculum ! con/ruction is sub/antially empirical1 guid$ 9 ! case ( a skill$ op]ator by a sort ( insight d]iv$ f exp]i5ce4 ,/>t+ f a sub/antially sph]ical convexity ( v ne>ly ! requir$ curvature ! figure is gradually modifi$ z 9 ! e>li] example until te/ ) ! truly p>abolic mirror %[s a flawless image = ! comb9ation4 ,! tru? is t no conic surface ( revolution save ! sph]e c be gr\nd to true figure by any rigor\s geometrical me?od4 ,! result mu/ dep5d on ! skill ) :i* "o by ma*9e or h& c gauge m9ute dep>tures f ! sph]e4 ,attempts h be5 made by ! late ,pr(essor ,s*w>z*ild & o!rs to improve ! corrections ( reflectors s z to increase ! field b !y dem& ei!r v difficult curvatures impos$ on bo? mirrors1 or ! int]position ( l5ses1 & h ?us f> rea*$ no practical result4 ,,ref]5ces ,,s*w>z*ild3 ,unt]su*ung5 #b1 ,geom41 ,opt4 ,,ii4 ,,sampson ,obs]vatory #cf1 #bdh4 ,,codd+ton3 8,reflexion & ,refraction ( ,light40 ,,h]s*el3 8,light40 ,,taylor3 8,appli$ ,optics40 ,,s\?all3 8,geometrical ,optics40 ,,m>t93 ,ann4 ,sci4 de l',ecole ,normale1 #ahgg1 ,supplem5t4 ,,mos]3 ,zeit4 f4 ,in/rum5t5kunde1 #ahhg4 ,,h>t+3 ,zeit4 f4 ,in/41 #ahii4 ,,h>t+3 ,zeit4 f4 ,in/41 #ahih4 ,,von ,,hoegh3 ,zeit4 f4 ,in/41 #ahii4 ,,/e9heil ,,voit3 8,appli$ ,optics40 ,,collect$ ,,rese>*es1 ,national ,physical ,laboratory1 ,vol4 #ad4 ,,glei*53 8,lehrbu* d4 ,geometris*e ,optik40 ,,note4--,9 deal+ ) optical =mul look out = ! algebraic signs4 ,writ]s v>y 9 _! conv5tions reg>d+ !m & x sometimes is z difficult to le>n h[ to apply a =mula z to d]ive x f ! />t4 ,also1 especially 9 optical pat5ts1 look out = cam\flage1 z omitt+ to specify an optical con/ant1 giv+ examples involv+ glasses n produc$ by any manufactur]1 & ev5 specify+ curves lead+ to absurd prop]ties4 ,x is a good idea to *eck up ! a*romatization & focal l5g? be=e gett+ too tru/ful ( a num]ical design4 ,,*apt] ,v ,,m\nt+s ,a /eady & conv5i5t m\nt+ is j z necess>y to ! successful use ( ! telescope z is a good objective4 ,no satisfactory obs]vations = any purpose c be made ) a telescope un/eadily m\nt$ & n provid$ ) adju/m5ts enabl+ x to be mov$ smoo?ly & easily 9 foll[+ a cele/ial object4 ,broadly1 telescope m\nts may be divid$ into two g5]al classes1 alt-azimu? & equatorial4 ,! =m] class is1 z its "n sugge/s1 arrang$ to be turn$ 9 azimu? ab\t a v]tical axis1 & 9 altitude ab\t a horizontal axis4 ,su* a m\nt+ may be made ( extreme simplicity1 b obvi\sly x requires two motions 9 ord] to foll[ up any object 9 ! field1 = ! app>5t motion ( ! heav5ly bodies is 9 circles ab\t ! cele/ial pole z an axis1 & consequ5tly incl9$ f ! v]tical by ! latitude ( ! place ( obs]vation4 ,po9t+ a telescope ) motions ab\t a v]tical & horizontal axis only1 !re=e means t1 z a /> moves 9 its app>5t pa?1 x w drift away f ! telescope bo? 9 azimu? & 9 altitude1 & require to be foll[$ by a d\ble motion4 ,alt-azimu? m\nts may be divid$ into ?ree g5]al gr\ps accord+ to _! con/ruction4 ,! fir/ & simple/ ( !m is ! pill>-&-claw /& %[n 9 ,figure #fi4 ,?is consi/s ( a v]tical pill> support$ on a /rong tripod1 usually ( brass or iron1 & provid$ at its top ) a long conical be>+ c>ry+ at its upp] extremity a h+$ jo9t1 be>+ a b> to :i* ! telescope is screw$ z %[n 9 ! illu/ration4 ,if prop]ly made ! upp] jo9t comprises a circul> plate c>ry+ ! b> & held betwe5 two *eek pieces ) means = tak+ up we>1 & provid+ j en\gh friction to p]mit ( easy adju/m5t ( ! telescope1 :i* c be swung 9 altitude f ne> ! z5i? to ! horizon or bel[1 & turn$ ar\nd its v]tical axis 9 any direction4 ,:5 well made a /& ( ?is k9d is /eady & smoo? work+1 readily capable ( c>ry+ a telescope up to ab\t #c in*es ap]ture4 ,x ne$s = its prop] use a firm sub-support = ! ?ree /rong h+$ legs ( ! pill>4 ,?is is conv5i5tly made z a v solid /ool ) spread+ legs1 or a plank ( suffici5t size may be firmly bolt$ to a well set po/4 ,illu/ration3 ,,fig4 #fi4--,table ,m\nt ) ,sl[ ,motion4 ,?us arrang$ ! m\nt is a v s]viceable "o = small in/rum5ts4 ,its /ability1 h[ev]1 dep5ds on ! base ^u :i* x is set4 ,! writ] once unwisely attempt$ to ga9 conv5i5ce by remov+ ! legs ( ! /& & screw+ its body firmly ^u a v sub/antial tripod4 ,! result 0 a complete failure 9 /ead9ess1 ow+ to ! r long lev] arm furni%$ by ! height ( ! pill>2 & ! in/rum5t1 :i* _h be5 admirably /eady orig9ally1 vibrat$ abom9ably :5 t\*$ = focuss+4 ,! p>ticul> /& "h %[n is furni%$ ) a rack motion 9 altitude :i* is a consid]able conv5i5ce 9 foll[+4 ,m r>ely adju/able /eady+ rods atta*$ to ! objective end ( ! in/rum5t are br\ght d[n to its base1 b = a telescope l>ge en\gh to require ?is a bett] m\nt is g5]ally desirable4 ,n[ & !n an alt-azimu? head ( j ! sort us$ 9 ! pill>-&-claw /& is actually fitt$ on a tall tripod1 b su* an arrangem5t is usually f\nd only 9 *eap in/rum5ts & = su* tripod m\nt+s o!r fitt+s are desirable4 ,illu/ration3 ,,fig4 #gj4--;,alt-azimu? ,m\nt1 ,cl>k ,type ,t4 ,! second =m ( alt-azimu? m\nt1 is altoge!r ( m sub/antial con/ruction4 ,! v]tical axis1 usually tap]$ & c>efully gr\nd 9 its be>+s1 c>ries an oblique =k 9 :i* ! telescope tube is c>ri$ on trunnions = its v]tical motion4 ,! incl9ation ( ! =k$ head enables ! telescope to be po9t$ directly t[>d ! z5i? & ! :ole m\nt =ms ! head ( a well made tripod4 ,figure #gj %[s an excell5t type ( ?is =m ( m\nt z us$ = ! ,cl>k ,type ,t telescope1 design$ = bo? t]re/rial & a/ronomical obs]vation4 ,9 ?is p>ticul> arrangem5t ! telescope lies 9 an alum9um cradle c>ri$ on ! trunnions1 f :i* x c be readily remov$ by loos5+ ! ?umb screws & op5+ ! cradle4 ,illu/ration3 ,,fig4 #ga4--;,alt-azimu? ) ,full ,sl[ ,motions4 ,x c also be set longitud9ally = balance 9 ! cradle if any atta*m5ts are to be plac$ ^u ei!r end4 ,"h ! adju/m5t = ! height ( ! in/rum5t is provid$ bo? 9 ! spread ( ! tripod & 9 ! adju/able legs4 ,s m\nt$ a telescope ( #c or #d in*es ap]ture is easily h&l$ & capable ( r5d]+ v good s]vice ei!r = t]re/rial or cele/ial "w4 ,inde$ ! ,cl>ks h made in/rum5ts up to #f in*es ap]ture1 m\nt$ = special s]vice 9 ?is simple mann]4 ,= cele/ial use :]e fairly high p[]s may be requir$ ?is or any simil> m\nt c be readily furni%$ ) sl[ motions ei!r 9 azimu? or altitude or bo?4 ,figure #ga %[s a #d in* telescope & m\nt by ,zeiss ?us equipp$4 ,"s alt-azimu? m\nts are also provid$ ) a v]tical rack motion to br+ ! telescope to a conv5i5t height )\t di/urb+ ! tripod4 ,a good alt-azimu? m\nt su* z is %[n 9 ,figs4 #gj & #ga is by no means to be despis$ = use ) telescopes ( #c or #d in* ap]ture4 ,! sole inconv5i5ce to be consid]$ is t ( ! two motions requir$ 9 foll[+4 ,) well fitt$ sl[ motions ?is is n really s]i\s = ord9>y obs]v+ ) mod]ate p[]s1 = "o c "w v com=tably up to p[]s ( #aej or #bjj diamet]s keep+ ! object easily 9 view2 b ) ! high] p[]s 9 :i* ! field is v small1 only a few m9utes ( arc1 ! d\ble motion becomes r a nuisance & x is extremely inconv5i5t ev5 ) l[ p[]s 9 sweep+ = an object ! place ( :i* is n exactly kn[n4 ,!re are 9 fact two di/9ct k9ds ( foll[+ necess>y 9 a/ronomical obs]vations4 ,fir/1 ! m]e keep+ ( ! object some:]e 9 ! field1 & second1 hold+ x some:at rigor\sly 9 position1 z 9 mak+ close obs]vations ( detail or micrometrical measurem5ts4 ,:5 ?is f9] foll[+ is necess>y ! soon] "o gets away f alt-azimu? m\nts ! bett]4 ,/ill ano!r =m ( alt-azimu? m\nt is %[n 9 ,fig4 #gb appli$ = a ,newtonian reflector ( #f or #h in*es ap]ture4 ,"h ! ov]hung =k c>ry+ ! tube on trunnions is support$ on a /\t fix$ tripod1 to :i* x is pivot$ at ! front1 & x is provid$ at ! re> ) a firm be>+ on a sector borne by ! tripod4 ,at ! front a rod ) slid+ co>se1 & screw f9e1 adju/m5t provides ! necess>y motion 9 altitude4 ,! :ole =k is swung ab\t its pivot ov] ! sector be>+ by a cross screw turn$ by a rod ) a univ]sal jo9t4 ,?is m\nt /rongly sugge/s ! orig9al "o ( ,hadley1 ,fig4 #af1 & is mo/ firm & s]viceable4 ,a reflector ?us m\nt$ is rem>kably conv5i5t 9 t ! eyepiece is always 9 a mo/ accessible position1 ! view always horizontal1 & ! adju/m5ts always )9 easy rea* ( ! obs]v]4 ,illu/ration3 ,,fig4 #gb4--;,alt-azimu? ,newtonian ,reflector4 ,:5ev] x is necess>y to foll[ an object closely1 z 9 us+ a micromet] & "s o!r auxili>ies1 ! alt-azimu? m\nt is tr\blesome & "s modification adju/able by a s+le motion1 pref]ably made automatic by clockwork1 becomes necess>y4 ,! fir/ /ep 9 ?is direction is a v simple "o inde$4 ,suppose "o 7 to tilt ! azimu? axis s t x po9t$ to ! cele/ial pole1 ab\t :i* all ! />s appe> to revolve4 ,!n evid5tly ! telescope be+ once po9t$1 a /> c\ld be foll[$ m]ely by turn+ ! tube ab\t ?is tilt$ axis4 ,( c\rse "o c\ld n easily rea* "s objects ne> ! pole )\t1 p]haps1 f\l+ ! m\nt1 b 9 g5]al ! sky is )9 rea* & a s+le motion foll[s ! />1 v easily if ! orig9al m\nt _h a sl[ motion 9 azimu?4 ,?is is 9 fact ! simple/ =m ( equatorial m\nt1 sometimes call$ p>allactic4 ,figure #gc %[s ! pr9ciple appli$ to a small reflector4 ,an oblique block ) its angle adju/$ to ! co-latitude ( ! place drops ! v]tical axis into l9e ) ! pole1 & ! major "p ( ! cele/ial vault is !n )9 easy rea*4 ,x may be reg>d$ z ! transition /ep f ! alt-azimu? to ! true equatorial4 ,x is r>ely us$ = refractors1 & ! fir/ attempt at a real equatorial m\nt 0 9 fact made by ,james ,%ort ,f4 ,r4 ,s4 9 m\nt+ "s ( 8 small ,gregorians4#ae ,z a matt] ( record ?is is %[n1 f ,%ort's own pap] be=e ! ,royal ,society 9 #agdi1 9 ,fig4 #gd4 #ae ,in/rum5ts ) a pol> axis 7 us$ by ,s*e9] z e>ly z #afbg2 by ,roem] ab\t ?ree qu>t]s ( a c5tury lat]1 & previ\sly _h be5 employ$1 us+ sights r ?an telescopes1 by ! ,*9ese2 b ^! 7 f> f be+ equatorials 9 ! mod]n s5se4 ,illu/ration3 ,,fig4 #gc4--,p>allactic ,m\nt = ,reflector4 ,a glance %[s a /& app>5tly mo/ complicat$1 b clos] exam9ation discloses t x is m]ely an equatorial on a table /& ) a sweep 9 decl9ation ov] a v wide arc1 & q complete arrangem5ts = sett+ to ! exact latitude & azimu?4 ,! p>ticul> in/rum5t %[n 0 ( #d in*es ap]ture & ab\t #ah in*es long & 0 "o ( sev]al produc$ by ,%ort at ab\t ?is epo*4 ,illu/ration3 ,,fig4 #gd4--,%ort's ,equatorial ,m\nt4 ,9 ! in/rum5t z %[n !re is fir/ an azimu? circle ,a ,a support$ on a base ,b ,b ,b ,b hav+ levell+ screws 9 ! feet4 ,imm$iately "u ! azimu? circle is m\nt$ a compass ne$le = approximate ori5tation1 & ! circle is adju/able by a tang5t screw ,c4 ,c>ri$ by ! azimu? circle on a be>+ support$ by f\r pill>s is a latitude circle ,d ,d = ! adju/m5t ( ! pol> axis1 ) a sl[ motion screw ,e4 ,! latitude circle c>ries a "r asc5sion circle ,f ,f1 ) a sl[ motion ,g1 & ?is 9 turn c>ries on f\r pill>s ! decl9ation circle ,h ,h1 & its axis adju/able by ! sl[ motion ,k4 ,to ?is decl9ation circle is secur$ ! ,gregorian reflector ,l ,l :i* s]ves z ! obs]v+ telescope4 ,all ! circles are provid$ ) v]ni]s z well z sl[ motions4 ,& ! in/rum5t is1 s to speak1 a univ]sal "o = all ! purposes ( an equatorial1 & :5 ! pol> axis is set v]tical equally adapt$ = use z a transit in/rum5t1 !odolite1 azimu? in/rum5t1 or level1 s9ce ! circles are provid$ ) suitable levels4 ,?is m\nt 0 really a v neat & complete piece ( "w = ! purpose int5d$1 al?\gh sc>cely suitable = m\nt+ any b a small in/rum5t4 ,a v simil> con/ruction 0 us$ lat] by ,ramsd5 = a small refractor4 ,x is obvi\s t ! rea* ( ! telescope :5 us$ z an equatorial is some:at limit$ 9 ! m\nt j describ$4 ,9 mod]n con/ructions ! telescope is s m\nt$ t x may be turn$ readily to any "p ( ! sky1 al?\gh (t5 ! pol> axis mu/ be swung ?r\gh #ahj 9 ord] to pass freely f ! extreme s\!rn to ! extreme nor!rn heav5s4 ,! two motions necess>y are ^? 9 "r asc5sion to foll[ ! heav5ly bodies 9 _! app>5t c\rse1 & 9 decl9ation to rea* an object at any p>ticul> angul> di/ance f ! pole4 ,!re are always provid$ adju/m5ts 9 azimu? & = latitude ov] at lea/ a small arc1 b ^! adju/m5ts are altoge!r rudim5t>y z comp>$ ) ! wide sweep giv5 by ,%ort4 ,! fundam5tal con/ruction ( ! equatorial involves two axes work+ at "r angles position$ l a capital ,t4 ,! upright ( ! ,t is ! pol> axis1 fitt$ to a sleeve & be>+ ! cross ( ! ,t1 :i* is holl[ & provides ! be>+ = ! decl9ation axis1 :i* aga9 c>ries at "r angles to itself ! tube ( ! telescope4 ,:5 ! sleeve :i* c>ries ! upright ( ! ,t po9ts to ! pole ! telescope tube c evid5tly be swung to cov] an object at any altitude1 & c !n be turn$ on its pol> axis s z to foll[ t object 9 its app>5t diurnal motion4 ,! front =k ( a bicycle set at ! prop] angle ) a cross axis replac+ ! h&le b>s has m ?an once done good s]vice ,illu/ration3 ,,fig4 #ge4--,section ( ,mod]n ,equatorial4 9 an em]g5cy4 ,figure #ge %[s 9 section a mod]n equatorial m\nt = a m$ium siz$ telescope4 ,! m\nt+ %[n 9 ,fig4 #ge1 by ,zeiss1 is ?or\ghly typical ( rec5t practice 9 in/rum5ts ( mod]ate size4 ,! g5]al =m ( ?is equatorial comes /raight d[n to u f ,fraunh(]'s m\nt+ ( ! ,dorpat in/rum5t4 ,x consi/s ess5tially ( two axes cross$ exactly at "r angles4 ,p1 ! pol> axis1 is align$ exactly ) ! pole1 & is support$ on a holl[ iron pi] provid$ at its top ) ! latitude block ,l to :i* ! be>+s ( ,p are bolt$4 ,d ! decl9ation axis supports ! telescope tube ,t4 ,! tube is c\nt]pois$ z reg>ds ! pol> axis by ! weight a1 & z reg>ds ! decl9ation axis by ! weights b b4 ,at ,a1 ! upp] section ( ! pi] c be set 9 exact azimu? by adju/+ screws1 & at ! base ( ! l[] section ! screws at ,b4 ,b4 all[ "s adju/m5t 9 latitude4 ,to su* m]e rudim5ts are ! azimu? & altitude circles ( ,%ort's m\nt r$uc$4 ,at ! upp] end ( ! pol> axis is fitt$ ! ge> :eel g1 driv5 by a worm f ! clockwork at ,c to foll[ ! />s 9 _! c\rse4 ,at ! l[] end ( ! same axis is ! h\r circle h1 graduat$ = "r asc5sion1 & a h& :eel = quick adju/m5t 9 ,r4 ,a4 ,at d is ! decl9ation circle1 :i* is read1 & set1 by ! telescope t ) a "r angl$ prism at its upp] end1 :i* saves ! obs]v] f leav+ ! eye piece = small *anges4 ,f is ! usual f9d]1 :i* %\ld be appli$ to e telescope ( #c in*es ap]ture & above4 ,x %\ld be ( l[ p[]1 ) ! l>ge/ practicable field1 & has commonly an ap]ture or t ( ! ma9 objective1 big en\gh to pick up readily objects to be exam9$ & by its co>se cross wires to br+ !m neatly into ! field4 ,at m & n are ! clamp+ screws = ! "r asc5sion & decl9ation axes respectively1 :ile o & p control ! respective tang5t screws = f9e adju/m5t 9 ,r4 ,a4 & ,dec4 aft] ! axes are clamp$4 ,?is m\nt has really all ! me*anical ref9em5ts ne$$ 9 mu* l>g] in/rum5ts & repres5ts ! class ( p]man5tly m\nt$ telescopes us$ 9 a fix$ obs]vatory4 ,! ord9>y small telescope is provid$ ) a m\nt ( ! same g5]al type b mu* simpl] &1 s9ce x is n 9 a fix$ obs]vatory1 has m lib]al adju/m5ts 9 azimu? & altitude to provide = *anges ( location4 ,figure #gf %[s 9 "s detail ! admirable portable equatorial m\nt+ us$ by ! ,cl>ks = in/rum5ts up to ab\t #e or #f in*es ap]ture4 ,five in*es is practically ! divid+ l9e betwe5 portable & fix$ telescopes4 ,9 fact a #e in* telescope ( /&>d con/ruction ) equatorial m\nt+ is actually too heavy = practical portability on a tripod /&4 ,! ,cl>ks h turn$ out really portable in/rum5ts ( ?is ap]ture1 ( relatively %ort focus & ) alum9um tube fitt$ to ! m\nt+ /&>d = a #d in* telescope1 b ! ord9>y #e in* equipm5t ( ! usual focal l5g? des]ves a p]man5t placem5t4 ,9 ?is m\nt ! %ort tap]$ pol> axis ,p is socket$ betwe5 ! *eeks ,a1 & tight5$ 9 any requir$ position by ! h& screws ,b4 ,! /\t decl9ation axis ,d be>s ! telescope & ! c\nt]weight ,c4 ,sett+ circles 9 ,r4 ,a4 & ,dec41 p & d respectively1 are c>ri$ on ! two axes1 & ea* axis has a worm :eel & tang5t screw op]at$ by a univ]sal jo9t to give ! necess>y sl[ motion4 ,illu/ration3 ,,fig4 #gf4--;,cl>k ,adju/able ,equatorial ,m\nt4 ,! worm :eels c>ry _! respective axes ?r\gh friction be>+s & ! c\nt] pois+ is s exact t ! in/rum5t c be quickly swung to any "p ( ! sky & ! sl[ motion pick$ up on ! in/ant4 ,! wide sweep ( ! pol> axis all[s imm$iate conv]sion into an alt-azimu? = t]re/rial use1 or adju/m5t = any latitude4 ,a graduat$ latitude arc is cu/om>ily engrav$ on "o ( ! *eck pieces to facilitate ?is adju/m5t4 ,ord9>ily portable equatorials on tripod m\nts are n provid$ ) circles1 & h only a s+le sl[ motion1 t 9 ,r4 ,a4 ,a decl9ation circle1 h[ev]1 facilitates sett+ up ! in/rum5t accurately & is conv5i5t = locat+ an object to be swept = 9 ,r4 ,a4 :i* mu/ (t5 be done if "o has n sid]eal "t at h&4 ,9 ,fig4 #gf a ?umb screw und]nea? ! tripod head unclamps ! m\nt s t x may be at once adju/$ 9 azimu? )\t %ift+ ! tripod4 ,z a rule ,am]ican /&s = fix$ equatorials h ! clock drive enclos$ 9 ! holl[ pill> :i* c>ries ! equatorial head z %[n 9 ! reflector ( ,fig4 #ce1 & 9 ! ,cl>k m\nt = refractors ( m$ium size %[n 9 ,fig4 #gg4 ,"h a neat quadrangul> pill> c>ries an equatorial m\nt+ 9 pr9ciple v mu* l ,fig4 #gf1 b big en\gh to c>ry telescopes ( #h to #aj in*es ap]ture4 ,x has univ]sal adju/m5t 9 latitude1 s t x c be us$ 9 ei!r hemisph]e1 ! clock & its driv+ weight are enclos$ 9 ! pill> & sl[ motions are provid$ = f9d+ 9 ,r4 ,a4 & ,dec4 ,! adju/m5t 9 azimu? is made by mov+ ! pill> on its base-plate1 :i* is bolt$ to ! pi]4 ,! conv5i5t connections = accurate foll[+ & ! p[]ful clock make ! m\nt especially good = photographic telescopes ( mod]ate size & ! :ole equipm5t is mo/ conv5i5t & workmanlike4 ,x is wor? not+ t ! circles are provid$ ) graduations t are pla9 r ?an m9ute1 9 accordance ) mod]n practice4 ,9 ^! days ( cele/ial photography equatorials are seldom us$ = det]m9+ positions except ) ! micromet]1 & graduat$ circles !re=e1 prim>ily us$ m]ely = f9d+1 %\ld be1 above all ?+s1 easy to read4 ,all portable m\nts are m]ely simplifications ( ! obs]vatory type ( ,fig4 #ge1 :i*1 ) ! addition ( v>i\s labor sav+ devices is appli$ to ne>ly all l>ge refractors & to _m reflectors z well4 ,!re is a modifi$ equatorial m\nt sometimes kn[n z ! 8,engli%0 equatorial 9 :i* ! pol> axis is long & support$ on two pi]s diff]+ en\gh 9 height to give ! prop] latitude angle1 ! decl9ation axis be+ ab\t midway ( ! pol> axis4 ,a bit ( ! sky is cut (f by ! tall] pi]1 & ! type is n especially advantage\s unless 9 support+ a v heavy in/rum5t1 too heavy to be readily ov]hung 9 ! usual way4 ,illu/ration3 ,,fig4 #gg4--,univ]sal ,obs]vatory ,m\nt 7,cl>k #i-;in*74 ,9 su* case "s =m ( ! 8,engli%0 m\nt+ is v important to secur+ fre$om f flexure & !reby ! p]fection ( driv+ 9 ,r4 ,a4 s important to photographic "w4 ,! #gb in* ,dom9ion ,obs]vatory reflector & ! #ajj in* ,hook] telescope at ,mt4 ,wilson are ?us m\nt$1 ! =m] on a c\nt]pois$ decl9ation axis crosswise ! pol> axis1 ! orig9al 8,engli%0 type2 ! latt] on trunnions )9 a long clos$ =k :i* c>ries ! pol> be>+s at its ends4 ,illu/ration3 ,,fig4 #gh4--;,engli% ,equatorial ,m\nt 7,hook] #ajj-;in* ,telescope74 ,figure #gh %[s ! latt] in/rum5t1 ( #ajj in*es cle> ap]ture & ( #db feet pr9cipal focal l5g?1 increas$ to #ace feet :5 us$ z a ,cassegra9ian4 ,x is ! imm5se /ability ( ?is m\nt t has enabl$ x to c>ry ! long cross gird] be>+ ! int]f]omet] rec5tly us$ 9 measur+ ! diamet]s ( ! />s4 ,note ! m]cury-flotation drum at ea* end ( ! pol> axis4 ,! mirrors 7 figur$ by ! skillful h&s ( ,mr4 ,rit*ey4 ,illu/ration3 ,,fig4 #gi4--;,engli% ,equatorial ,m\nt 7#gb-;in* ,dom9ion ,obs]vatory ,telescope74 ,figure #gi gives 9 outl9e ! proportions & m\nt+ ( ! beautiful in/rum5t 9 s]vice at ! ,dom9ion ,obs]vatory1 ne> ,victoria1 ,b4 ,c4 ,! mirror & its auxili>ies 7 figur$ by ,bra%e> & ! v elegant m\nt+ 0 by ,w>n] & ,swasey4 ,! ma9 mirror is ( #cj feet pr9cipal focus4 ,! #bj in* hyp]boloidal mirror ext5ds ! focus z a ,cassagra9ian to #ajh feet4 ,! me*anical /ability ( ^! ,engli% m\nts = v l>ge in/rum5ts has be5 amply demon/rat$ by ?is1 z by ! ,hook] #ajj in* reflector4 ,!y suff] less f flexure ?an ! ,fraunh(] m\nt :]e great weights are to be c>ri$1 al?\gh ! latt] is m conv5i5t & g5]ally useful = in/rum5ts ( mod]ate size4 ,x is h>d to say too mu* ( ! me*anical skill t has made ^! two colossal telescopes s completely successful z in/rum5ts ( rese>*4 ,illu/ration3 ,,fig4 #hj4--;,a/rographic ,m\nt ) ,b5t ,pi]4 ,! inconv5i5ce ( hav+ to sw+ ! telescope tube to cle> ! pi] at c]ta9 po9ts 9 ! ,r4 ,a4 foll[+ is (t5 a s]i\s nuisance 9 photographic "w requir+ long exposures1 & may wa/e valuable "t 9 visual "w4 ,sev]al rec5t =ms ( equatorial m\nt h !re=e be5 devis$ to all[ ! telescope complete fre$om ( revolution 9 ,r4 ,a41 sw++ cle> ( ev]y?+4 ,"o su* =m is %[n 9 ,fig4 #hj :i* is a /&>d a/rographic m\nt = a ,bra%e> d\blet & guid+ telescope4 ,! pi] is /rongly ov]hung 9 ! direction ( ! pol> axis f> en\gh to all[ ! in/rum5t to foll[ ?r\gh = any requir$ p]iod1 ev5 to resum+ op]ations on ano!r night )\t a %ift ( work+ position4 ,illu/ration3 ,,fig4 #ha4--,op5 ,=k ,m\nt+4 ,ano!r =m1 ev5 simpl] & f\nd to be extremely satisfactory ev5 = r l>ge in/rum5ts1 is ! op5 pol> =k m\nt4 ,"h ! pol> axis ( an ord9>y =m is cont9u$ by a wide & /iff ca/+ 9 ! =m ( a =k )9 :i* ! tube is c>ri$ on sub/antial trunnions1 giv+ x complete fre$om ( motion4 ,! op5 =k m\nt 9 its simple/ =m1 c>ry+ a helio/at mirror1 is %[n 9 ,fig4 #ha4 ,"h ,a is ! =k1 ,b ! pol> axis1 c>ri$ on an adju/able sector = v>iation 9 latitude1 ,c ! decl9ation axis c>ry+ ! mirror ,d 9 its cell1 ,e ! sl[ motion 9 decl9ation1 & ,f t 9 ,r4 ,a4 ,bo? axes c be unclamp$ = quick motion & ! ,r4 ,a4 axis c readily be driv5 by clock or electric motor4 ,! resemblance to ! fully develop$ ,engli% equatorial m\nt ( ,fig4 #gh is obvi\s1 b ! pres5t arrangem5t gives entirely free sw+ to a %ort in/rum5t1 is conv5i5tly adju/able1 & altoge!r workmanlike4 ,x c easily c>ry a %ort focus cele/ial cam]a up to #f or #h in*es ap]ture or a reflector ( #d or #e feet focal l5g?4 ,9 ,fig4 #agc1 ,*ap4 ,x a pair ( ^! same m\nts are %[n at ,h>v>d ,obs]vatory4 ,! ne>] "o1 c>ry+ a cele/ial cam]a1 is expos$ to view4 ,x is provid$ ) a sl[ motion & clamp 9 decl9ation1 & ) an electric drive 9 ,r4 ,a41 quickly unclamp$ = sw++ ! cam]a4 ,x works v smoo?ly1 its weight is tak5 by a v simple self adju/+ ?ru/ be>+ at ! l[] end ( ! pol> axis1 & altoge!r x is ab\t ! simple/ & *eape/ equatorial m\nt ( fir/ class quality t c be devis$ = c>ry+ in/rum5ts ( mod]ate l5g?4 ,sev]al o!rs are 9 use at ! ,h>v>d ,obs]vatory & v simil> ones ( a l>g] gr[? c>ry ! #bd in* ,newtonian reflector !re us$ = /ell> photography & ! #af in* ,metcalf photographic d\blet4 ,illu/ration3 ,,fig4 #hb4--,m\nt+ ( ,mt4 ,wilson #fj-;in* ,reflector4 ,illu/ration3 ,,fig4 #hc4--,! #fj-;in* z ,cassegra9ian1 ,f #ajj4 ,9 fact ! op5 =k m\nt1 :i* 0 develop$ by ! late ,dr4 ,common1 is v well suit$ to ! m\nt+ ( big reflectors4 ,x 0 fir/ adapt$ by him to 8 #c ft4 reflector & lat] us$ = 8 two #e ft4 mirrors1 & m rec5tly = ! #e ft4 in/rum5t at ,mt4 ,wilson1 & a good _m o!rs ( rec5t make4 ,dr4 ,common 9 ord] to secure ! easie/ possible motion 9 ,r4 ,a4 devis$ ! plan ( float+ mo/ ( ! weight assum$ by ! pol> axis 9 m]cury4 ,figure #hb is1 diagrammatically1 ?is =k m\nt z work$ out by ,rit*ey = ! #e ,mt4 ,wilson reflector4 ,"h ,a is ! lattice tube1 ,b ! pol> axis1 ,c ! =k & ,d ! holl[ /eel drum :i* floats ! axis 9 ! m]cury tr\gh ,e4 ,! great mirror is "h %[n work$ z a simple ,newtonian ( #be ft4 focal l5g?4 ,z a matt] ( fact x is us$ mu* ( ! "t z a ,cassegranian4 ,to ?is end ! upp] section ( tube c>ry+ ! oblique mirror is remov$ & a %ort] tube c>ry+ any "o ( ?ree hyp]boloidal mirrors is put 9 its place4 ,fig4 #hc is ! normal arrangem5t = visual or photographic "w on ! long focus1 #ajj ft4 ,! dott$ l9es %[ ! pa? ( ! rays & x w be notic$ t ! great mirror is n p]=at$ z 9 ! usual ,cassegra9ian con/ruction1 b t ! rays are br\ght out by a diagonal flat4 ,figure #hd is a simil> arrangem5t us$ = /ell> spectroscopy ) a small flat & an equival5t focus ( #hj ft4 ,9 ,fig4 #he a radically diff]5t s*eme is c>ri$ out4 ,! hyp]boloidal mirror n[ us$ gives an equival5t focus ( #aej ft41 & ! auxili>y flat is arrang$ to turn on an axis p>allel to ! decl9ation axis s z to s5d ! reflect$ beam d[n ! holl[ pol> axis into a spectrograph vault bel[ ! s\!rn end ( ! axis4 ,obvi\sly "o _c "w ne> ! pole ) ?is arrangem5t b only ?r\gh "s #ge z indicat$ by ! dott$ l9es4 ,! =k m\nt is n at all univ]sal = reflectors1 z has already be5 se51 & ,cassegra9ian ( mod]ate size are v commonly m\nt$ exactly l refractors4 ,illu/ration3 ,,fig4 #hd4--,! #fj-;in* z ,cassegra9ian1 ,f #hj4 ,illu/ration3 ,,fig4 #he4--,! #fj-;in* z ,pol> ,cassegra9ian1 ,f #aej4 ,we n[ come to a gr\p ( m\nts :i* h 9 common ! fundam5tal idea ( a fix$ eyepiece1 & incid5tally bett] protection ( ! obs]v] aga9/ ! rigors ( long w9t] nights :5 ! see+ may be at its be/ b ! effici5cy ( ! obs]v] is greatly dim9i%$ by discom=t4 ,"s ( ! arrangem5ts are also ( value 9 facilitat+ ! use ( long focus objectives & mirrors & escap+ ! co/ ( ! l>ge domes :i* o!rwise w\ld be requir$4 ,p]haps ! e>lie/ example ( ! class is f\nd 9 ,c>ol9e ,h]s*el's comet seek]1 %[n 9 ,fig4 #hf4 ,?is 0 a ,newtonian reflector ( ab\t #f in*es ap]ture m\nt$ 9 a fa%ion t is almo/ self explanatory4 ,x 01 l all ,h]s*el's telescopes1 an alt-azimu? b in/ead ( be+ pivot$ 9 altitude ab\t ! mirror or ! c5t] ( gravity ( ! :ole tube1 x 0 pivot$ on ! eyepiece location & ! tube 0 c\nt]balanc$ z %[n s t x c\ld be v easily adju/$ 9 altitude :ile ! :ole frame turn$ 9 azimu? ab\t a v]tical po/4 ,?us ! obs]v] c\ld /& or sit at ease sweep+ 9 a v]tical circle1 & m]ely _h to move ar\nd ! po/ z ! azimu? 0 *ang$4 ,! arrangem5t is n )\t advantages1 & 0 _m ye>s lat] adopt$ ) modifications ( detail by ,dr4 ,j4 ,w4 ,drap] = ! fam\s in/rum5t ) :i* he advanc$ s notably ! art ( cele/ial photography4 ,! same fundam5tal idea ( free+ ! obs]v] f cont9ual climb+ ab\t to rea* ! eyepiece has be5 c>ri$ out 9 v>i\s equatorially m\nt$ comet seek]s4 ,a v good example ( ! type is a big comet seek] by ,zeiss1 %[n 9 ,fig4 #hg4 ,! fundam5tal pr9ciple is t ! ocul> is at ! int]section ( ! pol> & decl9ation axis1 ! telescope tube be+ ov]hung well beyond ! nor? end ( ! =m] & c\nt]balanc$ on ! latt]4 ,! obs]v] c !re=e sit 9 8 swivel *air & )\t /irr+ f x sweep rapidly ov] a v wide expanse ( sky4 ,?is p>ticul> in/rum5t is probably ! l>ge/ ( regul> comet seek]s1 #h in*es 9 cle> ap]ture & #eb in*es focal l5g? ) a triple objective to ensure ! necess>y corrections 9 us+ s great a relative ap]ture4 ,9 ?is figure #a is ! base ) corrections 9 altitude & azimu?1 #b ! c\nt]poise ( ! :ole telescope on its base1 #c ! pol> axis & ,r4 ,a4 circle1 #d ! ov]hung decl9ation axis & its circle1 #e ! c\nt]poise 9 decl9ation1 #f ! pol> c\nt]poise1 & #g ! ma9 telescope tube4 ,! h&:eel %[n m]ely op]ates ! ge> = revolv+ ! dome )\t leav+ ! obs]v+ *air4 ,! next /ep beyond ! eyepiece fix$ 9 g5]al position is s to locate x t ! obs]v] c be ?or\ghly protect$ )\t includ+ ! optical p>ts ( ! telescope 9 su* wise z to injure _! p]=mance4 ,"o _c successfully obs]ve ?r\gh an op5 w9d[ on acc\nt ( ! air curr5ts due to temp]ature diff]5ces1 & 9 an obs]vatory dome1 unheat$ z x is1 mu/ wait aft] ! %utt] is op5$ until ! temp]ature is fairly /eadi$4 ,except = ^! comet seek]s practically all ( ! class make use ( "o or two auxili>y reflections to br+ ! image into ! requir$ direction1 & 9 g5]al ! field ( possible view is some:at curtail$ by ! m\nt+4 ,?is is less ( a disadvantage ?an x w\ld appe> at fir/ ?\ght1 =1 to beg9 )1 obs]vations )9 #bj ( ! horizon or !reab\ts are g5]ally unsatisfactory1 & ! advantages ( a /able & conv5i5t long focus in/rum5t are s notable z = _m purposes q to outweigh "s loss ( sky-space4 ,illu/ration3 ,,fig4 #hf4--;,c>ol9e ,h]s*el's ,comet ,seek]4 ,! simple/ ( ! fix$ eyepiece gr\p is ! pol> telescope ( :i* ! rudim5ts are well %[n 9 ,fig4 #hh1 a m\nt describ$ by ,sir ,h[>d ,grubb 9 #ahhj1 & an example ( :i* 0 in/all$ a little lat] 9 ! ,craw=d ,obs]vatory 9 ,cork4 ,"h ! pol> axis ,a is ! ma9 tube ( ! telescope1 & 9 front ( ! objective ,b1 is held 9 a =k ! decl9ation cradle & mirror ,c1 by :i* any object )9 a wide sweep ( decl9ation c be br\ght into ! field & held !re by h& or clockwork ?r\gh rotat+ ! pol> tube4 ,illu/ration3 ,,fig4 #hg4--,m\nt+ ( ,l>ge ,comet ,seek]4 ,look$ at f ano!r slant x is a pol> helio/at1 ( :i* ! telescope =ms ! driv+ axis 9 ,r4 ,a4 ,! :ole m\nt 0 a sub/antial ca/+ on :eels :i* ran on a pair ( rails4 ,= use ! in/rum5t 0 roll$ to a specially arrang$ w9d[ & ?r\gh x until ov] its regul> be>+s on a pi] j outside4 ,a few turns ( ! :eel ,d l[]$ x ^u ^!1 & ! back ( ! frame !n clos$ ! op5+ 9 ! wall leav+ ! in/rum5t 9 ! op51 & ! eye end inside ! room4 ,! example fir/ built 0 ( only #d in*es ap]ture b prov$ its case admirably z a mo/ useful & conv5i5t in/rum5t4 ,?is m\nt ) v>i\s o!rs ( ! fix$ eyepiece class may be reg>d$ z d]iv$ f ! horizontal photoheliographs us$ at ! #ahgd transit ( ,v5us & subsequ5tly at _m total sol> eclipses4 ,giv5 an equatorially m\nt$ helio/at l ,fig4 #ha & x is evid5t t ! beam f x may be turn$ into a horizontal telescope plac$ 9 ! m]idian1 7or = t matt] 9 any conv5i5t direction7 & held !re by rotation ( ! mirror 9 ,r4 ,a41 b also 9 decl9ation1 save 9 ! case :]e ! beam travels along ! ext5sion ( ! pol> axis4 ,illu/ration3 ,,fig4 #hh4--;,grubb's ,orig9al ,pol> ,telescope4 ,= ! brief exposure p]iods orig9ally ne$$ & ! sl[ v>iation ( ! sun 9 decl9ation ?is helio/atic telescope 0 easily kept 9 adju/m5t4 ,! orig9al in/rum5ts 7 ( #e in*es ap]ture & #dj ft4 focal l5g?1 & ! #g in* helio/at mirror 0 provid$ ) ord9>y equatorial clockwork4 ,set up ) ! telescope po9t+ along ! pol> axis no cont9u\s v>iation 9 decl9ation is ne$$ & ! clock drive holds ! field /eadily1 z 9 any o!r equatorial4 ,figure #hi %[s diagrammatically ! #ab in* pol> telescope us$ = m ?an tw5ty ye>s pa/ at ! ,h>v>d ,obs]vatory4 ,! m\nt 0 design$ by ,mr4 ,w4 ,p4 ,g]ri% ( ! ,h>v>d /aff & conta9s _m +5i\s features4 ,unlike ,fig4 #hh ?is is a fix$ m\nt1 ) ! eye-end com=tably h\s$ 9 a room on ! second floor ( ! ma9 obs]vatory build+1 & ! l[] be>+ on a sub/antial pi] to ! s\?w>d4 ,illu/ration3 ,,fig4 #hi4--;,diagram ( ,g]ri% ,pol> ,telescope4 ,9 ! figure1 ,a is ! eye end1 ,b ! ma9 tube ) ! objective at its l[] end & prolong$ by a =k support$ by ! be>+ ,c & ,d is ! decl9ation mirror s5d+ ! beam upw>d4 ,! :ole is rotat$ 9 ,r4 ,a4 by an electric clock drive1 & all ! necess>y adju/m5ts are made f ! eye end4 ,a view ( ! ext]ior is %[n 9 ,fig4 #ij1 ) ! mirror & objective uncov]$4 ,! rock+ arm at ! objective end1 op]at$ by a small w9* beside ! ocul>1 sw+s cle> bo? mirror & objective caps 9 a few seconds1 & ! telescope is !n ready = use4 ,its focal l5g? is #af ft4 #aj in*es & x gives a sweep 9 decl9ation ( approximately #hj4 ,x gives excell5t def9ition & has prov$ a mo/ useful in/rum5t4 ,a second pol> telescope 0 set up at ! ,h>v>d ,obs]vatory /ation 9 ,m&eville1 ,jamaica1 9 ! autumn ( #aijj4 ,?is 0 int5d$ prim>ily = lun> photography & 0 provid$ ) a #ab in* objective ( #ace ft4 #d in*es focal l5g? & an #ah in* helio/at ) electric clock drive4 ,illu/ration3 ,,fig4 #ij4--;,g]ri% ,pol> ,telescope1 ,h>v>d ,obs]vatory4 ,inasmu* z all in/rum5ts ( ?is class necess>ily rotate ! image z ! mirror turns1 ! tail-piece ( ?is telescope is also m\nt$ = rotation by a simil> drive s t ! image is /ation>y on ! plate bo? 9 position & ori5tation4 ,z ,m&eville is 9 ,n4 lat4 #ah #ja ! telescope is conv5i5tly ne> ! horizontal4 ,! obs]vatory ( ,yale ,univ]sity has a l>ge in/rum5t ( ?is class1 ( #ej feet focal l5g?1 ) a #ae-;in* photographic objective & a #aj-;in* visual guid+ objective work+ toge!r f ! same helio/at4 ,despite its simplicity & conv5i5ce ! pol> telescope has an obvi\s defect 9 its v mode/ sweep 9 decl9ation1 only to be increas$ by ! use ( an exceptionally l>ge mirror4 ,x is n !re=e rem>kable t ! fir/ s]i\s attempt at a fix$ eyepiece in/rum5t = g5]al use turn$ to a diff]5t con/ruction ev5 at ! co/ ( an additional reflection4 ,?is 0 ! equatorial c\d devis$ by ,m4 ,loewy ( ! ,p>is ,obs]vatory 9 #ahhb4 7,fig4 #ia47 ,9 ! diagram ,a is ! ma9 tube :i* =ms ! pol> axis1 & ,b ! eye end "u %elt]1 ) all accessories at ! obs]v]'s h&4 ,b ! tube is brok5 by ! box cas+ ,c conta9+ a mirror rigidly support$ at #de to ! axis ( ! ma9 tube & ( ! side tube ,d1 :i* is c\nt]balanc$ & is 9 effect a holl[ decl9ation axis c>ry+ ! objective ,e at its out] end4 ,illu/ration3 ,,fig4 #ia4--;,diagram ( ,equatorial ,c\d4 ,9 lieu ( ! telescope tube usually c>ri$ on ?is decl9ation axis we h ! #de mirror1 ,f1 turn+ 9 a sleeve conc5tric ) ! objective1 :i*1 hav+ a lat]al ap]ture1 virtually gives ! objectives a full sweep 9 decl9ation1 save z ! upp] pi] cuts x (f4 ,! :ole in/rum5t is clock driv5 9 ,r4 ,a41 & has ! usual circles & sl[ motions all h&ily manipulat$ f ! eye end4 ,! equatorial c\d is und5iably complicat$ & co/ly1 b z con/ruct$ by ,h5ry ,frres x actually p]=ms admirably ev5 "u sev]e te/s1 & has be5 sev]al times duplicat$ 9 ,fr5* obs]vatories4 ,! fir/ c\d erect$ 0 ( #aj in*es ap]ture & 0 soon foll[$ by "o ( #bc4f in*es ap]ture & #ei ft4 focus1 :i* is ! l>ge/ yet built4 ,/ill ano!r m\nt+ sugge/ive ( bo? ! pol> telescope & ! c\d is due to ,sir ,h[>d ,grubb1 ,fig4 #ib4 ,"h z 9 ! c\d ! upp] "p ( ! pol> axis1 ,a1 is ! telescope tube :i* leads into a solid cas+ ,b1 ab\t :i* a sub/antial =k1 ,c1 is pivot$4 ,?is =k is ! ext5sion ( ! side tube ,d1 :i* c>ries ! objective1 & ?us has free sw+ 9 decl9ation ?r\gh an angle limit$ by ! ro( ( ! obs]v+ room above1 & ! proximity ( ! horizon bel[4 ,its useful sw+1 z 9 ! pol> telescope1 is limit$ by ! dim5sions ( ! mirror ,e1 :i* receives ! cone ( rays f ! objective & turns x up ! pol> tube to ! eyepiece4 ,?is mirror is ge>$ to turn at half ! rate ( ! tube ,d s t ! angle ,d ,e ,a is cont9ually bisect$4 ,illu/ration3 ,,fig4 #ib4--;,grubb ,modifi$ ,c\d4 ,9 po9t ( fact ! sole ga9 9 ?is con/ruction is ! r$uction 9 ! size ( mirror requir$1 by reason ( ! dim9i%$ size ( ! cone ( rays :5 x rea*es ! mirror4 ,! plan has be5 v successfully work$ out 9 ! f9e a/rographic telescope ( ! ,cambridge ,obs]vatory ( #ab in*es ap]ture & #ai4c ft4 focal l5g?4 ,z 9 ! o!r in/rum5ts ( ?is g5]al class ! adju/m5ts are all conv5i5tly made f ! eye end4 ,! ,cambridge in/rum5t has a triple photo-visual objective ( ! =m design$ by ,mr4 ,h4 ,d4 ,taylor & ! side tube1 :5 n 9 use1 is turn$ d[n to ! horizontal & cov]$ 9 by a l[ :eel$ h\s+ c>ri$ on a track4 ,! sky space cov]$ is f #ae above ! pole to ne> ! horizontal4 ,x is obvi\s t v>i\s pol> & c\d =ms ( reflector are q practicable & inde$ "o su* arrangem5t is %[n 9 connection ) ! #fj in* ,mt4 ,wilson reflector1 b we are "h conc]n$ only ) ! *ief types ( m\nt+ :i* h actually prov$ _! usefulness4 ,none ( ! arrangem5ts :i* require ! use ( additional l>ge reflect+ surfaces are exempt f dang] ( impair$ def9ition4 ,only sup]latively f9e workman%ip & skill 9 m\nt+ c save !m f di/ortion & a/igmatism due to flexure & w>p+ ( ! mirrors1 & su* tr\bles h n infrequ5tly be5 enc\nt]$4 ,to a some:at v>iant type belong sev]al valuable con/ructions :i* utilize 9 ! auxili>y reflect+ sy/em ! clo/at r ?an ! pol> helio/at or its equival5t4 ,! clo/at is simply a plane mirror m\nt$ ) its plane fix$ 9 t ( a pol> axis :i* rotates once 9 #dh h\rs1 i4e41 at half ! app>5t rate ( ! />s4 ,illu/ration3 ,,fig4 #ic4--;,diagram ( ,sn[ ,horizontal ,telescope4 ,a telescope po9t$ at su* a mirror w hold ! />s motionless 9 its field z if ! firmam5t 7 halt$ la ,jo%ua4 ,b if a *ange ( view is want$ ! telescope mu/ be %ift$ 9 altitude or azimu? or bo?4 ,?is is altoge!r inconv5i5t1 s t z a matt] ( practice a second plane mirror is us$ to turn ! /eady beam f ! clo/at into any desir$ direction4 ,by ?us %ift+ ! mirror in/ead ( ! telescope1 ! latt] c be p]man5tly fix$ 9 ! mo/ conv5i5t location1 at ! co/ ( "s add$ exp5se & loss ( light4 ,fur!r1 ! image does n rotate z 9 case ( ! pol> helio/at1 :i* is (t5 an advantage4 ,an admirable type ( ! fix$ telescope ?us con/itut$ is ! ,sn[ telescope at ,mt4 ,wilson 7,cont4 f ! ,sol> ,obs4 #b1 ,hale74 ,fig4 #ic f ?is pap] %[s ! equipm5t 9 plan & elevation4 ,! topography ( ! m\nta9 top made x desirable to lay out ! axis ( ! build+ #ae ,e4 ( ,n4 & slop+ d[nw>d #e t[>d ! ,n4 ,at ! "r h& end ( ! figure is %[n ! clo/at pi]1 #bi ft4 high at its ,s end4 ,?is pi] c>ries ! clo/at mirror prop]1 #cj in*es 9 diamet]1 on rails a a accurately ,e4 & ,w4 to all[ = slid+ ! in/rum5t s t its field may cle> ! second>y mirror ( #bd in*es diamet] :i* is on an alt-azimu? =k m\nt+ & also slides on rails b b4 ,! telescope "h is a pair ( p>abolic mirrors ea* ( #bd in*es ap]ture & ( #fj ft4 & #ade ft4 focus respectively4 ,! beam f ! second>y clo/at mirror passes fir/ ?r\gh ! spectrographic laboratory %[n to ! left ( ! ma9 pi]1 & 9 ?r\gh a long & n>r[ %elt] h\se to "o ( ^! mirrors2 ! "o ( longe/ focus on longitud9al focuss+ rails e e1 ! o!r on simil> rails c c1 ) provision = slid+ sidewise at d to cle> ! way = ! long] beam4 ,! ocul> end ( ?is rem>kable telescope is ! spectrographic laboratory :]e ! beam c be turn$ into ! p]man5tly m\nt$ in/rum5ts1 = ! details ( :i* ! orig9al pap] %\ld be consult$4 ,! purpose ( ?is brief description is m]ely to %[ ! beautiful facility ) :i* a clo/atic telescope may be adapt$ to a/rophysical "w4 ,obvi\sly an objective c\ld be put 9 ! clo/at beam = any purpose = :i* x might be desirable4 ,su* 9 fact is ! arrangem5t ( ! t[] telescopes at ! ,mt4 ,wilson ,obs]vatory4 ,9 ^! in/rum5ts we h ! ord9>y clo/at arrangem5t turn$ on end = ! sake ( gett+ ! *ief optical p>ts well above ! gr\nd :]e1 remov$ f ! heat$ surface1 ! def9ition is g5]ally improv$4 ,to be sure ! focus is at or ne> ! gr\nd level1 b ! upw>d air curr5ts cause mu* less di/urbance ?an ! crosswise ones 9 ! ,sn[ telescope4 ,! head ( ! fir/ t[] telescope is %[n 9 ,fig4 #id4af ,a is ! clo/at mirror prop] #ag in*es 9 diamet] & #ab in*es ?ick1 ,b ! second>y mirror #ab in*es 9 ! %ort] axis ( ! ellipse1 #bb in*es 9 ! long]1 & also #ab in*es ?ick4 ,c is ! #ab in* objective ( #fj ft4 focus1 & ,d ! focuss+ ge> work$ by a /eel ribbon f bel[4 #af ,contributions f ! ,sol> ,obs4 #bc1 ,hale1 :i* %\ld be se5 = details4 ,illu/ration3 ,,fig4 #id4--;,head ( #fj-;in* ,t[] ,telescope4 ,?is in/rum5t be+ = sol> rese>* ! mirrors are arrang$ = conv5i5t work+ ) ! sun fairly l[ on ei!r horizon :]e ! def9ition is at its be/1 & c be %ift$ accord+ly1 to ! same end z 9 ! ,sn[ telescope4 ,!re is also provision = %ift+ ! objective lat]ally at a uni=m rate f bel[1 to provide = ! use ( ! app>atus z spectro-heliograph4 ,! t[] is ( ! w9dmill type & prov$ to be fairly /eady 9 spite ( its height1 high w9ds be+ r>e on ,mt4 ,wilson4 ,! great ?ickness ( ! mirrors 9 ! ef=t to escape di/ortion des]ves notice4 ,!y actually prov$ to be too ?ick to give !rmal conductivity suffici5t to prev5t di/ortion4 ,illu/ration3 ,,fig4 #ie4--,port]'s ,pol> ,reflector4 ,9 ! lat] #aej t[] telescope ! mirrors are relatively less ?ick1 & a v int]e/+ modification has be5 introduc$ 9 ! t[]1 9 t x consi/s ( a lattice memb] = memb] )9 ano!r ext]ior lattice1 s t ! op5 /ructure is reta9$1 :ile ea* memb] t supports ! optical p>ts is %ield$ f ! w9d & sudd5 temp]ature *ange by its correspond+ out] %ea?4 ,/ill ano!r =m ( m\nt+ to give ! obs]v] access to a fix$ eyepiece "u %elt] is f\nd 9 ! +5i\s pol> reflector by ,mr4 ,russell ,w4 ,port] ( :i* an example ) ma9 mirror ( #af in*es diamet] & #ae ft4 #f in*es focal l5g? 0 erect$ by him a few ye>s ago4 ,fig4 #ie is entirely descriptive ( ! arrangem5t :i* f ,mr4 ,port]'s acc\nt seems to h work$ extremely well4 ,! *ief difficulty enc\nt]$ 0 cond5sation ( moi/ure on ! mirrors1 :i* 9 "s climates is v difficult to prev5t4 ,illu/ration3 ,,fig4 #if4--;,diagram ( ,h>tness ,turret ,telescope4 ,x is int]e/+ to note t ,mr4 ,port]'s fir/ plan 0 to use ! in/rum5t z a ,h]s*elian ) its focus ?r[n bel[ ! sid]o/at at ,f1 b ! tilt+ ( ! mirror1 :i* 0 work$ at ,f_/#aa4f1 produc$ excessive a/igmatism ( ! images1 & ! plan 0 ab&on$ 9 favor ( ! ,newtonian =m %[n 9 ! figure4 ,at ,f_/#be or !reab\ts ! e>li] s*eme w\ld probably h succe$$ well4 ,/ill ano!r fix$ eyepiece telescope ( d>+ & successful design is ! turret telescope ( ! ,hon4 ,j4 ,e4 ,h>tness ( :i* ! inv5tor erect$ a f9e example ( #aj in* ap]ture at ,spr+field1 ,v]mont4 ,! telescope is 9 ?is case a refractor1 & ! feature ( ! m\nt is t ! pol> axis is exp&$ into a turret )9 :i* ! obs]v] sits com=tably1 look+ into ! ocul> :i* lies 9 ! divid$ decl9ation axis & is suppli$ f a reflect+ prism 9 ! ma9 beam f ! objective ,figure #if %[s a diagram ( ! m\nt & obs]vatory4 ,"h a is ! pol> turret1 bb ! be>+s ( ! decl9ation axis1 c ! ma9 tube1 d its support1 & e ! ocul> end4 ,optically ! telescope is m]ely an ord9>y refractor us$ ) a "r angl$ prism a little l>g] & fur!r up ! tube ?an usual4 ,! turret is ent]$ 9 ?is in/ance f bel[1 ?r\gh a tunnel f ! inv5tor's resid5ce4 ,! telescope z %[n 9 ,fig4 #if has a #aj in* ,bra%e> objective ( f9e optical quality1 & ! light is turn$ into ! ocul> tube by a "r angl$ prism only #b in*es 9 ! face4 ,?is is an entirely practicable size = a reflect+ prism & ! light lo/ is n mat]ially 9 excess ( t lo/ 9 ! ord9>y 8/> diagonal0 s necess>y = ! obs]vation ( />s ne> ! z5i? 9 an ord9>y equatorial4 ,! only obvi\s difficulty ( ! con/ruction is ! support ( ! v l>ge pol> axis4 ,be+ an accompli%$ me*anical eng9e]1 ,mr4 ,h>tness work$ out ! details ( ?is design v successfully al?\gh ! mov+ p>ts weigh$ "s #b tons4 ,! ocul> is n absolutely fix$ ) ref]5ce to ! obs]v] b is always conv5i5tly plac$1 & ! p]=mance ( ! in/rum5t is report$ z excell5t 9 e respect1 :ile ! %elt]+ ( ! obs]v] f ! rigors ( a ,v]mont w9t] is altoge!r admirable4 ,figure #ig %[s ! complete obs]vatory z x /&s4 ,obvi\sly ! high] ! latitude ! easi] is ?is p>ticul> con/ruction1 :i* l5ds itself readily to l>ge in/rum5ts & has ! additional advantage ( free+ ! obs]v] f ! insect pe/s :i* are extremely tr\blesome 9 w>m wea!r ov] a l>ge "p ( ! _w4 ,?is runn+ acc\nt ( m\nt+s makes no claim at complet5ess4 ,x m]ely %[s ! devices 9 common use & "s :i* po9t ! way to fur!r progress4 ,! ma9 requirem5ts 9 a m\nt are /ead9ess1 & smoo?ness ( motion4 ,ev5 an alt-azimu? m\nt ) its ne$ ( two motions1 if smoo? work+ & /eady1 is pref]able to a %aky & j]ky equatorial4 ,rememb] t ! ,h]s*els did immortal "w )\t equatorial m\nt+s1 & us$ high p[]s at t4 ,a clock driv5 equatorial is a great conv5i5ce & practically indisp5sable = ! photographic "w t makes s l>ge a "p ( mod]n a/ronomy1 b = eye obs]vations "o gets on v fairly )\t ! clock4 ,illu/ration3 ,,fig4 #ig4--;,h>tness ,turret ,obs]vatory f ! ,n4 ,e4 ,circles arc a necessity 9 all b ! small telescopes us$ on portable tripods1 o!rwise mu* "t w be wa/$ 9 f9d+4 ,9 any ev5t d n skimp on ! f9d]1 :i* %\ld be ( ample ap]ture & wide field1 say ! ap]ture ( ! ma9 objective1 & #c to #e 9 field4 ,sup]ior def9ition is ne$less1 light1 & sky room en\gh to locate objects quickly be+ ! fundam5tal requisites4 ,z a f9al ^w see t all ! adju/m5ts are )9 easy rea* f ! eyepiece1 s9ce an object once lo/ f a high p[] ocul> (t5 proves tr\blesome to locate aga94 ,,ref]5ces ,,*amb]s' ,a/ronomy1 ,vol4 ,,ii4 ,f4 ,l4 ,o4 ,,wadswor?3 ,ap4 ,j41 #e1 #acb4 ,rany>d's m\nts = reflectors4 ,g4 ,w4 ,,rit*ey3 ,ap4 ,j41 #e1 #adc4 ,support+ l>ge specula4 ,g4 ,e4 ,,hale3 ,cont4 ,sol> ,obs4 #b4 ,! 8,sn[0 horizontal telescope4 ,g4 ,e4 ,,hale3 ,cont4 ,sol> ,obs4 #bc4 ,! #fj ft4 t[] telescope4 ,j4 ,w4 ,,drap]3 ,smi?sonian ,contrib4 #cd4 ,m\nt+ ( 8 l>ge reflector4 ,g4 ,w4 ,,rit*ey3 ,smi?sonian ,contrib4 #ce4 ,m\nt+ ( ! ,mt4 ,wilson #fj in* reflector4 ,,sir ,h4 ,,grubb3 ,tr4 ,roy4 ,dubl9 ,soc4 ,s]4 #b4 #c4 ,pol> ,telescopes4 ,,sir ,r4 ,s4 ,,ball3 ,m4 ,n4 #ei1 #aeb4 ,photographic pol> telescope4 ,a4 ,a4 ,,common3 ,mem4 ,r4 ,a4 ,s41 #df1 #agc4 ,m\nt+ ( 8 #c ft4 reflector4 ,r4 ,w4 ,,port]3 ,pop4 ,a/41 #bd1 #cjh4 ,pol> reflect+ telescope4 ,,james ,,h>tness3 ,trans4 ,a4 ,s4 ,m4 ,e41 #aiaa1 ,turret ,telescope4 ,,sir ,,david ,,gill3 ,enc4 ,brit41 #aa? ,ed4 ,telescope4 ,admirable summ>y ( m\nts4 ,,*apt] ,,vi ,,eye ,,pieces ,! eyepiece ( a telescope is m]ely an in/rum5t = magnify+ ! image produc$ by ! objective or mirror4 ,if "o looks ?r\gh a telescope )\t its eyepiece1 draw+ ! eye back f ! focus to its ord9>y di/ance ( di/9ct vision1 ! image is cle>ly se5 z if susp5d$ 9 air1 or x c be receiv$ on a bit ( gr\nd glass4 ,x appe>s l>g] or small] ?an ! object se5 by ! nak$ eye1 9 proportion z ! focal l5g? ( ! objective is l>g] or small] ?an ! di/ance to :i* ! eye has to drop back to see ! image cle>ly4 ,?is real image1 ! quality ( :i* dep5ds on ! exactness ( correction ( ! objective or mirror1 is !n to be magnifi$ s mu* z may be desirable1 by ! eyepiece ( ! in/rum5t4 ,9 broad t]ms1 !n1 ! eyepiece is a simple microscope appli$ to ! image ( an object in/ead ( ! object itself4 ,& look+ at ! matt] 9 ! simple/ way ! magnify+ p[] ( any simple l5s dep5ds on ! focal l5g? ( t l5s comp>$ ) ! ord9>y see+ di/ance ( ! eye4 ,if ?is be tak5 at #aj in*es z x (t5 conv5tionally is1 !n a l5s ( #a in* focus br+s cle> vision d[n to an in* f ! object1 increases ! app>5t angle cov]$ by ! object #aj times & h5ce gives a magnify+ p[] ( #aj4 ,b if ! objective has a focal l5g? ( #ajj in*es ! image1 z we h j se51 is already magnifi$ #aj times z ! nak$ eye sees x1 h5ce ) an objective ( #ajj in*es focus & a #a in* eyepiece ! total magnification is #ajj diamet]s4 ,& ?is expresses ! g5]al law1 = if we took ! normal see+ di/ance ( ! nak$ eye at "s o!r value ?an #aj in*es1 say #ab in*es !n we %\ld h to reckon ! image z magnifi$ by #h times s f> z ! objective in*es is conc]n$1 b #ab times due to ! #a in* eyepiece1 & s =?4 ,?us ! magnify+ p[] ( any eyepiece is ,f_/f :]e ,f is ! focal l5g? ( ! objective or mirror & f t ( ! eyepiece4 ,! focal di/ance ( ! eye q drops out ( ! reckon+4 ,all ^! facts appe> v quickly if "o explores ! image f an objective ) a slip ( gr\nd glass & a pocket l5s4 ,an ord9>y cam]a tells ! same /ory4 ,a di/ant object :i* cov]s #a w cov] on ! gr\nd glass #a reckon$ on a radius equal to ! focal l5g? ( ! l5s4 ,if ?is is equal to ! ord9>y di/ance ( cle> vision1 an eye at ! same di/ance w see ! image 7or ! di/ant object7 cov]+ ! same #a4 ,! geometry ( ! situation is z foll[s3 ,let o ,fig4 #e1 ,*ap4 #a1 be ! objective4 ,?is l5s1 z 9 an ord9>y cam]a1 =ms an inv]t$ image ( an object ,a ,b at its focus1 z at a b1 & = any po9t1 z a1 ( ! image !re is a correspond+ po9t ( ! object ly+ on ! /raight l9e f ,a to t po9t ?r\gh ! c5t]1 c1 ( ! objective4 ,a pair ( rays #a1 #b1 div]g+ f ! object po9t ,a pass ?r\gh rim & c5t] ( o respectively & meet 9 ,a4 ,aft] cross+ at ?is po9t !y fall on ! eye l5s e1 & if a is ne>ly 9 ! pr9cipal focus ( e1 ! rays #a & #b w em]ge sub/antially p>allel s t ! eye w unite !m to =m a cle> image4 ,n[ if ,f is ! focal l5g? ( o1 & f t ( a1 ! object =m+ ! image subt5ds at ! c5t] ( ! objective1 o1 an angle ,a c ,b1 & = a di/ant object ?is w be s5sibly ! angle "u :i* ! eye sees ! same object4 ,if ,l is ! half l9e> dim5sion ( ! image1 ! eye sees half ! object cov]+ ! angle ^: tang5t is ,l_/,f4 ,simil>ly half ! image ab is se5 ?r\gh ! eye l5s e z cov]+ a half angle ^: tang5t is ,l_/f4 ,s9ce ! magnify+ p[] ( ! comb9ation1 m1 is directly z ! ratio ( increase 9 ?is tang5t ( ! visual angle1 :i* measures ! image dim5sion m ,f_/f1 z be=e ,fur!r1 z all ! light :i* comes 9 p>allel ?r\gh ! :ole op5+ ( ! objective =ms a s+le conical beam conc5trat+ into a focus & !n div]g+ to ent] ! eye l5s1 ! diamet] ( ! cone com+ ?r\gh ! eye l5s mu/ be> ! same relation to ! diamet] ( o1 t f does to ,f4 ,any less diamet] ( e w cut (f "p ( ! em]g+ light2 any m w %[ an em]g5t beam small] ?an ! eye l5s1 :i* is g5]ally ! case4 ,h5ce if we call p ! diamet] ( ! bright p5cil ( light :i* we see com+ ?r\gh ! eye l5s !n = t p>ticul> eye l5s1 m o_/p ,t is1 f p,f_/o :i* is q ! easie/ way ( measur+ ! focal l5g? ( an eyepiece4 ,po9t ! telescope t[>d ! cle> sky1 focus+ = a di/ant object s t ! em]g5t p5cil is %>ply def9$ at ! ocul>1 & !n measure its diamet] by ! help ( a f9e scale & a pocket l5s1 tak+ c>e t scale & em]g5t p5cil are simultane\sly 9 %>p focus & %[ no p>allax z ! eye is %ift$ a bit4 ,?is bright circle ( ! em]g+ beam is actually ! projection by ! eye l5s ( ! focal image ( ! objective ap]ture4 ,?is me?od ( measur+ p[] is easy & r accurate4 ,b x leads to tr\ble if ! measur$ diamet] ( ! objective is 9 fact contract$ by a /op any:]e along ! pa? ( ! beam1 z occasionally happ5s4 ,exam9e ! telescope c>efully ) ref]5ce to ?is po9t be=e ?us te/+ ! p[]4#ag #ag ,a m precise me?od1 dep5d+ on an actual measurem5t ( ! angle subt5d$ by ! diamet] ( ! eyepiece diaphragm z se5 ?r\gh ! eye end ( ! ocul> & its comp>ison ) ! same angul> diamet] reckon$ f ! objective1 is giv5 by ,s*aeb]le4 ,m4 ,n4 #dc1 #big4 ,! eye l5s ( ,fig4 #e is a simple d\ble convex "o1 su* z 0 us$ by ,*ri/oph] ,s*e9] & 8 contempor>ies4 ,) a fir/ class objective or mirror ! simple eye l5s su* z is %[n 9 ,fig4 #ih;a is by no means to be despis$ ev5 n[4 ,sir ,william ,h]s*el always pref]r$ x = high p[]s1 & speaks ) evid5t contempt ( obs]v]s :o sacrific$ its advantages to ga9 a bigg] field ( view4 ,let u try to fa?om ! reason = 8 vigor\sly express$ op9ion1 /rongly back$ up by exp]i5c$ obs]v]s l ! late ,t4 ,w4 ,webb & ,mr4 ,w4 ,f4 ,d5n+4 ,fir/ ( all a s+le l5s saves ab\t #aj ( ! light4 ,ea* surface ( glass ?r\gh :i* light passes transmits #ie to #if ( t light1 s t a s+le l5s transmits approximately #ij1 two l5ses #ha & s on4 ,?is loss may be en\gh to det]m9e ! visibility ( an object4 ,sir ,wm4 ,h]s*el f\nd t fa9t objects invisible ) ! ord9>y two l5s eyepiece came to view ) ! s+le l5s4 ,probably ! actual loss is less s]i\s ?an its effect on see+ conditions4 ,! loss is due sub/antially to reflection at ! surfaces1 & ! light ?us reflect$ is scatt]$ close to1 or into1 ! eye & produces /ray light 9 ! field :i* injures ! contra/ by :i* fa9t objects become visible4 ,9 "s eyepieces ! =m ( ! surfaces is su* t reflect$ light is /rongly conc5trat$ :]e ! eye sees x1 =m+ a 8gho/0 q bright en\gh greatly to int]f]e ) ! vision ( delicate contra/s4 ,! s+le l5s has a v small %>p field1 h>dly #aj 9 angul> ext5t1 ! image fall+ (f rapidly 9 quality z x dep>ts f ! axis4 ,if plano-convex1 z is ! eye l5s ( common two-l5s ocul>s1 x works be/ ) ! curv$ side to ! eye1 i4e41 rev]s$ f its usual position1 ! sph]ical ab]ration be+ mu* less 9 ?is position4 ,illu/ration3 ,,fig4 #ih4--,simple ,ocul>s4 ,h]s*el's report ( bett] def9ition ) a s+le l5s ?an ) an ord9>y two l5s ocul> speaks ill = ! quality ( ! latt] !n available4 ,( c\rse ! s+le l5s gives "s *romatic ab]ration1 g5]ally ( small acc\nt ) ! n>r[ p5cils ( light us$ 9 high p[]s4 ,a some:at bett] =m ( eye l5s occasionally us$ is ! s-call$ ,codd+ton l5s1 really devis$ by ,sir ,david ,brew/]4 ,?is1 ,fig4 #ih;b1 is d]iv$ f a glass sph]e ) a ?ick equatorial belt remov$ & a groove cut d[n c5trally leav+ a diamet] ( less ?an half ! radius ( ! sph]e4 ,! focus is1 = ord9>y cr[n glass1 #c_/b ! radius ( ! sph]e1 & ! field is a little improv$ ov] ! simple l5s1 b x falls (f r rapidly1 ) consid]able color t[>d ! edge4 ,! obvi\s /ep t[>d full] correction ( ! ab]rations :ile reta9+ ! advantages ( ! simple l5s is to make ! ocul> a*romatic1 l a m9ute objective1 ?us correct+ at once ! *romatic & sph]ical ab]rations ov] a reasonably l>ge field4 ,z ! compon5ts are cem5t$ ! loss ( light at _! common surface is negligible4 ,figure #ih;c %[s su* a l5s4 ,if correctly design$ x gives an admirably %>p field ( #ae to #bj1 colorless & ) v little di/ortion1 & is well adapt$ = high p[]s4 ,illu/ration3 a b ,,fig4 #ii4--,triple ,cem5t$ ,ocul>s4 ,/ill bett] results 9 field & or?oscopy c be atta9$ by go+ to a triple cem5t$ l5s1 simil> to ! objective ( ,fig4 #eg4 ,triplets ?us con/itut$ are made abroad by ,zeiss1 ,/e9heil & o!rs1 :ile 9 ?is c\ntry an admirable triplet design$ by ,pr(essor ,ha/+s is made by ,baus* ,lomb4 ,illu/ration3 ,,fig4 #ajj4--,pa? ( ,rays ,?r\gh ,huyg5ian ,ocul>4 ,su* l5ses give a beautifully flat & %>p field ov] an angle ( #bj to #cj1 q colorless & or?oscopic4 ,fig4 #ii;a1 a =m us$ by ,/e9heil1 is an excell5t example ( ! con/ruction & a mo/ useful ocul>4 ,! late ,r4 ,b4 ,tolles made su* triplets1 ev5 d[n to in* focus1 :i* gave admirable results4 ,a highly specializ$ =m ( triplet is ! s-call$ monoc5tric ( ,/e9heil ,fig4 #ii;b4 ,its peculi>ity is less 9 ! fact t all ! curves are /ruck f ! same c5t] ?an 9 ! great ?ickness ( ! front fl9t & ! cr[n1 :i*1 z 9 "s photographic l5ses1 give add$ facilities = flatt5+ ! field & elim9at+ di/ortion4 ,! monoc5tric eyepiece has a high reputation = ke5 def9ition & is admirably a*romatic & or?oscopic4 ,! %>p field is ab\t #cb1 r ! l>ge/ giv5 by any ( ! cem5t$ comb9ations4 ,all ^! optically s+le l5ses are q free ( gho/s1 r$uce scatt]$ light to a m9imum1 & leave little to be desir$ 9 precise def9ition4 ,! weak po9t ( ! :ole tribe is ! small field1 :i*1 despite ,h]s*el's op9ion1 is a real disadvantage = c]ta9 k9ds ( "w & wa/es ! obs]v]'s "t unless 8 facilities = close sett+ are m ?an usually good4 ,h5ce ! g5]al use ( ocul>s ( ! two l5s types1 all ( !m giv+ relatively wide fields1 "s ( !m faultless also 9 def9ition & or?oscopy4 ,! e>lie/ =m1 ,fig4 #ajj1 is ! v useful & common "o us$ by ,huyg5s & be>+ 8 "n1 ?\gh p]haps indep5d5tly devis$ by ,campani ( ,rome4 ,probably f\r out ( five a/ronomical eyepieces belong to ?is class4 ,! ,huyg5ian ocul> accompli%es two useful results--fir/1 x gives a wid] %>p field ?an any s+le l5s1 & second x comp5sates ! *romatic ab]ration1 :i* o!rwise mu/ be remov$ by a composite l5s4 ,x usually consi/s ( a plano-convex l5s1 convex side t[>d ! objective1 :i* is br\ght inside ! objective focus & =ms an image 9 ! plane ( a re> diaphragm1 & a simil> eye l5s ( %ort] focus by :i* ?is image is exam9$4 ,fig4 #ajj %[s ! c\rse ( ! rays--,a be+ ! field l5s1 ,b ! diaphragm & ,c ! eye l5s4 ,let #a1 #b1 be rays :i* are incid5t ne> ! m>g9 ( ,a4 ,ea*1 9 pass+ ?r\gh ! l5s1 is disp]s$1 ! blue be+ m refract$ ?an ! r$4 ,bo? rays come to a g5]al focus at ,b1 &1 cross+1 div]ge slightly t[>ds ,c4 ,b1 on rea*+ ,c1 ray #a1 t 0 ne>] ! m>g9 & ! m refract$ because 9 a zone ( great] pit*1 n[ falls on ,c ! ne>] its c5t]1 & is less refract$ ?an ray #b :i* /rikes ,c ne>] ! rim4 ,if ! curvatures ( ,a & ,c are prop]ly relat$ #a & #b em]ge f ,c p>allel to ea* o!r & ?us unite 9 =m+ a di/9ct image4 ,n[ foll[ ?r\gh ! two bran*es ( l m>k$ lr1 & lv1 ! r$ & violet compon5ts4 ,ray lv1 ! m refrangible1 /rikes ,c ne>] ! c5t]1 & is ! less refract$1 em]g+ f ,c sub/antially p>allel ) its mate lr1 h5ce bl5d+ ! r$ & violet images1 if1 be+ ( ! same glass1 ,a & ,c h suitably relat$ focal l5g?s & sep>ation4 ,z a matt] ( fact ! condition = ?is *romatic comp5sation is d 7f f7_/#b :]e d is ! di/ance betwe5 ! l5ses & f1 f1 _! respective focal l5g?s4 ,if ?is condition ( a*romatism be comb9$ ) t ( equal refraction at ,a & ,c1 favorable to m9imiz+ ! sph]ical ab]ration1 we f9d f #c;f & d #b;f4 ,?is is ! conv5tional ,huyg5ian ocul> ) an eye l5s ! focus ( ! field l5s1 spac$ at d\ble ! focus ( ! eye l5s1 ) ! diaphragm midway4 ,9 practice ! ratio ( foci v>ies f #a3c to #a3b or ev5 #a3a4e1 ! exact figure v>y+ ) ! am\nt ( ov]correction 9 ! objective & "u-correction 9 ! eye t has to be dealt )1 :ile ! value ( d %\ld be adju/$ by actual trial on ! telescope to obta9 ! be/ color correction practicable4 ,"o _c use any *ance ocul> & expect ! f9e/ results4 ,illu/ration3 ,,fig4 #aja;a4--,airy & ,mitt5zuey ,ocul>s4 ,! ,huyg5ian eyepieces are (t5 ref]r$ to z 8negative0 inasmu* z !y _c be us$ directly z magnifi]s1 al?\gh deal+ effectively ) an image r ?an an object4 ,! /atem5t is also (t5 made t !y _c be us$ ) cross wires4 ,?is is incorrect1 = :ile !re is noticeable di/ortion t[>d ! edge ( ! wide field1 to say no?+ ( a/igmatism1 9 & ne> ! c5t] ( ! field ! situation is a good deal bett]4 ,c5tral cross wires 9 ! plane ( ! diaphragm are entirely suitable = alignm5t ( ! in/rum5t1 & ov] a mod]ate ext5t ( field ! di/ortion is s small t a micromet] scale 9 ! plane ( ! diaphragm gives v good approximate measurem5ts1 & inde$ is widely us$ 9 microscopy4 ,x %\ld be not$ t ! a*romatism ( ?is type ( eyepiece is comp5satory r ?an real4 ,"o _c at ! same "t br+ ! images ( v>i\s colors to ! same size1 & also to ! same plane4 ,z failure 9 ! latt] respect is comp>atively unimportant1 ! ,huyg5ian eyepiece is adju/$ s f> to comp5sate ! pa?s ( ! v>i\s rays z to br+ ! color$ images to ! same size1 & 9 po9t ( fact ! result is v good4 ,! field ( ! conv5tional =m ( ,huyg5ian ocul> is fully #dj1 & ! def9ition1 p>ticul>ly c5trally1 is v excell5t4 ,!re are no p]ceptible gho/s produc$1 & :ile "s #aj ( light is lo/ by reflection 9 ! extra l5s x is diffus$ 9 ! g5]al field & is damag+ only z x injures ! contra/ ( fa9t objects4 ,! !ory ( ! ,huyg5ian eyepiece 0 elaborately giv5 by ,littr[1 7,memoirs ,r4 ,a4 ,s4 ,vol4 #d1 p4 #eii71 :]e9 ! some:at intricate geometry ( ! situation is fully discuss$4 ,v>i\s modifications ( ! ,huyg5ian type h be5 devis$ & us$4 ,figure #aja;a is ! ,airy =m devis$ z a result ( a some:at full ma!matical inve/igation by ,sir ,george ,airy1 lat] ,a/ronom] ,royal4 ,its peculi>ity lies 9 ! =m ( ! l5ses :i* pres]ve ! usual #c3a ratio ( focal l5g?s4 ,! field l5s is a positive m5iscus ) a noticeable am\nt ( concavity 9 ! re> face :ile ! eye l5s is a 8cross$0 l5s1 ! out] curvature be+ ab\t ( ! inn] curvature4 ,! m>g9al field 9 ?is ocul> is a little bett] ?an 9 ! conv5tional ,huyg5ian4 ,illu/ration3 ,,fig4 #aja;b4--,airy & ,mitt5zwey ,ocul>s4 ,a common] modification n[-a-days is ! ,mitt5zwey =m1 ,fig4 #aja;b4 ,?is is usually made ) #b3a ratio ( focal l5g?s1 & ! field l5s /ill a m5iscus1 b less conspicu\sly concave ?an 9 ! ,airy =m4 ,! eye l5s is ! usual plano-convex4 ,x is widely us$1 especially abroad1 & gives p]haps z l>ge available field z any ocul> yet devis$1 approximately #ej1 ) pretty good def9ition out to ! m>g94 ,f9ally1 we come to ! solid eyepiece ,fig4 #ajb;a1 devis$ by ! late ,r4 ,b4 ,tolies ne>ly ?ree qu>t]s ( a c5tury ago1 & & (t5 made by him bo? = telescopes & microscopes4 ,x is practically a ,huyg5ian eyepiece made out ( a s+le cyl9d] ( glass ) a curvature ratio ( #a3a betwe5 ! eye & ! field l5s4 ,a groove is cut ar\nd ! long l5s at ab\t its l5g? f ! v]tex ( ! field end4 ,?is s]ves z a /op1 r$uc+ ! diamet] ( ! l5s to ab\t "o-half its focal l5g?4 ,x is 9 fact a ,huyg5ian eyepiece free f ! loss ( light 9 ! usual con/ruction4 ,x gives a wide field1 m ext5sive ?an 9 ! ord9>y =m1 ) exquisite def9ition4 ,x is really a mo/ admirable =m ( eyepiece :i* %\ld be us$ f> m ?an is n[ ! case4 ,! late ,dr4 ,bra%e> is on record z believ+ t all negative eyepieces less ?an in* focus %\ld be made 9 ?is =m4 ,illu/ration3 a b ,,fig4 #ajb4--,tolles' ,solid & ,comp5sat$ ,ocul>s4 ,s f> z ! writ] c asc]ta9 ! only reason t x is n m us$ is t x is some:at m difficult to con/ruct ?an ! two l5s =m1 = its curvatures & l5g? mu/ be v accurately adju/$4 ,x is consequ5tly unpopul> ) ! con/ruct+ optician 9 spite ( its conspicu\s m]its4 ,x gives no gho/s1 & ! fa9t reflection at ! eye end is widely spread s t if ! ext]ior ( ! cyl9d] is well black5$1 z x %\ld be1 x gives exceptional fre$om f /ray light4 ,/ill ano!r v>iety ( ! ,huyg5ian ocul> sometimes useful is analog\s to ! comp5sat+ eyepiece us$ 9 microscopy4 ,if1 z commonly is ! case1 a telescope objective is ov]-correct$ = color to correct = ! *romatism ( ! eye 9 l[ p[]s1 ! high p[]s %[ /rong ov] correction1 ! blue focus be+ long] ?an ! r$1 & ! blue image !re=e ! l>g]4 ,if n[ ! field l5s ( ! ocul> be made ( heavy fl9t glass & ! sep>ation ( ! l5ses suitably adju/$1 ! /rong] refraction ( ! field l5s = ! blue pulls up ! blue focus & br+s its image to sub/antially ! dim5sions ( ! r$1 s t ! eye l5s p]=ms z if !re 7 no ov]correction ( ! objective4 ,! writ] has exp]im5t$ ) an ocul> ( ?is sort z %[n 9 ,fig4 #ajb;b & f9ds t ! color correction is1 z might be expect$1 greatly improv$ ov] a ,mitt5zwey ocul> ( ! same focus 7 in*74 ,!re w\ld be mat]ial advantage 9 ?us v>y+ ! ocul> color correction to suit ! p[]4 ,9 ! ,huygh5ian eyepiece ! equival5t focal l5g? ,f is giv5 by1 ,f #b;ff_/7f f7 :]e f & f are ! focal l5g?s ( ! field & eye l5ses respectively4 ,?is assumes ! normal spac+1 d1 ( half ! sum ( ! focal l5g?s1 n always adh]$ to by con/ructors4 ,! p]fectly g5]al case1 z = any two comb9$ l5ses is1 ,f ff#a_/7;f f#a-;d7 ,illu/ration3 ,,fig4 #ajc4--,pa? ( ,rays ,?r\gh ,ramsd5 ,ocul>4 ,to obta9 a flatt] field1 & p>ticul>ly "o free f di/ortion ! con/ruction devis$ by ,ramsd5 is commonly us$4 ,?is consi/s1 ,fig4 #ajc1 ( two plano convex l5ses ( equal focal l5g?1 plac$ ) _! plane faces outw>d1 at a di/ance equal to1 or some:at less ?an1 _! common focal l5g?4 ,! =m] spac+ is ! "o :i* gives ! be/ a*romatic comp5sation s9ce z be=e ! condition = a*romatism is d 7f f7 ,:5 ?us spac$ ! plane surface ( ! field l5s is exactly 9 ! focus ( ! eye l5s1 ! comb9$ focus ,f is ! same z t ( ei!r l5s1 s9ce z j %[n 9 any additive comb9ation ( two l5ses ,f ff_/7f f-d7 & :ile ! field is flat & colorless1 e speck ( du/ on ! field l5s is (f5sively 9 view4 ,x is !re=e usual to make ?is ocul> 9 ! =m sugge/$ by ,airy1 9 :i* some?+ ( ! a*romatic correction is sacrific$ to obviate ?is difficulty1 & to obta9 a bett] balance ( ! residual ab]rations4 ,! pa? ( ! rays is %[n 9 ,fig4 #ajc4 ,! l5ses ,a & ,b are ( ! same focal l5g? b are n[ spac$ at ( ?is l5g? ap>t4 ,! two neighbor+ rays #a1 #b1 com+ ?r\gh ! objective f ! di/ant object meet at ! objective focus 9 a po9t1 a1 ( ! image plane a b4 ,!nce1 div]g+1 !y are s refract$ by ,a & ,b z to leave ! latt] sub/antially p>allel s t bo? appe> to proce$ f ! po9t c1 ( ! image plane c1 d1 9 ! pr9cipal focus ( ,b4 ,f ! ord9>y equation = ! comb9ation1 ,f f4 ,! comb9ation focusses f back ( ! pr9cipal focus ( ! objective1 & ! position ( ! eye is ,f back ( ! eye l5s1 :i* is ano!r reason = %ort5+ ! l5s spac+4 ,at long] spac+ ! eye di/ance is inconv5i5tly r$uc$4 ,?us con/itut$1 ! ,ramsd5 ocul>1 kn[n z 8positive0 f its capability = use z a magnifi] ( actual objects1 gives a good flat field free f di/ortion ov] a field ( ne>ly #ce & at "s loss ( def9ition a little m4 ,x is ! =m mo/ commonly us$ = micromet] "w4 ,9 all optical in/rum5ts ! ab]rations increase z "o dep>ts f ! axis1 s t angul> field is r a loose t]m dep5d+ on ! maximum ab]rations t c be tol]at$4#ah #ah ,! angul> field a is def9$ by tan a _/,f :]e is1 num]ically1 ! radius ( ! field %>p en\gh = ! purpose 9 h&1 & ,f ! effective focal l5g? ( ! ocul>4 ,( ! ,ramsd5 ocul> !re are _m modifications4 ,sometimes f & f are made unequal or !re is dep>ture f ! simple plano-convex =m4 ,m (t5 ! l5ses are made a*romatic1 ?us gett+ rid ( ! v p]ceptible color 9 ! simpl] =m & mat]ially help+ ! def9ition4 ,figure #ajd;a %[s su* an a*romatic ocul> z made by ,/e9heil4 ,! g5]al arrangem5t is z 9 ! ord9>y ,ramsd51 b ! %>p field is slightly enl>g$1 a good #cf1 & ! def9ition is improv$ q noticeably4 ,a some:at analog\s =m1 b consid]ably modifi$ 9 detail1 is ! ,kelln] ocul>1 ,fig4 #ajd;b4 ,x 0 devis$ by an optician ( t "n1 ( ,wetzl>1 :o exploit$ x "s ?ree qu>t]s ( a c5tury s9ce 9 a little bro*ure entitl$ 8,das or?oskopis*e ,okul>10 z notable a bla/ ( 8hot air0 z ev] came f a mod]n publicity ag5t4 ,z made today ! ,kelln] ocul> consi/s ( a field l5s :i* is commonly plano-convex1 plano side out1 b sometimes cross$ or ev5 equiconvex1 comb9$ ) a consid]ably small] eye l5s :i* is an ov]-correct$ a*romatic4 ,! focal l5g? ( ! field l5s is approximately #g_/d ,f1 t ( ! eye l5s #d_/c ,f1 sep>at$ by ab\t ,f4 ,?is ocul> has its front focal plane v ne> ! field l5s1 sometimes ev5 )9 its sub/ance1 & a r %ort eye di/ance1 b x gives admirable def9ition & a usable field ( v great ext5t1 colorless & or?oscopic to ! edge4 ,! writ] has "o ( #b focus1 ) an a*romatic triplet z eye l5s1 :i* gives an admirable field ( q #ej4 ,illu/ration3 ,,fig4 #ajd4--;,a*romatic & ,kelln] ,ocul>s4 ,! ,kelln] is decid$ly valuable z a wide field positive ocul>1 b x has 9 common ) ! two j previ\sly describ$ a sometimes unpleasant gho/ ( bright objects4 ,?is arises f light reflect$ f ! inn] surface ( ! field l5s1 & back aga9 by ! front surface to a focus4 ,?is focus commonly lies n f> back ( ! field l5s & q too ne> to ! focus ( ! eye l5s = com=t4 ,x %\ld be wat*$ = 9 go+ aft] fa9t objects ) ocul>s ( ! types not$4 ,a decid$ly bett] =m ( positive ocul> is ! mod]n or?oscopic z made by ,/e9heil & ,zeiss1 ,fig4 #aje;a4 ,x consi/s ( a triple a*romatic field l5s1 a d5se fl9t betwe5 two cr[ns1 ) a plano-convex eye l5s ( mu* %ort] focus 7 to 7 almo/ 9 contact on its convex side4 ,! field triplet is heavily ov]-correct$ = color1 ! front focal plane is ne>ly ,f ahead ( ! front v]tex ( ! field l5s1 & ! eye di/ance is notably great] ?an 9 ! ,kelln]4 ,! field is above #dj1 beautifully flat1 %>p1 & or?oscopic1 free ( tr\blesome gho/s4 ,on ! :ole ! writ] is incl9$ to rate x z ! be/ ( two-l5s ocul>s4 ,!re %\ld also "h be m5tion$ a v useful long relief ocul>1 (t5 us$ = artill]y sights1 & %[n 9 ,fig4 #aje;b4 ,x consi/s l ,fig4 #ajd;a1 ( a pair ( a*romatic l5ses1 b !y are plac$ ) ! cr[ns almo/ 9 contact & are frequ5tly us$ ) a simple plano convex field l5s ( mu* long] focus1 to r5d] ! comb9ation m fully or?oscopic4 ,! field1 especially ) ! field l5s1 is wide1 q #dj z app>5t angle = ! :ole in/rum5t1 & ! eye di/ance is r\ghly equal to ! focal l5g?4 ,x is a =m ( ocul> t might be v advantage\sly us$ 9 f9d]s1 :]e "o (t5 has to assume uncom=table angles ( view1 & long relief is valuable4 ,illu/ration3 a b ,,fig4 #aje4--,or?oscopic & ,long ,relief ,ocul>s4 ,:atev] ! app>5t angul> field ( an ocul> may be1 ! real angul> field ( view is obta9$ by divid+ ! app>5t field by ! magnify+ p[]4 ,?us ! au?or's big ,kelln]1 j m5tion$1 gives a p[] ( #bj ) ! objective = :i* x 0 design$1 h5ce a real field ( #b1 :ile a second1 p[] #fe1 gives a real field ( h>dly #jdj1 ! app>5t field 9 ?is case be+ a trifle ov] #dj4 ,!re is no escap+ ?is relation1 s t high p[] always implies small field4 ,! limit ( app>5t field is due to increas+ errors away f ! axis1 /rong curvature ( ! field1 & p>ticul>ly a/igmatism 9 ! out] zones4 ,! eye itself c take 9 only ab\t #dj s t m ?an ?is1 :ile atta9able1 c only be utiliz$ by pe]+ ar\nd ! m>g9al field4 ,= l[ p[]s ! usable field is help$ out by ! accommodation ( ! eye1 b 9 ocul>s ( %ort focus ! curvature ( field is ! limit+ factor4 ,! radius ( curvature ( ! image is1 9 a s+le l5s approximately #c_/b ,f1 & 9 ! common two l5s =ms ab\t ,f4 ,9 consid]+ ?is matt] ,conrady has %[n 7,m4 ,n4 #gh #dde7 t = a total field ( #dj ! %>pness ( field fails at a focal l5g? ( ab\t #a in* = normal p[] ( accommodation4 ,! be/ a*romatic comb9ations r$uce ?is limit to ab\t in*4 ,at focal l5g?s bel[ ?is ! %>pe/ field is obta9able only ) r$uc$ ap]ture4 ,!re is an int]e/+ possibility ( build+ an ana/igmatic ocul> on ! l9es ( ! mod]n photographic l5s1 :i* ,conrady sugge/s1 b ! ne$ ( wide field 9 high p[]s is h>dly press+ en\gh to /imulate rese>*4 ,illu/ration3 ,,fig4 #ajf4--,ord9>y ,t]re/rial ,ocul>4 ,f9ally we may pass to ! v simple adjunct ( mo/ small telescopes1 ! t]re/rial ocul> :i* inv]ts ! image & %[s ! l&scape "r side up4 ,:atev] its exact =m x consi/s ( an inv]t+ sy/em :i* erects ! inv]t$ image produc$ by ! objective alone1 & an eyepiece = view+ ?is erect$ image4 ,9 its common =m x is compos$ ( f\r plano-convex l5ses arrang$ z 9 ,fig4 #ajf4 ,"h ,a & ,b = ! inv]t+ pair & ,c & ,d a modifi$ ,huyg5ian ocul>4 ,! image f ! objective is =m$ 9 ! front focus ( ,,ab :i* is practically an inv]t$ ocul>1 & ! erect$ image is =m$ 9 ! usual way betwe5 ,c & ,d4 ,! app>5t field is fairly good1 ab\t #ce1 & :ile slightly bett] corrections c be ga9$ by us+ l5ses ( specially adju/$ curvatures1 z ,airy has %[n1 ^! are seldom appli$4 ,! *ief objection to ?is erect+ sy/em is its l5g?1 "s t5 times its equival5t focus4 ,n[ & !n to save light & ga9 field1 ! erector is a s+le cem5t$ comb9ation & ! ocul> l ,fig4 #ii;a or ,fig4 #ajb;a4 ,fig4 #ajg %[s a t]re/rial eyepiece s arrang$1 f an example by ! late ,r4 ,b4 ,tolles4 ,:5 c>efully design$ an app>5t field ( #dj or m c be secur$1 ) great brilliancy1 & ! l5g? ( ! erect+ sy/em is mod]ate4 ,v mu* ak9 9 pr9ciple is ! eyepiece microscope1 su* z is made by ,zeiss to give v>iable p[] & a conv5i5t position ( ! eye 9 connection ) fil> micromet]s1 ,fig4 #ajh4 ,x is provid$ ) a focuss+ coll> & its draw tube all[s v>y+ p[] j z 9 case ( an ord9>y microscope4 ,9 fact eyepiece microscopes h long be5 n[ & !n us$ to advantage = high p[]s4 ,!y are easi] on ! eye1 & give great] eye di/ance ?an ! exce$+ly small eye l5ses ( %ort focus ocul>s1 & us+ a solid eyepiece & s+le l5s objective lose no m light ?an an ord9>y ,huyg5ian ocul>4 ,! erect resultant image is occasionally a conv5i5ce 9 a/ronomical use4 ,illu/ration3 ,,fig4 #ajg4--,tolles ,triplet ,inv]t+ ,sy/em4 ,illu/ration3 ,,fig4 #ajh4--,microscope z ,ocul>4 ,illu/ration3 ,,fig4 #aji4--8;,davon0 ,in/rum5t4 ,q analog\s to ! eyepiece microscope is ! s-call$ 8,davon0 micro-telescope4 ,orig9ally develop$ z an atta*m5t = ! sub/age ( a microscope to give l>ge images ( objects at a little di/ance x has gr[n also into a sep>ate h& telescope1 monocul> or b9ocul>1 = g5]al purposes4 ,! atta*m5t ?us develop$ is %[n complete 9 ,fig4 #aji4 ,d is m]ely a well correct$ objective set 9 a m\nt provid$ ) ample /ops4 ,! image is view$ by an ord9>y microscope or special eyepiece microscope ,a1 z ! case may be1 furni%$ ) rack focuss+ at ,a & assembl$ ) ! objective by means ( ! c>efully c5t]$ c\pl+ ,c4 ,x furni%es a compact & p[]ful in/rum5t1 v suitable = t]re/rial or m9or a/ronomical uses1 l ! ,tolles' %ort-focus h& telescopes already m5tion$4 ,:5 prop]ly design$ telescopes ( ?is sort give ne>ly ! field ( prism glasses1 weigh mu* less & lose f> less light = ! same effective p[] & ap]ture4 ,!y also h "u fairly high p[]s r ! advantage 9 ! matt] ( def9ition1 o!r ?+s be+ equal4 ,,*apt] ,,vii ,,h& ,,telescopes ,,& ,,b9ocul>s ,! h& telescope f9ds comp>atively little use 9 obs]v+ cele/ial bodies4 ,x is usually q too small = any except v limit$ applications1 & _c be giv5 suffici5t p[] )\t be+ difficult to keep /eady except by ! aid ( a fix$ m\nt+4 ,/ill1 = c]ta9 "w1 especially ! obs]vation ( v>iable />s1 x f9ds useful purpose if suffici5tly compact & ( good light-ga!r+ p[]4 ,!re is mo/ decid$ly a limit to ! magnify+ p[] :i* c be giv5 to an in/rum5t held 9 ! h& )\t mak+ ! outfit too un/eady to be s]viceable4 ,any?+ beyond #h to #aj diamet]s is highly tr\blesome1 & requires a rudim5t>y m\nt or at lea/ /eady+ ! h& aga9/ some?+ 9 ord] to obs]ve ) com=t4 ,! long] ! in/rum5t ! m difficult x is to manage1 & ! be/ results ) h& telescopes are to be obta9$ ) %ort in/rum5ts ( relatively l>ge diamet] & l[ p[]4 ,! ord9>y field glass ( ,galilean type comes imm$iately to m9d & 9 fact ! field glass is & has be5 mu* us$4 ,z ord9>ily con/ruct$ x is optically r crude = a/ronomical purposes4 ,! objectives are r>ely well figur$ or accurately c5t]$ & a bright /> usually appe>s z a wobbly fl>e r ?an a po9t4 ,fur!rmore ! field is g5]ally small1 & ( q unev5 illum9ation f c5tre to p]iph]y1 s t great caution has to be ex]cis$ 9 judg+ ! brightness ( a />1 accord+ to its position 9 ! field4 ,! l5s diamet] possible ) a field glass ( ord9>y con/ruction is limit$ by ! limit$ di/ance betwe5 ! eyes1 :i* mu/ be well c5t]$ on ! eyepieces to obta9 cle> vision4 ,! int]-pupill>y di/ance is g5]ally a scant #b in*es s t ! cle> ap]ture ( "o ( ! objectives ( a field glass is r>ely c>ri$ up to #b in*es4 ,! be/ field glasses h ea* objective a triple cem5t$ l5s1 & ! concave l5ses also triplets1 ! arrangem5t ( p>ts be+ t %[n 9 ,fig4 #aaj4 ,glasses ( ?is sort r>ely h a magnify+ p[] above #e4 ,9 select+ a field glass ) ! idea ( us+ x on ! sky try x on a bright />1 real or artificial1 & if ! image ) c>eful focuss+ does n pull d[n to a pretty small & uni=m po9t take no fur!r int]e/ 9 ! in/rum5t4 ,illu/ration3 ,,fig4 #aaj4--,optical ,p>ts ( ,field ,glass4 ,! advantage ( a b9ocul> in/rum5t is popul>ly mu* exagg]at$4 ,x gives a some:at delusive appe>ance ( brilliancy & cle>ness :i* is psy*ological r ?an physical4 ,dur+ ! late w> a v c>eful rese>* 0 made at ! in/ance ( ! ,unit$ ,/ates ,gov]nm5t to det]m9e ! actual value ( a b9ocul> field glass aga9/ a monocul> "o ( exactly ! same type1 ! latt] be+ *eap]1 light]1 & 9 _m respects mu* h&i]4 ,! diff]5ce f\nd 9 po9t ( actual see+ all sorts ( objects "u v>y+ conditions ( illum9ation 0 s small z to be practically negligible4 ,an increase ( less ?an #e p] c5t 9 magnify+ p[] enabl$ "o to see ) ! monocul> in/rum5t ev]y?+ t c\ld be se5 ) ! b9ocul>1 equally well1 & x is altoge!r probable t 9 ! matt] ( see+ f9e detail ! diff]5ce w\ld be ev5 less ?an 9 g5]al use1 s9ce x is n altoge!r easy to get ! two sides ( a b9ocul> work+ toge!r effici5tly or to keep !m s aft]w>ds4 ,!re has be51 !re=e1 a def9ite field = monocul> h& telescopes ( good quality & mod]ate p[] & su* are manufactur$ by "s ( ! be/ ,cont95tal mak]s4 ,su* in/rum5ts h sometimes be5 %ort5$ by build+ !m on ! exact pr9ciple ( ! telephoto l5s1 :i* gives a relatively l>ge image ) a %ort cam]a ext5sion4 ,illu/ration3 ,,fig4 #aaa4--,/e9heil ,%ort5$ ,telescope4 ,a mu* %ort5$ telescope1 z made by ,/e9heil = sol> photographic purposes1 is %[n 9 ,fig4 #aaa4 ,?is in/rum5t ) a total l5g? ( ab\t #b feet & a cle> ap]ture ( #b in*es gives a sol> image ( in* diamet]1 correspond+ to an ord9>y glass ( m ?an d\ble t total l5g?4 ,q ! same pr9ciple has be5 appli$ to t]re/rial telescopes by ! same mak]1 giv+ aga9 an equival5t focus ( ab\t d\ble ! l5g? ( ! :ole in/rum5t4 ,?is id5tical pr9ciple has (t5 be5 us$ 9 ! s-call$ ,b>l[ l5s1 a negative l5s plac$ betwe5 objective & eyepiece & giv+ increas$ magnification ) small increase ( l5g?2 also photographic enl>g]s ( sub/antially simil> function h f\nd consid]able use4 ,a highly effici5t h& telescope = a/ronomical purposes might be con/ruct$ along ?is l9e1 ! great %ort5+ ( ! in/rum5t mak+ x possible to use some:at high] p[]s ?an ! ord9>y )\t too mu* loss ( /ead9ess4 ,!re is also con/ruct$ a b9ocul> = /rictly a/ronomical use consi/+ ( a pair ( small h& comet-seek]s4 ,"o ( ^! little in/rum5ts is %[n 9 ,fig4 #aab4 ,x has a cle> diamet] ( objectives ( #a in*1 magnification ( #e1 & a brilliant & ev5 field ( #g ap]ture4 ,! objectives are triplets l ,fig4 #eg1 already ref]r$ to1 ! ocul>s a*romatic d\blets ( ! type ( ,fig4 #ajd;a4 ,) ! exception ( ^! specializ$ a/ronomical field glasses ! mo/ useful & g5]ally available h& in/rum5t is ! prism glass n[ 9 v g5]al use4 ,x is bas$ on rev]sal ( ! image by int]nal total reflection 9 two prisms hav+ _! reflect+ surfaces p]p5dicul> ea* to ! o!r4 ,! rudim5ts ( ! process lie 9 ! simple rev]sion prism %[n 9 diagram 9 ,fig4 #aac4 ,illu/ration3 ,,fig4 #aab4--;,a/ronomical ,b9ocul>4 ,?is is no?+ m nor less ?an a "r angl$ glass prism set ) its hypo!nuse face p>allel & ) its sides at #de to ! optical axis ( ! in/rum5t4 ,rays fall+ ^u "o ( its refract+ faces at an angle ( #de are refract$ ^u ! hypo!nuse face1 are !re totally reflect$ & em]ge f ! second face ( ! prism p>allel to _! orig9al c\rse4 ,inspection ( ,fig4 #aac %[s t an elem5t l ,a ,b p]p5dicul> to ! plane ( ! hypo!nuse face is inv]t$ by ! total reflection s t x takes ! position ,a ,b4 ,x is equally cle> t an elem5t exactly p]p5dicul> to ,a ,b w be reflect$ f ! hypo!nuse face flatwise z x 71 & w em]ge )\t its ends be+ rev]s$ s t ! net effect ( ?is s+le reflection is to inv]t ! image )\t rev]s+ x lat]ally at ! same "t4 ,on ! o!r h& if a second prism be plac$ beh9d ! fir/1 flat ^u its side1 ) its hypo!nuse face occupy+ a plane exactly p]p5dicul> to t ( ! fir/ prism1 ! l9e ,a,b w be refract$1 totally reflect$ & refract$ aga9 out ( ! prism )\t a second inv]sion1 :ile a l9e p]p5dicul> to ,a,b w be refract$ endwise on ! hypo!nuse face ( ! second prism & w be inv]t$ z 0 ! l9e ,a ,b at ! />t4 ,illu/ration3 ,,fig4 #aac4--,rev]sion ,prism4 ,consequ5tly two prisms ?us plac$ w completely inv]t ! image1 produc+ exactly ! same effect z ! ord9>y inv]t+ sy/em ,fig4 #ajf4 ,! simple rev]sion prism is useful z furni%+ a means1 :5 plac$ ov] an eye l5s1 & rotat$1 ( revolv+ ! image on itself1 a proc$ure occasionally conv5i5t1 especially 9 /ell> photometry4 ,! two prisms toge!r con/itute a true inv]t+ sy/em & h be5 utiliz$ 9 t function1 b !y give a r small angul> field & h nev] come into a mat]ial am\nt ( use4 ,! exact effect ( ?is comb9ation1 kn[n hi/orically z ,dove's prisms1 is %[n pla9ly 9 ,fig4 #aad4 ,! fir/ actual prismatic inv]t+ sy/em 0 due to ,m4 ,porro1 :o inv5t$ x ab\t ! middle ( ! la/ c5tury1 & lat] br\ght x out comm]cially "u ! "n ( 8,lunette ,napoleon ,troisime10 z a glass = milit>y purposes4 ,illu/ration3 ,,fig4 #aad4--;,dove's ,prisms4 ,illu/ration3 ,,fig4 #aae4--,porro's ,prism ,sy/em4 ,! prism sy/em ( ?is /rik+ =m ( in/rum5t is %[n 9 ,fig4 #aae4 ,x 0 compos$ ( ?ree "r angle prisms ,a1 ,b1 & ,c4 ,a pres5t$ a ca!tus face to ! objective & ,b a ca!tus face to ! ocul>4 ,obvi\sly a v]tical elem5t br\ght 9 along a f ! objective w\ld be reflect$ at ! hypo!nuse face b1 to a position at "r angles to ! orig9al "o1 w\ld ent] ! hypo!nuse face ( ,c & !nce aft] two reflections at c & d flatwise & )\t *ange ( direction w\ld em]ge1 ent] ! l[] ca!tus face ( ,b & by reflection at ! hypo!nuse face e ( ,b w\ld be turn$ ano!r #ij mak+ a complete rev]sion z reg>ds up & d[n at ! eye plac$ at f4 ,an elem5t initially at "r angles to ! "o j consid]$ w\ld ent] ,a1 be reflect$ flatwise1 9 ! faces ( ,c be twice reflect$ endwise1 !reby completely inv]t+ x1 & w\ld aga9 be reflect$ flatwise f ! hypo!nuse face ( ,c1 !reby effect+1 z ! pa? ( ! rays indicat$ pla9ly %[s1 a complete inv]sion ( ! image4 ,focuss+ 0 v simply atta9$ by a screw motion affect+ ! prism ,c & ! :ole affair 0 9 a small flat case1 ! ext]nal appe>ance & size ( :i* is indicat$ 9 ,fig4 #aaf4 ,illu/ration3 ,,fig4 #aaf4--,lunette ,napoleon ,troisime4 ,illu/ration3 ,,fig4 #aag4--,porro's ,fir/ ,=m ( ,prisms4 ,f ocul> to objective ! l5g? 0 ab\t an in* & a half4 ,x 0 ( #aj p[] & took 9 a field ( #de y>ds at a di/ance ( #ajjj y>ds4 ,"h = ! fir/ "t we f9d a prismatic inv]t+ sy/em ( /rictly mod]n type4 ,& x is int]e/+ to note t if "o _h wi%$ to make a b9ocul> 8,lunette ,napoleon ,troisime0 he w\ld inevitably h produc$ an in/rum5t ) enhanc$ /]eoscopic effect l ! mod]n prism field glass by ! m]e ef=t to dodge ! obs]v]'s nose4 ,some:at e>li] ,m4 ,porro _h arrang$ 8 prisms 9 ! pres5t conv5tional =m ( ,fig4 #aag1 :]e two "r angle prisms h _! faces position$ 9 p>allel planes1 b turn$ ar\nd by #ij z 9 ,fig4 #aad4 ,! ray trac$ ?r\gh ?is conv5tional sy/em %[s t exactly ! same inv]sion occurs "h z 9 ! orig9al ,porro con/ruction1 & ?is =m is ! "o :i* has be5 mo/ commonly us$ = prismatic inv]sion & is conv5i5tly kn[n z ,porro's fir/ =m1 x actually hav+ be5 antec$5t 9 pr9ciple & practice to ! 8,lunette ,napoleon ,troisime40 ,! orig9al publi%$ description ( ,porro's "w1 translat$ f 8,cosmos0 ,vol4 #b1 p4 #bbb 7#aheb7 et seq4 is "h annex$ z x sets =? ! orig9 ( ! mod]n prism glass 9 unmi/akable t]ms4 ,cosmos1 ,vol4 #b1 p4 #bbb4--8,we h wi%$ = "s "t to make kn[n to our read]s ! preci\s advantages ( ! 8longue-vue cornet0 or tlmetre ( ,m4 ,porro4 ,ord9>y spyglasses or t]re/rial telescopes ( small dim5sions are at lea/ #cj or #dj cm4 long :5 ext5d$ to give di/9ct vision ( di/ant objects4 ,! l5g? is consid]ably r$uc$ by sub/itut+ = a fix$ tube multiple tubes slid+ into ea* o!r4 ,b ! draw+ out :i* ?is sub/itution necessitates is a some:at grave inconv5i5ce2 "o _c po9t ! telescope )\t arrang+ x & los+ "t4 ,= a long "t we h wi%$ x 7 possible to h ! p[] ( view+ di/ant objects1 ) telescopes v %ort & )\t draw4 ,m4 ,porro's 8longue-vue cornet0 seems to u to solve completely ?is difficult & important problem4 ,its con/ruction re/s ^u an exce$+ly +5i\s artifice :i* lit]ally folds triply ! axis ( ! telescope & ! lum9\s ray s t by ?is fact alone ! l5g? ( ! in/rum5t is r$uc$ by two-?irds4 ,let u try to give an idea ( ?is con/ruction3 ,beh9d ! telescope objective ,m4 ,porro places a rectangul> isosceles prism ( :i* ! hypo!nuse is p]p5dicul> to ! optic axis4 ,! lum9\s rays f ! object fall ^u ! rectangul> faces ( ?is prism1 are twice totally reflect$1 & return ^u !mselves p>allel to _! orig9al direction3 half way to ! po9t :]e !y w\ld =m ! image ( ! object1 !y are arre/$ by a second prism entirely simil> to ! fir/1 :i* returns !m to _! orig9al direction & s5ds !m to ! eyepiece ?r\gh :i* we obs]ve ! real image4 ,if ! rectangul> faces ( ! second prism 7 p>allel to ! faces ( ! fir/1 ?is real image w\ld be inv]t$--! telescope w\ld be an a/ronomical & n a t]re/rial telescope4 ,b ,m4 ,porro be+ an optician em95tly dextr\s1 well div9$ t to effect ! re9v]sion x suffic$ to place ! rectangul> faces ( ! second prism p]p5dicul> to ! correspond+ faces ( ! fir/ by turn+ !m a qu>t] revolution ^u !mselves4 ,9 effect1 a qu>t] revolution ( a reflect+ surface is a half revolution = ! image1 & a half revolution ( ! image evid5tly c>ries ! bottom to ! top & ! "r to ! left1 effect+ a complete inv]sion4 ,z ! image is ?us r$ress$ indep5d5tly ( ! eyepiece "o c evid5tly view x ) a simple two-l5s ocul> :i* decreases /ill fur!r ! l5g? ( ! telescope s t x is f9ally r$uc$ to ab\t a qu>t] ( t ( a telescope ( equal magnify+ p[]1 field & cle>ness4 ,! new telescope is !n a true pocket telescope ev5 ) a magnify+ p[] ( #aj or #ae4 ,its dim5sions 9 l5g? & bulk are ^? ( a field glass usually magnify+ only #d to #f times4 ,! m draws1 ! m bo!r1--x "h suffices to turn a little ?umbscrew to f9d 9 an in/ant ! po9t ( %>pe/ vision4 ,9 brilliancy necess>ily cut d[n a little1 n by ! d\ble total reflection1 :i* z is well kn[n does n lose light1 b by ! quadruple passage across ! sub/ance ( ! two prisms1 ! cornet 9 %>pness & amplification ( ! images c comp>e ) ! be/ hunt+ telescopes ( ! celebrat$ optician ,ploessl ( ,vi5na4 ,m4 ,porro has con/ruct$ ^u ! same pr9ciples a m>9e telescope only #ae c4m4 long ) an objective ( #dj m4m4 ap]ture :i* replaces an ord9>y m>9e glass #gj c4m4 long4 ,he has done /ill bett]1--a telescope only #cj c4m4 long c>ries a #fj m4m4 objective & c be made by turns a day & a night glass1 by sub/itut+ by a simple movem5t ( ! h& & )\t dism\nt+ any?+1 "o ocul> = ! o!r4 ,its brilliancy & magnification ( a doz5 times ) ! night ocul>1 ( tw5ty-five times ) ! day ocul>1 p]mits obs]v+ )\t difficulty ! eclipses ( ! satellites ( ,jupit]4 ,?is is evid5tly imm5se progress4 ,"o ( ! mo/ illu/ri\s ( ,g]man physici/s1 ,m4 ,dove ( ,b]l91 gave 9 #ahea ! "n ( rev]sion prism to ! comb9ation ( two prisms plac$ normally "o beh9d ! o!r s t _! correspond+ faces 7 p]p5dicul>4 ,he pres5t$ ?is disposition z an important new discov]y made by himself4 ,he d\btless did n "k t ,m4 ,porro1 :o des]ves all ! honor ( ?is *>m+ application1 _h realiz$ x long be=e him40 ,a little lat] ,m4 ,porro produc$ :at is commonly ref]r$ to z ,porro's second =m1 :i* is d]iv$ directly f annex+ ,a ,fig4 #aae to ! correspond+ half ( ,c z a s+le prism1 ! o!r half ( ,b be+ simil>ly annex$ to ! prism ,c1 ?us =m+ two sph5oid prisms1 su* z are %[n 9 ,fig4 #aah :i* may be m\nt$ sep>ately or may h _! faces cem5t$ toge!r to save loss ( light by reflections4 ,! sph5oid prisms h _h ! reputation ( be+ mu* m difficult to con/ruct ?an ! pla9 "r angl$ prisms ( ! o!r =ms %[n4 ,9 po9t ( fact !y are n p>ticul>ly difficult to make & ! be/ inv]t+ eye pieces = telescopes are n[ con/ruct$ ) sph5oid prisms l ^? j describ$4 ,illu/ration3 ,,fig4 #aah4--,porro's ,second ,=m4 ,illu/ration3 ,,fig4 #aai4--;,cl>k ,prismatic ,eyepiece4 ,?is p>ticul> arrangem5t l5ds itself v readily to a fairly compact & symmetrical m\nt+1 z is well %[n 9 ,fig4 #aai :i* is ! t]re/rial prismatic eyepiece z con/ruct$ by ! ,alvan ,cl>k corporation = application to v>i\s a/ronomical telescopes ( _! manufacture4 ,a glance at ! cut %[s ! compactness ( ! arrangem5t1 :i* actually %ort5s ! l9e> di/ance betwe5 objective & ocul> by ! am\nt ( ! pa? ( ! ray ?r\gh ! prisms in/ead ( l5g!n+ ! di/ance z 9 ! common t]re/rial eyepiece4 ,! field moreov] is mu* l>g] ?an t atta9able by a con/ruction l ,fig4 #aaj1 ext5d+ to some?+ ov] #dj1 & !re is no /rong t5d5cy = ! illum9ation or def9ition to fall (f ne> ! edge ( ! field4 ,9 ! practical con/ruction ( prism field glasses ! two "r angl$ prisms are usually sep>at$ by a mod]ate space z 9 ,porro's orig9al in/rum5ts s z to %ort5 ! actual l5g? ( ! prism telescope by fold+ ! ray ^u itself z 9 ,fig4 #abj1 :i* is a typical mod]n b9ocul> ( ?is class4 ,illu/ration3 ,,fig4 #abj4--,section ( ,prism ,b9ocul>4 ,! pa? ( ! rays is pla9ly %[n & ! mann] 9 :i* ! prisms fold up ! total focal l5g? ( ! objective is q obvi\s4 ,! add$ /]eoscopic effect obta9$ by ! arrangem5t ( ! two sides ( ! in/rum5t is practically a v mat]ial ga94 ,x gives admirable modell+ ( ! visible field1 a p]ception ( di/ance :i* is at lea/ v noticeable & a c]ta9 p[] ( p5etration1 z ?r\gh a mass ( und]bru%1 :i* results f ! objectives to a c]ta9 ext5t see+ ar\nd small objects s t "o or ! o!r ( !m gives an image ( some?+ beyond4 ,= ne> objects !re is "s exagg]ation ( /]eoscopic effect b on ! :ole = t]re/rial use ! net ga9 is decid$ly 9 evid5ce4 ,a well made prism b9ocul> is an extremely useful in/rum5t = obs]vation ( ! heav5s1 provid$ ! objectives are ( fair size1 & ! prisms big en\gh to receive ! :ole beam f ! objective1 & well execut$ en\gh to give a ?or\ghly good image ) a flat field4 ,! weak po9ts ( ! prism glass are great loss ( light ?r\gh reflection at ! usual #aj air-glass surfaces & ! g5]al pres5ce ( annoy+ gho/s ( bright objects 9 ! field4 ,mo/ su* b9ocul>s h ,kelln] eyepieces :i* are peculi>ly bad1 z we h se51 ) respect to reflect$ images1 & pres5t ! plane surface ( ! la/ prism to ! plane front ( ! field l5s4 ,rec5tly "s con/ructors h utiliz$ ! or?oscopic eyepiece1 ,figure #aje;a1 z a sub/itute ) great advantage 9 ! matt] ( reflections4 ,! loss ( light 9 ! prism glass is really a s]i\s matt]1 betwe5 reflection at ! surfaces & absorption 9 ! ?ick masses ( glass necess>y 9 ! prisms4 ,if ( any size ! transmitt$ light is n mu* ov] "o-half ( t receiv$1 v seldom above #fj4 ,if ! in/rum5t is prop]ly design$ ! app>5t field is 9 ! neighborhood ( #de1 sub/antially flat & fairly ev5ly illum9at$4 ,w>n+ %\ld "h be giv5 h[ev] t _m b9ocul>s are on ! m>ket 9 :i* ! field is f> f flat & equally f> f be+ uni=m4 ,9 _m in/ances ! prisms are too small to take ! :ole bundle ( rays f ! objective back to ! image plane )\t cutt+ d[n ! m>g9al light consid]ably4 ,ev5 :5 ! field is app>5tly q flat ?is fault ( unev5 illum9ation may exi/1 & 9 a glass = a/ronomical uses x is highly objectionable4 ,be=e pick+ out a b9ocul> = a /udy ( ! sky make v c>eful trial ( ! field ) respect to flatness & clean def9ition ( objects up to ! v edge4 ,!n judge z accurately z y may ( ! uni=mity ( illum9ation1 if possible by obs]vation on two />s ab\t ! radius ( ! field ap>t4 ,x %\ld be possible to obs]ve !m 9 any "p ( ! field )\t detectable *ange 9 _! app>5t brilliancy4 ,if ! objectives are easily removable unscrew "o ( !m to obta9 a cle> idea z to ! actual size ( ! prisms4#ai ,look out1 too1 = gho/s ( bright />s4 #ai ,!re are b9ocul>s on ! m>ket :i* are to outw>d appe>ance prism glasses1 b :i* are really ord9>y op]a glasses m\nt$ ) int5t to deceive1 sometimes be>+ a slight v>iation on ! "n ( "s well kn[n mak]4 ,! objectives ( prism glasses usually run f in* to #a in* 9 diamet]1 & ! p[]s f #f to #ab4 ,! bigg] ! objectives ! bett]1 provid$ ! prisms are ( ample size1 :ile high] p[] ?an #f or #h is g5]ally unnecess>y & disadvantage\s4 ,occasional glasses ( magnify+ p[] #ab to #bj or m are to be f\nd b su* p[]s are inconv5i5tly great = an in/rum5t us$ )\t support4 ,d n =get t a fir/ class monocul> prism glass is extremely conv5i5t & satisfactory 9 use1 to say no?+ ( be+ consid]ably less 9 price ?an ! in/rum5t = two eyes4 ,a monocul> prism glass1 by ! way1 makes an admirable f9d] :5 fitt$ ) co>se cross l9es 9 ! eyepiece4 ,x is p>ticul>ly well suit$ to small telescopes )\t circles4 ,illu/ration3 ,,fig4 #aba4--;,b9ocul> ) ,extreme ,/]eoscopic ,effect4 ,num]\s modifications ( ,porro's inv]t+ prisms h be5 made adapt+ !m to diff]5t specific purposes4 ,( ^! a s+le famili> example w suffice z %[+ ! way 9 :i* ! ,porro prism sy/em c be treat$ by m]e re>rangem5t ( ! prismatic elem5ts4 ,9 ,fig4 #aba is %[n a special ,zeiss b9ocul> capable ( extreme /]eoscopic effect4 ,x is =m$ ( two ,porro prism telescopes ) ! rays br\ght into ! objectives at "r angles to ! axis ( ! in/rum5t by a "r angl$ prism ext]nal to ! objective4 ,! ap]tures ( ^! prisms appe> po9t+ =w>d 9 ! cut4 ,z %[n !y are 9 a position ( maximum /]eoscopic effect4 ,be+ h+$ ! tubes c be swung up f ! horizontal position1 9 :i* latt] ! objectives are sep>at$ by some?+ l eight times ! int]ocul> di/ance4 ,! /]eoscopic effect ) ! tubes horizontal is ( c\rse greatly exagg]at$ s t x enables "o to =m a fair judgm5t z to ! relative position ( some:at di/ant objects1 a feature useful 9 locat+ %ell bur/s4 ,! optical /ructure ( "o ( ! pair ( telescopes is %[n 9 ,fig4 #abb 9 :i* ! c\rse ( ! ent]+ ray c be trac$ ?r\gh ! ext]ior prism ( ! objective & ! rema9d] ( ! rev]s+ tra9 & !nce ?r\gh ! eyepiece4 ,?is prism erect+ sy/em is obvi\sly d]iv$ f ! 8,lunette ,napoleon ,troisime0 by br++ d[n ! prism ,b ^u ! correspond+ half ,a & cem5t+ x !reto1 mean:ile plac+ ! objective imm$iately "u ,a4 ,"o occasionally meets prismatic inv]t+ sy/ems diff]+ consid]ably f ! ,porro =ms4 ,p]haps ! be/ kn[n ( ^! is ! s call$ ro( prism due to ,pr(4 ,abb1 ,fig4 #abc1 & occasionally useful 9 t ! ent]+ & em]g+ rays lie 9 ! same /raight l9e1 ?us =m+ a direct vision sy/em4 ,look+ at x z we did at ! ,porro sy/em a v]tical elem5t 9 front ( ! prism is rev]s$ 9 reflection f ! two surfaces a & b1 :ile a correspond+ horizontal elem5t is reflect$ flatwise s f> z ^! are conc]n$1 b is turn$ end = end by reflection at ! ro( surfaces c & d1 ?us giv+ complete inv]sion4 ,9 practice ! prism is made z %[n1 9 ?ree p>ts1 two ( !m "r angl$ prisms1 ! ?ird conta9+ ! ro( surfaces4 ,! extreme precision requir$ 9 figur+ ! ro( =ms a consid]able ob/acle to ! production ( su* prisms 9 quantity & :ile !y h f\nd conv5i5t use 9 c]ta9 special in/rum5ts l gunsights1 :]e direct vision is useful1 !y are n ext5sively employ$ = g5]al purposes1 al?\gh bo? monocul> & b9ocul> in/rum5ts h be5 con/ruct$ by _! aid4 ,illu/ration3 ,,fig4 #abb4--,pa? ( ,ray 9 ,fig4 #aba4 ,illu/ration3 ,,fig4 #abc4--;,abb ,ro( ,prism4 ,"o o!r v>iety ( prism involv+ ! ro( pr9ciple has f\nd "s application 9 field glasses manufactur$ by ! firm ( ,h5soldt4 ,! prism =m us$ is %[n 9 ,fig4 #abd4 ,?is l o!r =ms ( ro( prism is less easy to make ?an ! conv5tional ,porro type4 ,num]\s inv]t+ & lat]ally reflect+ prisms are 9 use = specific purposes4 ,"s ( !m are highly +5i\s & rem>kably well adapt$ = _! use1 b h>dly c be said to =m a mat]ial portion ( telescope practice4 ,!y belong r to ! te*nique ( special in/rum5ts l gunsights & p]iscopes1 :ile "s ( !m h be5 devis$ *iefly z +5i\s sub/itutes = ! simpl] ,porro =ms4 ,mo/ prism telescopes bo? monocul> & b9ocul> are g5]ally made on "o or ! o!r ( ! ,porro =ms4 ,?is is p>ticul>ly true ( ! l>ge b9ocul>s :i* are occasionally con/ruct$4 ,porro's second =m ) ! sph5oid prisms seems to be be/ adapt$ to cases :]e %ort5+ ( ! in/rum5t is n a p>am\nt consid]ation4 ,= example1 "s ,zeiss %ort focus telescopes are regul>ly made 9 b9ocul> =m1 & suppli$ ) inv]t+ sy/ems compos$ ( two sph5oid prisms1 & ) ocul>s con/ruct$ on ! exact pr9ciple ( ! triple nose-piece ( a microscope1 s t ?ree p[]s are imm$iately available to ! obs]v]4 ,illu/ration3 ,,fig4 #abd4--;,h5soldt ,prism4 ,/ill less commonly b9ocul> telescopes ( consid]able ap]ture are con/ruct$1 prim>ily = a/ronomical use1 be+ provid$ ) prismatic inv]sion = t]re/rial employm5t1 b m p>ticul>ly 9 ord] to ga9 by ! lat]al displacem5t ( a ,porro sy/em ! space necess>y = two objectives ( consid]able size4 ,z we h already se51 ! practical diamet] ( objectives 9 a b9ocul> is limit$ to a trifle ov] #b in*es unless space is s ga9$4 ,! l>ge/ prismatic b9ocul> z yet con/ruct$ is "o made ye>s ago by ! ,cl>ks1 ( #f in*es objective ap]ture & #ib in*es focal l5g?4 ,s big & p[]ful an in/rum5t obvi\sly w\ld give admirable b9ocul> views ( ! heav5s & s accurately 0 x con/ruct$ t ! reports ( its p]=mance 7 exce$+ly good4 ,! same firm has made a good _m simil> b9ocul>s ( #c in* & above1 ( :i* a typical example ( #d in* ap]ture & #fj in* focal l5g? is %[n 9 ,fig4 #abe4 ,9 ?is case ! erect+ sy/ems 7 ( ,porro's fir/ =m1 & 7 provid$ ) ,kelln] ocul>s ( v wide field4 ,^! b9ocul>s con/ructions 9 in/rum5ts ( su* size1 h[ev] well made & agreeable = t]re/rial obs]vation1 h>dly ju/ify ! exp5se = purely a/ronomical use4 ,illu/ration3 ,,fig4 #abe4--;,cl>k #d ,b9ocul> ,telescope4 ,,*apt] ,,viii ,,accessories ,aside f ! ord9>y equipm5t ( ocul>s v>i\s accessories =m an important "p ( ! obs]v]'s equipm5t1 _! numb] & "* dep5d+ on ! in/rum5t 9 use & ! purposes to :i* x is devot$4 ,illu/ration3 ,,fig4 #abf4--,/> ,diagonal4 ,fir/ 9 g5]al usefulness are sev]al special =ms ( eyepiece equipm5t supplem5t>y to ! usual ocul>s4 ,at ! head ( ! li/ is ! ord9>y /> diagonal = ! easi] view+ ( objects ne> ! z5i? "h %[n 9 ,fig4 #abf4 ,x is m]ely a tube1 ,a1 fitt+ ! draw tube ( ! telescope1 ) a slott$ side tube ,b1 at a "r angle1 into :i* ! ord9>y ocul> fits1 & a "r angl$ prism ,c ) its two faces p]p5dicul> respectively to ! axes ( ! ma9 & side tubes1 & ! hypo!nuse face at #de to ea*4 ,! beam com+ d[n ! tube is totally reflect$ at ?is face & br\ght to focus at ! ocul>4 ,! l[] end ( ! tube is clos$ by a cap to exclude du/4 ,"o looks1 by help ( ?is1 horizontally at z5i? />s1 or1 if obs]v+ objects at r high altitude1 views !m at a com=table angle d[nw>d4 ,! prism mu/ be v accurately made to avoid injury to ! def9ition1 b loses only ab\t #aj ( ! light1 & adds greatly to ! com=t ( obs]v+4 ,( almo/ equal importance is ! sol> diagonal devis$ by ,sir ,john ,h]s*el1 ,fig4 #abg4 ,"h ! tube /ructure ,a1 ,b1 is q ! same z 9 ,fig4 #abf b ! "r angl$ prism is replac$ by a simple elliptical prism ,c ( small angle1 #aj or less1 ) its upp] face accurately plane & at #de to ! axes ( ! tubes1 re/+ on a l9+ tube ,d cut (f z %[n4 ,9 view+ ! sun only ab\t #e ( ! light 7& heat7 is reflect$ at ?is upp] surface to =m ! image at ! eye piece4 ,illu/ration3 ,,fig4 #abg4--,sol> ,diagonal4 ,any reflection f ! l[] poli%$ surface is turn$ aside out ( ! field1 :ile ! rema9d] ( ! radiation passes ?r\gh ! prism ,c & is conc5trat$ bel[ x4 ,to prev5t scor*+ ! obs]v] ! l[] end ( ! tube is capp$ at ,e1 b ! cap has side p]=ations to provide circulation = ! heat$ air4 ,us+ su* a prism1 ! remnant ( light reflect$ c be readily ton$ d[n by a neutral t9t$ glass ov] ! ocul>4 ,9 ! telescopes ( #c in*es & less ap]ture1 & ord9>y focal ratio1 a plane p>allel disc ( v d>k glass ov] ! ocul> gives suffici5t protection to ! eye4 ,?is glass is pref]ably ( neutral t9t1 & commonly is a scant #a_/af in* ?ick4 ,"s obs]v]s pref] o!r t9ts ?an neutral4 ,a gre5 & a r$ glass sup]impos$ give good results & s does a disc ( ! deepe/ %ade ( ! s-call$ ,noviweld glass1 :i* is simil> 9 effect4 ,) an ap]ture z l>ge z #c in*es a pair ( sup]impos$ d>k glasses is wor? :ile1 = ! two w n break simultane\sly f ! heat & !re w be "t to get ! eye away 9 safety4 ,a brok5 sun%ade is likely to co/ ! obs]v] a p]man5t scotoma1 bl9dness 9 a small area ( ! ret9a :i* w nei!r get bett] nor worse z "t goes on4 ,above #c in*es ap]ture ! sol> prism %\ld be us$ or1 if "o c>es to g to fully d\ble ! co/1 !re is no?+ m com=table to employ 9 sol> obs]vation ?an ! pol>iz+ eye piece1 ,fig4 #abh4 ,?is %[s s*ematically ! arrangem5t ( ! device4 ,x dep5ds on ! fact t a ray ( light fall+ on a surface ( common glass at an angle ( incid5ce ( approximately #eg is pol>iz$ by ! reflection s t :ile x is freely reflect$ if x falls aga9 on a surface p>allel to ! fir/1 x is absorb$ if x falls at ! same incid5ce on a surface at "r angles to ! fir/4 ,illu/ration3 ,,fig4 #abh4--;,diagram ( ,pol>iz+ ,eyepiece4 ,?us 9 ,fig4 #abh ! incid5t beam f ! telescope falls on ! black glass surface a at #eg incid5ce1 is aga9 reflect$ f ! p>allel mirror b1 & !n pass$ on1 p>allel to its orig9al pa?1 to ! l[] pair ( mirrors c1 d4 ,! purpose ( ! second reflection is to pol>ize ! residual light :i* ?r\gh ! conv]g5ce ( ! rays 0 incompletely pol>iz$ at ! fir/4 ,! l[] pair ( mirrors c1 d1 aga9 twice reflect ! light at ! pol>iz+ angle1 &1 9 ! position %[n1 pass x on to ! ocul> dim9i%$ only by ! f\r reflections4 ,b if ! second pair ( mirrors be rotat$ toge!r ab\t a l9e p>allel to b c z an axis ! transmitt$ light beg9s to fade out1 & :5 !y h be5 turn$ #ij1 s t _! planes are incl9$ #ij to a & b 7 #cc to ! plane ( ! pap]71 ! light is sub/antially ext+ui%$4 ,?us by m]ely turn+ ! second pair ( mirrors ! sol> image c be r$uc$ 9 brilliancy to any ext5t :atev]1 )\t modify+ its color 9 any way4 ,! typical =m giv5 to ! pol>iz+ eyepiece is simil> to ,fig4 #abi4 ,"h t#b is ! box conta9+ ! pol>iz+ mirrors1 a b1 fitt$ to ! draw tube1 b = obvi\s reasons ecc5tric ) x1 t#a is ! rotat+ box conta9+ ! 8analys+0 mirrors c1 d1 & a is ! ocul> turn+ ) x4 ,sometimes ! pol>iz+ mirrors are actually a pair ( ,h]s*el prisms z 9 ,fig4 #abf1 fac+ ea* o!r1 ?us gett+ rid ( mu* ( ! heat4 ,o!rwise ! :ole set ( mirrors is ( black glass to avoid back reflections4 ,9 simpl] con/ructions s+le mirrors are us$ z pol>iz] & analys]1 & 9 fact !re are _m v>iations on ! pol>iz+ sol> eyepiece involv+ ab\t ! same pr9ciples4 ,illu/ration3 ,,fig4 #abi4--,pol>iz+ ,sol> ,eyepiece4 ,9 any sol> eyepiece a set ( small diaphragms ) holes f p]haps #a_/fd in* up are useful 9 cutt+ d[n ! g5]al gl>e f ! surface outside ( t "u scrut9y4 ,^! may be dropp$ ^u ! regul> diaphragm ( ! ocul> or conv5i5tly arrang$ 9 a revolv+ diaphragm l t us$ ) ! old] photographic l5ses4 ,! measurem5t ( cele/ial objects has develop$ a l>ge gr\p ( important auxili>ies 9 ! micromet]s ( v v>i$ =ms4 ,! simple/ ne$s little description1 s9ce x consi/s m]ely ( a plane p>allel disc ( glass fitt+ 9 ! focus ( a positive ocul>1 & et*$ ) a network ( uni=m squ>es1 =m+ a reticulat$ micromet] by :i* ! di/ance ( "o object f ano!r c be e/imat$4 ,x c be readily calibrat$ by measur+ a kn[n di/ance or not+ ! "t requir$ = an equatorial /> to drift across ! squ>es p>allel to "o set ( l9es4 ,x gives m]ely a useful approximation1 & accurate measures mu/ be turn$ ov] to m precise in/rum5ts4 ,illu/ration3 ,,fig4 #acj4--;,diagram ( ,r+ ,micromet]4 ,! r+ micromet] due1 l s mu* o!r valuable app>atus1 to ,fraunh(]1 is conv5i5t & widely us$ = det]m9+ positions4 ,x consi/s1 z %[n 9 ,fig4 #acj1 ( an accurately turn$ opaque r+1 g5]ally ( ?9 /eel1 cem5t$ to a plane p>allel glass or o!rwise susp5d$ 9 ! c5t] ( ! eyepiece field4 ,! :ole r+ is g5]ally half to two ?irds ! wid? ( ! field & has a mod]ate radial wid? s t bo? ! +ress & ! egress ( a /> c be conv5i5tly tim$4 ,x dep5ds :olly on ! measurem5t ( "t z ! />s to be comp>$ drift across ! r+ :ile ! telescope is fix$1 & :ile a clock or *ronomet] op]at+ a s\nd] is a desirable adjunct "o c d pretty well ) a c\ple ( /op wat*es s9ce only diff]5tial times are requir$4 ,= full directions z to its use consult ,loomis' ,practical ,a/ronomy1 a book :i* %\ld be 9 ! libr>y ( e "o :o has ! lea/ int]e/ 9 cele/ial obs]vations4 ,suffice x to say "h t ! r+ micromet] is v simple 9 use1 & ! computation ( ! results is q easy4 ,9 ,fig4 #acj ,f is ! edge ( ! field1 ,r ! r+1 & a b1 ab1 ! pa?s ( ! />s s & s1 ! =m] well into ! field1 ! latt] j )9 ! r+4 ,! necess>y data comprise ! "t tak5 by ea* /> to transv]se ! r+1 & ! radius ( ! r+ 9 angul> measure1 :5ce ! diff]5ce 9 ,r4 ,a4 or ,dec1 c be obta9$4#bj #bj r ! radius ( ! r+1 is giv5 by1 r 7#ae_/b77t-t7 cos ,dec41 t-t be+ ! seconds tak5 = transit4 ,diff]5ce ( ,r4 ,a4 7t-t7 7,t-,t7 :]e 7,t-,t7 is ! "t tak5 = transit ( second />4 ,to obta9 diff]5ces ( decl9ation "o decl9ation %\ld be kn[n at lea/ approximately1 & ! second e/imat$ f its relative position 9 ! r+ or o!rwise4 ,!n ) ^! t5tative values proce$ z foll[s4 ,put x angle aob & x angle aob ,also let d approximate decl9ation ( s & d approximate decl9ation ( s ,!n s9 x 7#ae_/br7 cos d 7,t-,t7 s9 x 7#ae_/br7 cos d 7t-t7 & f9ally ,diff]5ce ( ,dec4 r 7cos x-cos x71 :5 bo? arcs are on ! same side ( c5t] ( r+4 ,if on opposite sides1 ,diff4 r 7cos x cos x74 ,illu/ration3 ,*amb]'s 8,a/ronomy0 7,cl>5don ,press74 ,,fig4 #aca4--;,d\ble ,image ,micromet]4 7,c\rtesy ( ,! ,cl>5don ,press47 ,!re is also n[ & !n us$ a squ>e b> micromet]1 consi/+ ( an opaque squ>e set ) a diagonal 9 ! l9e ( diurnal motion4 ,x is us$ 9 mu* ! same way z ! r+1 & ! r$uctions are sub/antially ! same4 ,x has "s po9ts ( conv5i5ce b is little us$1 probably on acc\nt ( ! great difficulty ( accurate con/ruction & ! requirem5t1 = advantage\s use1 t ! telescope %\ld be on a well adju/$ equatorial /&4#ba ,! r+ micromet] works reasonably well on any k9d ( /eady m\nt1 requires no illum9ation ( ! field & is 9 p]man5t work+ adju/m5t4 #ba 7,= full discussion ( ?is in/rum5t see ,*&l]1 ,mem4 ,am]4 ,acad4 ,arts ,sci4 #ahhe1 p4 #aeh74 ,/ill ano!r type ( micromet] capable ( use )\t a clock-drive is ! d\ble image in/rum5t4 ,9 its usual =m x is bas$ on ! pr9ciple t if a l5s is cut 9 two along a diamet] & ! halves are slightly displac$ along ! cut all objects w be se5 d\ble1 ea* half ( ! l5s =m+ its own set ( images4 ,conv]sely1 if "o *oses two objects 9 ! unit$ field ^! c be br\ght toge!r by slid+ ! halves ( ! l5s z be=e1 & ! ext5t ( ! movem5t ne$$ measures ! di/ance betwe5 !m4 ,any l5s 9 ! optical sy/em c be ?us us$1 f ! objective to ! eyepiece4 ,fig4 #aca %[s a v simple d\ble image micromet] devis$ by ,br[n+ _m ye>s ago4 ,"h ! l5s divid$ is a s-call$ ,b>l[ l5s1 a weak a*romatic negative l5s sometimes us$ l a telephoto l5s to l5g!n ! focus & h5ce v>y ! p[] ( a telescope4 ,?is l5s is %[n at ,a ) ! halves widely sep>at$ by ! d\ble ?read$ micromet] screw ,b1 :i* c>ries !m ap>t symmetrically4 ,! ocul> prop] is %[n at ,c4 ,d\ble image micromet]s are n[ ma9ly ( hi/orical int]e/1 & ! pr9ciple survives *iefly 9 ! heliomet]1 a telescope ) ! objective divid$1 & provid$ ) slid+ me*anism ( ! highe/ ref9em5t4 ,! special function ( ! heliomet] is ! direct micrometric measurem5t ( /ell> di/ances too great to be )9 ! practicable range ( a fil> micromet]--di/ances = example up to #a or ev5 m4 ,! obs]vations ) ! heliomet] are some:at labori\s & dem& r intricate corrections1 b are capable ( great precision4 7,see ,sir ,david ,gill's article 8,heliomet]0 9 ! ,enc4 ,brit4 #aa? ,ed474 ,at ! pres5t day cele/ial photography1 ) micrometric measurem5t ( ! result+ plates1 has gone f> 9 r5d]+ ne$less visual measurem5ts ( di/ances above a v few m9utes ( arc1 s t x is some:at d\btful :e!r a l>ge heliomet] w\ld aga9 be con/ruct$4 ,illu/ration3 ,,fig4 #acb4--;,fil> ,micromet]4 7,c\rtesy ( ,j4 ,b4 ,lipp9cott ,co47 ,! a/ronom]'s real arm ( precision is ! fil> micromet]4 ,?is is %[n 9 outl9e 9 ,fig4 #acb1 ! ocul> & ! plate t c>ries x be+ remov$ s z to display ! work+ p>ts4 ,x consi/s ( a ma9 frame aa1 c>ry+ a slide bb1 :i* is mov$ by ! screws & mill$ head ,b4 ,! slide bb c>ries ! v]tical spid] l9e mm1 & usually "o or m horizontal spid] l9es1 l9e mm is ! s-call$ fix$ ?read ( ! micromet]1 movable only z a conv5i5ce to avoid %ift+ ! telescope4 ,on bb moves ! micromet] slide cc1 c>ry+ ! movable spid] l9e nn & ! comb :i* records1 ) mm z ref]5ce l9e1 ! :ole revolutions ( ! micromet] screw ,c4 ,! ocul> sometimes has a slid+ motion ( its own on cc1 to get x position$ to ! be/ advantage4 ,9 use "o /> is set ^u mm by ! screw ,b & !n ,c is turn$ until nn bisects ! o!r />4 ,!n ! exact turns & fraction ( a turn c be read (f on ! comb & divid$ head ( ,c1 & r$uc$ to angul> measure by ! kn[n con/ant ( ! micromet]1 usually det]m9$ by ! "t ( passage ( a ne>ly equatorial /> along ! horizontal ?read :5 mm1 nn1 are at a def9ite sett+ ap>t4 7,!n r 7#ae7t-t7 cos d7_/,n :]e r is ! value ( a revolution 9 seconds ( arc1 ,n ! revolutions ap>t ( mm1 nn1 & t & d z h]et(ore47 ,v g5]ally ! :ole sy/em ( slides is fitt$ to a graduat$ circle1 to :i* ! fix$ horizontal ?read is diametral4 ,!n by turn+ ! micromet] until ! horizontal ?reads cut ! two objects "u comp>ison1 _! position angle ) ref]5ce to a graduat$ circle c be read (f4 ,?is angle is conv5tionally c\nt$ f #j to #cfj f nor? ar\nd ?r\gh ea/4 ,illu/ration3 ,,fig4 #acc4--;,fil> ,position ,micromet]4 ,figure #acc %[s ! micromet] con/ruct$ by ! ,cl>ks = _! #bd in* equatorial ( ! ,l[ell ,obs]vatory4 ,"h ,a is ! head ( ! ma9 micromet] screw ( :i* ! :ole turns are reckon$ on ! c\nt] ,h 9 lieu ( ! comb ( ,fig4 #acb4 ,b is ! trav]s+ screw = ! fix$ wire sy/em1 ,c ! clamp+ screw ( ! position circle1 ,d its sett+ p9ion1 ,e ! rack motion = %ift+ ! ocul>1 ,f ! read+ glass = ! position circle1 & ,g ! little electric lamp = bright wire illum9ation4 ,! p>ts correspond q exactly ) ! diagram ( ,fig4 #acb b ! in/rum5t is f> m elegant 9 design ?an ! e>li] =ms ( micromet] & =tunately rid ( ! oil lamps t 7 once 9 g5]al use4 ,a small electric lamp ) reflector ?r[s a little light on ! spid] l9es--j en\gh to %[ !m di/9ctly4 ,or sometimes a fa9t light is ?us diffus$ 9 ! field aga9/ :i* ! spid] l9es %[ d>k4 ,commonly ei!r type ( illum9ation c be us$ & modifi$ z occasion requires4 ,! fil> micromet] is seldom us$ on small telescopes1 s9ce to "w easily ) x ! in/rum5t %\ld be p]man5tly m\nt$ & clock-driv54 ,good "w 0 done by "s ( ! e>ly obs]v]s )\t ^! aids1 b at ! co/ ( inf9ite pa9s & mu* loss ( "t4 ,! clock drive is 9 fact a mo/ important adjunct ( ! telescope :5 us$ = o!r purposes ?an ord9>y visual obs]vations1 ?\gh = simple see+ a smoo? work+ sl[ motion 9 ,r4 ,a4 answ]s well4 ,! driv+ clock f ! horological view-po9t is rudim5t>y4 ,x consi/s ess5tially ( a weight-driv51 or sometimes spr+-driv51 drum1 turn+ by a simple ge> connection a worm :i* engages a c>efully cut ge> :eel on ! pol> axis1 :ile prev5t$ f runn+ away by ge>+ up to a fa/ runn+ fly-ball gov]nor1 :i* applies friction to hold ! clockwork d[n to its rate if ! spe$ rises by a m9ute am\nt4 ,!re is no p5dulum 9 ! ord9>y s5se1 ! regul>ity dep5d+ on ! uni=mity ( ! total friction--t due to ! drive plus t appli$ by ! gov]nor4 ,figure #acd %[s a simple & entirely typical driv+ clock by ,w>n] ,swasey4 ,"h ,a is ! ma9 drum ) its w9d+ ge> at ,b1 ,c is ! bevel ge>1 :i* is driv5 f ano!r c>ri$ by ,a1 & s]ves to turn ! worm %aft ,d2 ,e m>ks ! fly balls driv5 by ! multiply+ ge>+ pla9ly visible4 ,! gov]nor acts at a pr$et]m9$ rotation spe$ to lift ! sp9n+ friction disc ,f aga9/ its fix$ mate1 :i* c be adju/$ by ! screw ,g4 ,! fly-balls c be slightly %ift$ 9 effective position to complete ! regulation4 ,^! simple clocks1 ( :i* !re are _m species diff]+ ma9ly 9 ! details ( ! friction device1 are capable ( excell5t precision if ! "w ( driv+ ! telescope is kept light4 ,= l>ge & heavy in/rum5ts1 p>ticul>ly if us$ = photographic "w :]e great precision is requir$1 x is difficult to keep ! v>iations ( ! driv+ friction )9 ! range ( comp5sation furni%$ by ! gov]nor friction alone1 & 9 su* case rec\rse is (t5 tak5 to con/ructions 9 :i* ! fly balls act z relay to an electrically controll$ brake1 or 9 :i* ! driv+ p[] is suppli$ by an electric motor suitably gov]n$ ei!r cont9u\sly or p]iodically4 ,= su* "w indep5d5t h& guid+ me*anism is provid$ to supplem5t ! clockwork4 ,= equatorials ( ! smalle/ sizes1 say #c to #d in*es ap]ture1 spr+ op]at$ driv+ clocks are occasionally us$4 ,! g5]al plan ( op]ation is q simil> to ! common weight driv5 =ms1 & :]e ! weights to be c>ri$ are n excessive su* clocks d good "w & s]ve a v useful purpose4 ,illu/ration3 ,,fig4 #acd4--,typical ,driv+ ,clock4 7,c\rtesy ( ,! ,cl>5don ,press47 ,an excell5t type ( ! simple spr+ driv+ clock is %[n 9 ,fig4 #acf z con/ruct$ by ,zeiss4 ,"h #a is ! w9d+ ge>1 #b ! friction gov]nor1 & #c ! regulat+ ge>4 ,x w be se5 t ! friction /uds are c>ri$ by ! fly balls !mselves1 some:at z 9 ,fraunh(]s' con/ruction a c5tury s9ce1 & ! regulation is v easily & quickly made by adju/+ ! height ( ! conical friction surface above ! balls4 ,= heavi] "w ! same mak]s g5]ally use a p[]ful weight driv5 tra9 ) f\r fly-balls & electric seconds control1 sometimes ) ! addition ( electric motor sl[ motions to adju/ = ,r4 ,a4 9 bo? directions4 ,illu/ration3 ,,fig4 #ace4--;,cl>k ,driv+ ,clock4 ,figure #ace is a r p[]ful clock ( analog\s =m by ! ,cl>ks4 ,x diff]s a little 9 its me*anism & especially 9 ! friction ge> 9 :i* ! be>+ disc is pick$ up by a delicately set lat* & c>ri$ j long en\gh to effect ! regulation4 ,x is really rem>kable t clockworks ( s simple "* z ^! %\ld p]=m z well z exp]i5ce %[s t !y d4 ,9 a few in/ances clocks h dep5d$ on air-fans = _! regulat+ =ce1 some?+ aft] ! mann] ( ! driv+ ge> ( a phonograph1 b ?\gh r successful = light "w !y h f\nd little favor 9 ! task ( driv+ equatorials4 ,an excell5t type ( a second g5us is ! p5dulum controll$ driv+ clock due to ,sir ,david ,gill4 ,?is has a p[]ful weight-driv5 tra9 ) ! usual fly-ball gov]nor4 ,b ! friction ge> is controll$ by a contact-mak+ seconds p5dulum 9 ! mann] %[n diagrammatically 9 ,fig4 #acg4 ,two light lea!r tipp$ rods ea* controll$ by an electro magnet act ^u an auxili>y brake disc c>ri$ by ! gov]nor sp9dle :i* is set = normal spe$ ) "o brake rod be>+ lightly on x4 ,excit+ ! correspond+ magnet relieves ! pressure & accel]ates ! clock1 :ile excit+ ! o!r adds brak+ effect & sl[s x4 ,illu/ration3 ,,fig4 #acf4--,spr+ ,op]at$ ,driv+ ,clock4 ,9 ,fig4 #acg is %[n f ! orig9al pap]1 7,m4 ,n4 ,nov41 #ahgc71 ! v +5i\s selective control me*anism4 ,at ,p is susp5d$ ! contact-mak+ seconds-p5dulum mak+ mom5t>y contact by ! p9 ,q ) a m]cury globule at ,r4 ,^u a sp9dle ( ! clock :i* turns once a second is fix$ a vulcanite disc 1 1 1 4 ,?is has a rim ( silv] brok5 at ! po9ts 1 1 1 1 by ivory spac]s cov]+ #c ( circumf]5ce4 ,on ea* side ( ?is disc is ano!r1 small]1 & ) a complete silv] rim4 ,"o1 1 is %[n1 connect$ ) ! contact spr+ ,v2 its mate 1 on ! o!r side contacts ) ,u1 :ile a ?ird contact ,k be>s on ! l>g] disc4 ,! pair ( segm5ts 1 1 & 1 1 are connect$ to 1 ! o!r pair ( segm5ts to 4 ,n[ suppose ! discs turn+ ) ! arr[s3 ,if ,k re/s on "o ( ! insulat$ po9ts :5 ! p5dulum ?r[s ! batt]y ,c ,z into circuit no?+ happ5s4 ,if ! disc is ga9+ on ! p5dulum1 ,k1 in/ead ( re/+ on z %[n w contact ) segm5t 1 1 & actuate a relay via ,v1 excit+ ! appropriate brake magnet4 ,illu/ration3 ,,fig4 #acg4--,sir ,david ,gill's ,electric ,control4 ,if ! disc is los+1 ,k contacts ) segm5t 1 1 & curr5t w pass via & ,u to a relay t op]ates ! o!r brake magnet & lets ! clock accel]ate4 ,a f\r? disc 7n %[n7 on ! same sp9dle is entirely insulat$ on its edge except at po9ts correspond+ to & 1 & ) a contact spr+ l ,k4 ,if ! disc is nei!r ga9+ nor los+ :5 ! p5dulum makes contact1 curr5t fl[s via ?is f\r? disc & sets ! relay on ! mid-po9t ready to act :5 ne$$4 ,?is clock is ! prototype ( div]s electrically-brak$ driv+ clocks ) p5dulum control1 & prov$ beautifully precise 9 action1 l v>i\s k9dr$ devices con/ruct$ s9ce1 ?\gh ! :ole g5us is some:at exp5sive & intricate4 ,! mod]n t5d5cy 9 driv+ app>atus = telescopes1 p>ticul>ly l>ge in/rum5ts1 is to utilize an electric motor = ! s\rce ( p[]1 us+ a clock me*anism m]ely = ! purpose ( accurately regulat+ ! rate ( ! motor4 ,we ?us h ! driv+ clock 9 its simple/ =m z a purely me*anical device work$ by a s5sitive fly-ball gov]nor4 ,! next important type is t 9 :i* ! clock drive is precisely regulat$ by a p5dulum clock1 ! necess>y gov]n+ p[] be+ appli$ electrically z 9 ,fig4 #acg or sometimes me*anically4 ,f9ally we come to ! type n[ "u consid]ation :]e ! in/rum5t itself is motor driv5 & ! function ( ! clock is t ( regulat+ ! motor4 ,a v good example ( su* a drive is ! ,g]ri% app>atus us$ = practically all ! in/rum5ts at ! v>i\s ,h>v>d obs]vatory /ations1 & :i* has prov$ extremely successful ev5 = ! mo/ try+ "w ( cele/ial photography4 ,! s*ematic arrangem5t ( ! app>atus is %[n 9 ,fig4 #ach4 ,"h an electric motor %[n 9 diagram 9 #a1 ,fig4 #ach1 is ge>$ d[n to approximately ! prop] spe$ = turn+ ! "r asc5sion axis ( ! telescope4 ,x is suppli$ ) curr5t ei!r f a batt]y or 9 practice f ! electric supply :i* may be at h&4 ,?is motor is op]at$ on a #aaj volt circuit :i* supplies curr5t ?r\gh ! swit* #b :i* is controll$ by ! l[ voltage clock circuit runn+ ?r\gh ! magnet #c4 ,! clock circuit c be clos$ & op5$ at two po9ts1 "o controll$ by ! seconds p5dulum #e1 ! o!r at #g by ! /ud on ! tim+ :eel ge>$ to ! motor = "o revolution p] second4 ,!re is also a %unt ar\nd ! p5dulum break1 clos$ by ! magnet swit* at #f4 ,?is swit* is me*anically connect$ to ! swit* #b by ! rod #d1 s t ! pair op5 & close toge!r4 ,! control op]ates z foll[s3 ,/>t+ ) ! motor at re/1 ! clock circuit is swit*$ on1 swit*es #b1 #f be+ op5 & #g clos$4 ,at ! fir/ beat ( ! p5dulum #b1 #f closes & ! curr5t1 %unt$ across ! loop conta9+ #e1 holds #b clos$ until ! motor has />t$ & brok5 ! clock circuit at ! tim]4 ,! fly-:eel c>ries on until ! p5dulum aga9 closes ! p[] circuit via #b1 #f1 & curr5t /ays on ! motor until ! tim] has complet$ its revolution4 ,?is goes on z ! motor spe$s up1 ! p]iodic p[] supply be+ %ort5$ z ! tim] breaks x e>li] ow+ to ! accel]ation1 until ! motor comes to its /eady spe$ at :i* ! p[] is appli$ j long en\gh to ma9ta9 uni=mity4 ,if ! motor = any cause t5ds to ov]spe$ ! cut-(f is e>li]1 :ile sl[+ d[n produces a long] p[]-p]iod br++ ! spe$ back to normal4 ,! p[] p]iod is g5]ally to second4 ,! p[] suppli$ to ! motor is v small ev5 9 ! example "h %[n1 only #a amp]e at #aaj volts4 ,illu/ration3 ,,fig4 #ach4--;,diagram ( ,g]ri% ,electric ,control4 ,! actual proportion ( a revolution dur+ :i* curr5t is suppli$ ! motor is !re=e rigor\sly det]m9$ by ! clock p5dulum1 & ! motor is select$ s t its revolutions are exactly tim$ to ?is clock p5dulum :i* has no "w to d o!r ?an ! circuit clos+1 & c h5ce be regulat$ to keep accurate "t4 ,! small fly-:eel 7#i71 ! weight ( :i* is c>efully adju/$ ) respect to ! g5]al am\nt ( "w to be done1 atta*$ to ! motor %aft1 effectively /eadies its action dur+ ! process ( gov]nm5t4 ,?is ,g]ri% type has be5 v>i\sly modifi$ 9 detail to suit ! in/rum5ts to :i* x has be5 appli$1 always foll[+ h[ev] ! same fundam5tal pr9ciples4 ,illu/ration3 ,,fig4 #aci4--;,g]ri% ,drive on #bd in* ,reflector4 ,an admirable example ( ! application ( ?is drive is %[n 9 ,fig4 #aci1 ! #bd in* reflector at ! ,h>v>d ,obs]vatory4 ,! m\nt is a massive op5 =k1 & ! motor drive is se5 on ! "r ( ! m\nt4 ,!re are "h two motors1 ord9>y fan motors 9 size4 ,! "r h& motor c>ries ! fly-:eel & runs /eadily on "u ! p5dulum control4 ,! o!r1 connect$ to ! same diff]5tial ge> z ! driv+ motor1 s]ves m]ely = indep5d5t regulation & c be run 9 ei!r direction by ! obs]v] to spe$ or sl[ ! motion 9 ,r4 ,a4 ,^! examples ( clock drive are m]ely typical ( ^? :i* h prov$ to be successful 9 use = v>i\s s]vice1 light & heavy4 ,!re are almo/ innum]able v>iations on clocks con/ruct$ on "o or ano!r ( ! g5]al l9es "h indicat$1 s _m v>iations 9 fact t "o almo/ might say !re are few driv+ clocks :i* are n 9 "s degree special4 ,! t5d5cy at pres5t is = l>ge in/rum5ts v di/9ctly t[>d a motor-driv5 me*anism op]at+ on ! "r asc5sion axis1 & gov]n$ 9 "o ( a consid]able v>iety ( ways by an actual clock p5dulum4 ,= small] in/rum5ts ! old me*anical clock1 (t5 fitt$ ) electric brake ge> & n[ & !n p5dulum regulat$1 is capable ( v excell5t "w4 ,! pr9ciple ( ! spectroscope is rudim5t>ily simple1 9 ! famili> decomposition ( :ite light into ra9b[ colors by a prism4 ,"o gets ! ph5om5a neatly by hold+ a n>r[ slit 9 a l>ge piece ( c>dbo>d at arms l5g? & look+ at x ?r\gh a prism held ) its edge p>allel to ! slit4 ,if ! light 7 n :ite b ( a mixture ( def9ite colors ea* color pres5t w\ld be repres5t$ by a sep>ate image ( ! slit in/ead ( ! images be+ m]g$ into a cont9u\s color$ b&4 ,) ! sun z s\rce ! cont9u\s spectrum is cross$ by ! d>k l9es fir/ mapp$ by ,fraunh(]1 ea* repres5t+ ! absorption by a relatively cool ext]ior lay] ( "s sub/ance t at a high] temp]ature bel[ gives a bright l9e 9 exactly ! same position4 ,! actual con/ruction ( ! a/ronomical spectroscope v>ies greatly accord+ to its use4 ,9 obs]vations on ! sun ! di/ant slit is br\ght ne>] = conv5i5ce by plac+ x 9 ! focus ( a small objective po9t$ t[>d ! prisms 7! collimator7 & ! spectrum is view$ by a telescope ( mod]ate magnify+ p[] to disclose m ( detail4 ,also1 s9ce !re is extremely bright light available1 v great disp]sion c be us$1 obta9$ by sev]al or _m prisms1 s t ! spectrum is bo? fairly wide1 7! l5g? ( ! slit7 & extremely long4 ,9 try+ to get ! spectrum ( a /> ! s\rce is a po9t1 equival5t to an extremely m9ute l5g? ( a v n>r[ slit4 ,!re=e no actual slit is necess>y & ! *ief tr\ble is to get ! spectrum wide en\gh & bright en\gh to exam9e4 ,! simple/ =m ( /ell> spectroscope & ! "o 9 mo/ common use ) small telescopes is ! ocul> spectroscope arrang$ mu* l ,fig4 #adj4 ,?is fits into ! eye tube ( a telescope & ! ,mc,clean =m made by ,br[n+ ( ,london consi/s ( an ord9>y cas+ ) screw coll> ,b1 a cyl9drical l5s ,c1 a direct vision prism c1 f1 c1 & an eye-cap ,a4 ,! draw tube is focuss$ on ! /> image z ) any o!r ocul>1 & ! light is deliv]$ ?r\gh ,c to ! prism face ne>ly p>allel1 & !nce goes to ! eye1 aft] disp]sion by ! prism4 ,?is consi/s ( a c5tral prism1 f1 ( l>ge angle1 made ( extremely d5se fl9t1 to :i* are cem5t$ a pair ( prisms ( light cr[n c1 c1 ) _! bases turn$ away f t ( f4 ,we h already se5 t ! disp]sions ( glasses v>y v mu* m ?an _! refractions s t ) prop] *oice ( mat]ials & angles ! refraction ( f is entirely comp5sat$ = "s *os5 "p ( ! spectrum1 :ile its disp]sion q ov]p[]s t ( ! cr[n prisms & gives a fairly long available spectrum4 ,! cyl9drical l5s ,c m]ely s]ves to /ret* out ! t9y r\nd /> image into a %ort l9e !reby giv+ ! result+ spectrum wid? en\gh to exam9e com=tably4 ,! weak cyl9drical l5s is sometimes slipp$ ov] ! eye end ( ! prisms to give ! ne$$ wid? ( spectrum in/ead ( putt+ x ahead ( ! prisms4 ,a small in/rum5t ( ?is k9d us$ ) a telescope ( #c in*es to #e in*es ap]ture gives a fairly good view ( ! spectra ( />ts above second or ?ird magnitude1 ! qualities ( tol]ably bright comets & nebul & s =?4 ,! visibility ( /ell> spectra v>ies greatly accord+ to _! type1 ^? ) heavy broad b&s be+ easy to obs]ve1 :ile = ! same /ell> magnitude spectra ) _m f9e l9es may be q beyond exam9ation4 ,nev]!less a little ocul> spectroscope enables "o to see _m ?+s well wor? ! tr\ble ( obs]v+4 ,illu/ration3 ,,fig4 #adj4--,mc,clean ,ocul> ,spectroscope4 ,) ! l>g] in/rum5ts1 say #f or #h in*es1 "o c well take advantage ( ! great] light to use a spectroscope ) a slit1 :i* gives some:at %>p] def9ition & also an opportunity to measure ! spectrum produc$4 ,an excell5t type ( su* an in/rum5t is t %[n 9 ,fig4 #ada1 due to ,pr(essor ,abb4 ,! con/ruction is analog\s to ,fig4 #adj4 ,! ocul> is a ,huygh5ian "o ) ! slit me*anism 7controll$ by a mill$ head7 at ,a 9 ! usual place ( ! diaphragm4 ,! slit is !re=e 9 ! focus ( ! eye l5s1 :i* s]ves z collimat+ l5s4 ,above is ! direct vision sy/em ,j ) ! usual prisms :i* are slightly adju/able lat]ally by ! screw ,p & spr+ ,q4 ,at ,n is a t9y transp>5t scale ( wave l5g?s illum9at$ by a fa9t light reflect$ f ! mirror ,o1 & 9 ! focus ( ! little l5s ,r1 :i* transf]s x by reflection f ! front face ( ! prism to ! eye1 alongside ! edge ( ! spectrum4 ,"o !re=e sees ! spectrum m>k$ (f by a bright l9e wave-l5g? scale4 ,illu/ration3 ,,fig4 #ada4--;,abb ,ocul> ,spectroscope4 ,! pivot ,k & clamp ,l enable ! :ole to be swung side-wise s t "o c look ?r\gh ! wid5$ slit1 locate ! />1 close ! slit accurately ^u x & sw+ on ! prisms4 ,m is ! clamp 9 position angle4 ,sometimes a comp>ison prism is add$1 toge!r ) suitable means = ?r[+ 9 spectra ( gases or metals alongside t ( ! />1 ?\gh ^! ref9em5ts are m g5]ally res]v$ = in/rum5ts ( high] disp]sion4 ,to w9 ! advantage ( accurate c5t]+ ( ! /> 9 ! field ga9$ by ! sw+-out ( ! spectroscope 9 ,fig4 #ada simple in/rum5ts l ,fig4 #adj are sometimes m\nt$ ) an ord9>y ocul> 9 a d\ble nose-piece l t us$ = microscope objectives1 s t ei!r c be us$ at w4 ,any ord9>y pocket spectroscope1 ) or )\t scale or a comp>ison prism ov] "p ( ! slit1 c 9 fact be fitt$ to an adapt] & us$ ) ! /> focuss$ on ! slit & a cyl9drical l5s1 if necess>y1 z an eye-cap4 ,su* slit spectroscopes readily give ! *>act]i/ics ( /ell> spectra & ^? ( ! bright] nebul or ( comets4 ,!y enable "o to id5tify ! m typical l9es & comp>e !m ) t]re/rial s\rces1 & save = sol> "w are ab\t all ! amateur obs]v] f9ds use =4 ,= s]i\s rese>* a good deal m ( an in/rum5t is requir$1 ) a l>ge telescope to collect ! light1 & means = photograph+ ! spectra = p]man5t record4 ,! cumulative effect ( prolong$ exposures makes x possible easily to record spectra mu* too fa9t to see ) ! same ap]ture1 & exposures are (t5 ext5d$ to _m h\rs4 ,spectroscopes = su* use commonly employ d5se fl9t prisms ( ab\t #fj refract+ angle & refractive index ( ab\t #a4fe1 "o1 two1 or ?ree ( ^! be+ fitt$ to ! in/rum5t z occasion requires4 ,a f9e example by ,bra%e> is %[n 9 ,fig4 #adb1 arrang$ = visual "w on ! #bd in* ,l[ell refractor4 ,"h ,a is ! slit1 ,b ! prism box1 ,c ! obs]v+ telescope1 ,d ! micromet] ocul> ) electric lamp = illum9at+ ! wires1 & ,e ! l9k motion t keeps ! prism faces at equal angles ) collimator & obs]v+ telescope :5 ! angle betwe5 ^! is *ang$ to obs]ve diff]5t p>ts ( ! spectrum4 ,?is precaution is necess>y to ma9ta9 ! be/ ( def9ition4 ,:5 photographs are to be tak5 ! obs]v+ telescope is unscrew$ & a photographic l5s & cam]a put 9 its place4 ,if ! brightness ( ! object p]mits1 ?ree prisms are in/all$1 turn+ ! beam #ahj into a cam]a brac$ to ! same frame alongside ! slit4 ,= purely photographic "w1 too1 ! objective prism us$ by ,fraunh(] = ! e>lie/ obs]vation ( /ell> spectra is 9 wide use4 ,x is a prism fitt$ 9 front ( ! objective ) its refract+ faces mak+ equal angles ) ! telescope & ! region to be obs]v$1 respectively4 ,its great advantages are small loss ( light & ! ability to photograph _m spectra at once1 = all ! />s 9 ! cle> field ( ! in/rum5t leave _! images spread out into spectra ^u ! photographic plate4 ,figure #adc %[s su* an objective prism m\nt$ 9 front ( an a/rographic objective4 ,! prism is rotatable into any azimu? ab\t ! axis ( ! objective & by ! scale i & clamp+ screw r c h its refract+ face adju/$ ) respect to t axis to ! be/ position = photograph+ any "p ( ! spectrum4 ,su* an arrangem5t is typical ( ^? us$ = small in/rum5ts say f #c in*es to #f in*es ap]ture4 ,illu/ration3 ,,fig4 #adb4--,typical ,/ell> ,spectroscope4 ,= l>g] objectives ! prism is usually ( decid$ly small] angle1 &1 if ! light w>rants high disp]sion1 sev]al prisms 9 t&em are us$4 ,! objective prism does its be/ "w :5 appli$ to true photographic objectives ( ! portrait l5s type :i* yield a fairly l>ge field4 ,x is by means ( big in/rum5ts ( su* sort t ! spectra = ! magnific5t ,drap] ,catalogue h be5 secur$ by ! ,h>v>d ,obs]vatory1 mo/ly at ! ,arequipa /ation4 ,9 photograph+ ) ! objective prism ! spectra are commonly giv5 ! necess>y wid? = conv5i5t exam9ation by *ang+ j a trifle ! rate ( ! driv+ clock s t !re is a slight & gradual drift 9 ,r4 ,a4 ,! refract+ edge ( ! prism be+ turn$ p>allel to ! diurnal motion ?is drift v gradually & uni=mly wid5s ! spectrum to ! ext5t ( a few m9utes ( arc dur+ ! :ole exposure4 ,:5 "o comes to sol> spectroscopy "o meets an entirely diff]5t situation4 ,9 /ell> "w ! difficulty is to get en\gh light1 & ! t5d5cy is t[>d l>ge objectives ( relatively %ort focal l5g? & spectroscopes ( mod]ate disp]sion4 ,9 sol> /udies !re is ample light1 & ! ma9 ?+ is to get an image big en\gh to be scrut9iz$ 9 detail ) v great disp]sion4 ,illu/ration3 ,,fig4 #adc4--,simple ,objective ,prism4 ,especially is ?is true 9 ! /udy ( ! *romosph]ic flames t rim ! sol> disc & blaze ov] its surface4 ,to exam9e ^! effectively ! spectroscope %\ld h imm5se disp]sion ) a m9imum am\nt ( /ray light 9 ! field to int]f]e ) vision ( delicate details4 ,9 us+ a spectroscope l ,fig4 #adb1 if "o turn$ ! slit t[>d ! l&scape1 ! in/rum5t be+ remov$ f ! telescope & ! slit op5$ wide1 he c\ld pla9ly see its v>i\s features1 refract$ ?r\gh ! prism1 & appe>+ 9 su* color z correspond$ to ! "p ( ! spectrum 9 ! l9e ( ! obs]v+ telescope4 ,9 o!r words "o sees refract$ images q di/9ctly 9 spite ( ! b5d+ ( ! rays4 ,) high disp]sion ! image se5 is practically mono*romatic4 ,n[ if "o puts ! spectroscope 9 place1 br+s ! sol> image tang5t to ! slit & !n cauti\sly op5s ! slit1 he sees ! bright cont9u\s spectrum ( ! sky close to ! sun1 plus any light ( ! p>ticul> color = :i* ! obs]v+ telescope is set1 :i* may proce$ f ! edge ( ! sol> disc4 ,?us1 if ! sett+ is = ! r$ l9e ( hydrog5 7,c71 "o sees ! hydrog5 gl[ t plays 9 fi]y pill>s ( cl\d ab\t ! sun's limb q pla9ly ?r\gh ! op5$ slit1 on a backgr\nd ( light /ream+ f ! adjac5t sky4 ,to see mo/ cle>ly "o mu/ use great disp]sion to spread ?is backgr\nd out into insignificance1 mu/ keep o!r /ray light out ( ! field1 & limit 8 view to ! op5$ slit4 ,illu/ration3 ,,fig4 #add4--;,diagram ( ,ev]%$ ,sol> ,spectroscope4 ,to ^! ends e>ly sol> spectroscopes _h _m prisms 9 t&em1 up to a doz5 or s1 kept 9 prop] relation by complicat$ l9kages analog\s to ! simple "o %[n 9 ,fig4 #adb4 ,details c be f\nd 9 almo/ any a/ronomical "w ( #dj ye>s ago4 ,!y 7 highly effective 9 giv+ disp]sion b nei!r improv$ ! def9ition nor cut out light reflect$ back & =? f _! _m surfaces4 ,( late simpl] con/ructions h come into use ( :i* an excell5t type is ! spectroscope design$ by ,mr4 ,ev]%$ & %[n 9 diagram 9 ,fig4 #add4 ,"h ! pa? ( ! rays is f ! slit ?r\gh ! collimator objective1 !n ?r\gh a v p[]ful direct vision sy/em1 giv+ a disp]sion ( #cj ?r\gh ! visible spectrum1 !n by reflection f ! mirror ?r\gh a second su* sy/em1 & !nce to ! obs]v+ telescope4 ,! mirror is rotat$ to get v>i\s p>ts ( ! spectrum into view1 & ! micromet] screw t turns x gives means = mak+ accurate measurem5t ( wave l5g?s4 ,!re are b five reflect+ surfaces 9 ! prism sy/em 7= ! cem5t$ prism surfaces d n c\nt = mu*7 z aga9/ m ?an #bj 9 "o ( ! old] in/rum5ts ( simil> p[]1 & z 9 o!r direct vision sy/ems ! spectrum l9es are sub/antially /raight in/ead ( be+ /rongly curv$ z ) multiple s+le prisms4 ,! result is ! light1 compact1 & p[]ful spectroscope %[n complete 9 ,fig4 #ade1 equally well fitt$ = obs]v+ ! sun's prom95ces & ! detail$ spectrum f 8 surface4 ,illu/ration3 ,,fig4 #ade4--;,ev]%$ ,sol> ,spectroscope4 ,9 mo/ ( ! sol> spectroscopes made at ! pres5t "t ! prisms are replac$ by a diffraction grat+4 ,! orig9al grat+s made by ,fraunh(] 7 made ( wire4 ,two p>allel screws ( extremely f9e ?read =m$ two opposite sides ( a brass frame4 ,a v f9e wire 0 !n w\nd ov] ^! screws1 made fa/ by sold] on "o side ( ea*1 & !n cut away on ! o!r1 s z to leave a grat+ ( p>allel wires ) cle> spaces betwe54 ,today ! grat+ is g5]ally rul$ by an automatic rul+ eng9e ^u a poli%$ plate ( speculum metal4 ,! diamond po9t c>ri$ by ! eng9e cuts v smoo? & f9e p>allel furr[s1 commonly f #aj1jjj to #bj1jjj to ! in*4 ,! spaces betwe5 ! furr[s reflect brilliantly & produce diffraction spectra4#bb #bb ,= ! pr9ciple ( diffraction spectra see ,baly1 ,spectroscopy2 ,kays]1 ,h&bu* d4 ,specktroskoie or any ( ! l>g] textbooks ( physics4 ,:5 a grat+ is us$ in/ead ( prisms ! in/rum5t is commonly set up z %[n 9 ,fig4 #adf4 ,"h ,a is ! collimator ) slit ^u :i* ! sol> image light falls1 ,b is ! obs]v+ telescope1 & ,c ! grat+ set 9 a rotatable m\nt ) a f9e ?read$ tang5t screw to br+ any l9e accurately ^u ! cross wires ( ! ocul>4 ,illu/ration3 ,,fig4 #adf4--;,diagram ( ,grat+ ,spectroscope4 ,! grat+ gives a s]ies ( spectra on ea* side ( ! slit1 violet ends t[>d ! slit1 & ) deviations proportional to #a1 #b1 #c1 #d1 etc41 times ! wave l5g? ( ! l9e consid]$4 ,! spectra !re=e ov]lap1 ! ultra violet ( ! second ord] be+ sup]impos$ on ! extreme r$ ( ! fir/ ord] & s on4 ,color$ scre5s ov] ! slit or ocul> are us$ to get ! ov]ly+ spectra out ( ! way4 ,! grat+ spectroscopes are v advantage\s 9 furni%+ a wide range ( available disp]sions1 & 9 giv+ less /ray light ?an a prism tra9 ( equal p[]4 ,! spectra moreov] are v ne>ly 8normal10 i4e41 ! position ( ea* l9e is proportional to its wave l5g? in/ead ( ! blue be+ disproportionately long z 9 prismatic spectra4 ,9 exam9+ sol> prom95ces ! wid5$ slit ( a grat+ spectroscope %[s !m =e%ort5$ or /ret*$ to an am\nt dep5d+ on ! angul> position ( ! grat+1 b ! effect is easily reckon$4#bc #bc ,! effect on ! obs]v$ height ( a prom95ce is h h s9 c_/s9 t1 :]e h is ! real height1 h ! app>5t height1 c ! angle made by ! grat+ face ) ! collimator1 & t t ) ! telescope 7,fig4 #adf74 ,if ! slit is ne>ly clos$ "o sees m]ely a ?9 l9e1 irregul>ly bright accord+ to ! %ape ( ! prom95ce2 a %ift ( ! slit ) respect to ! sol> image %[s a new irregul> section ( ! prom95ce 9 ! same mono*romatic light4 ,^! simple ph5om5a =m ! basis ( "o ( ! mo/ important in/rum5ts ( sol> /udy--! spectro-heliograph4 ,?is 0 devis$ almo/ simultane\sly by ,g4 ,e4 ,hale & ,m4 ,desl&res ab\t #cj ye>s ago1 & enables photographs ( ! sun to be tak5 9 mono*romatic light1 %[+ n only ! prom95ces ( ! limb b gl[+ masses ( gas scatt]$ all ov] ! surface4 ,! pr9ciple ( ! in/rum5t is v simple4 ,! collimator ( a p[]ful grat+ spectroscope is provid$ ) a slit ! full l5g? ( ! sol> diamet]1 arrang$ to slide smoo?ly on a ball-be>+ c>riage cle> across ! sol> disc4 ,j 9 front ( ! photographic plate set 9 ! focus ( ! cam]a l5s is ano!r n>r[ slid+ slit1 :i*1 l a focal plane %utt]1 exposes /rip aft] /rip ( ! plate4 ,! two slits are ge>$ toge!r by a sy/em ( lev]s or o!rwise s t !y move at exactly ! same uni=m rate ( spe$4 ,?us :5 ! front slit is lett+ ?r\gh a mono*romatic section ( a prom95ce on ! sun's limb ! plate-slit is at an exactly correspond+ position4 ,:5 ! front slit is exactly across ! sun's c5t] s is ! plate slit1 at ea* elem5t ( movem5t expos+ a l9e ( ! plate to ! mono*romatic image f ! mov+ front slit4 ,! grat+ c ( c\rse be turn$ to put any requir$ l9e into action b x usually is set = ! ,k l9e 7calcium71 :i* is photographically v brilliant & %[s bright masses ( float+ vapor all ov] ! sun's surface4 ,figure #adg %[s an e>ly & simple type ( ,pr(essor ,hale's in/rum5t4 ,"h ,a is ! collimator ) its slid+ slit1 ,b ! photographic telescope ) its correspond+ slide & ,c ! lev] sy/em :i* connects ! slides 9 p]fectly uni=m alignm5t4 ,! s\rce ( p[] is a v accurately regulat$ wat] pressure cyl9d] m\nt$ p>allel ) ! collimator4 ,! result is a complete photograph ( ! sun tak5 9 mono*romatic light ( exactly def9$ wave l5g? & %[+ ! precise di/ribution ( ! gl[+ vapor ( ! correspond+ sub/ance4 ,s9ce ! spectro-heliograph ( ,fig4 #adg1 :i* %[s ! pr9ciple rem>kably well1 !re h be5 made _m modifications1 9 p>ticul> = adapt+ ! s*eme to ! great horizontal & v]tical fix$ telescopes n[ 9 use4 7,= details ( ^! see ,cont4 f ! ,sol> ,obs4 ,mt4 ,wilson1 ,nos4 #c1 #d1 #bc1 & o!rs74 ,! *ief difficulty always is to secure entirely smoo? & uni=m motion ( ! two mov+ elem5ts4 ,illu/ration3 ,,fig4 #adg4--;,hale's ,spectro-heliograph 7,e>ly ,=m74 ,s great & int]e/+ a bran* ( a/ronomy is ! /udy ( v>iable />s t "s =m ( photomet] %\ld be "p ( ! equipm5t ( e telescope 9 s]i\s use = cele/ial obs]vation4 ,an imm5se am\nt ( useful "w has be5 done by ,argel&]'s sy/ematic me?od ( eye obs]vation1 b x is f> f be+ precise en\gh to disclose _m ( ! mo/ important features ( v>iability4 ,! conv5tional way ( reckon+ by /ell> magnitudes is conducive to loose measurem5ts1 s9ce ea* magnitude ( diff]5ce implies a light ratio ( :i* ! log is #j4d1 i4e41 ea* magnitude is #b4eab times bright] ?an ! foll[+ "o4 ,z a result ( ?is way ( reckon+ ! light ( a /> ( mag4 #i4i diff]s f "o ( mag4 #aj4j n by "o p] c5t b by ab\t n9e4 ,h5ce to grasp light v>iations ( small ord] "o mu/ be able to measure f> bel[ #j4am4 ,illu/ration3 ,,fig4 #adh4-;,d\ble ,image ,/ell> ,photomet]4 ,! ord9>y laboratory photomet] enables "o to comp>e light s\rces ( any:]e ne> simil> color to a probable error ( well "u #j4a p] c5t1 b x all[s a comp>ison betwe5 %>ply def9$ juxtapos$ fields f ! two illum9ants1 a condition mu* m favorable to precision ?an ! comp>ison ( two po9ts ( light1 ev5 if fairly ne> toge!r4 ,/ell> photomet]s may 9 pr9ciple be divid$ into ?ree classes4 7#a7 ,^? 9 :i* two actual />s are br\ght into ! same field & comp>$ by v>y+ ! light f "o or bo? 9 a kn[n degree4 7#b7 ,^? :i* br+ a real /> into ! field alongside an artificial />1 & aga9 br+ ! two to equality by a kn[n v>iation1 usually comp>+ two or m />s via ! same artificial />2 7#c7 ^? :i* measure ! light ( a /> by a def9ite me?od ( ext+ui%+ x entirely or j to ! v]ge ( disappe>ance 9 a kn[n progression4 ,( ea* class !re are div]s v>ieties4 ,! type ( ! fir/ class may be tak5 z ! late ,pr(essor ,e4 ,c4 ,pick]+'s pol>iz+ photomet]4 ,its optical pr9ciple is %[n 9 ,fig4 #adh4 ,"h ! brightness ( two neighbor+ objects is comp>$ by pol>iz+ at #ij ap>t ! light receiv$ f ea* & r$uc+ ! result+ images to equality by an analyz+ ,nicol prism4 ,! photomet] is fully describ$1 )1 sev]al o!r pol>iz+ in/rum5ts1 9 ,h4 ,a4 ,vol4 ,,ii f :i* ,fig4 #adh is tak54 ,a is a ,nicol prism ins]t$ 9 ! ocul> ,b1 :i* revolves c>ry+ ) x a divid$ circle ,c read aga9/ ! index ,d4 ,9 ! tube ,e :i* fits ! eye end ( ! telescope1 is plac$ ! d\ble image qu>tz prism ,f capable ( slid+ ei!r way )\t rotation by pull+ ! cord ,g4 ,) ! objects to be comp>$ 9 ! same field1 two images ( ea* appe>4 ,by turn+ ! analyz+ ,nicol ! fa9t] image ( ! bright] c always be r$uc$ to equality ) ! bright] image ( ! fa9t]1 & ! am\nt ( rotation measures ! requir$ ratio ( brightness4#bd ,?is in/rum5t works well = objects ne> en\gh to be 9 ! same field ( view4 ,! di/ance betwe5 ! images c be adju/$ by slid+ ! prism ,f back & =?1 b ! available range ( view is limit$ to a small fraction ( a degree 9 ord9>y telescopes4 #bd ,if ,a be ! brightness ( "o object & ,b t ( ! o!r1 ! read+ ( ! index :5 "o image disappe>s & ! read+ :5 ! two images are equal !n ,a_/,b tan7-74 ,!re are f\r positions ( ! ,nicol1 #ij ap>t1 = :i* equality c be e/abli%$1 & usually all are read & ! mean tak54 7,h4 ,a4 ,,ii1 #a47 ,! m]idian photomet] 0 design$ to avoid ?is small scope4 ,! photometric device is sub/antially ! same z 9 ,fig4 #adh4 ,! objects comp>$ are br\ght into ! field by two exactly simil> objectives plac$ at a small angle s t ! images1 aft] pass+ ! d\ble image prism1 are sub/antially 9 co9cid5ce4 ,9 front ( ea* ( ! objectives is a mirror4 ,! in/rum5t po9ts 9 ! ea/ & we/ l9e & ! mirrors are at #de ) its axis4 ,"o br+s ,pol>is into ! field1 ! o!r by a motion ( rotation ab\t ! telescope axis c br+ any object 9 or close to ! m]idian into ! field alongside ,pol>is4 ,! images are !n comp>$ precisely z 9 ! prec$+ in/ance4#be ,!re are suitable adju/m5ts = br++ ! images into ! positions requir$4 #be ,= full description & me?od see ,h4 ,a4 ,vol4 #ad1 also ,miss ,furness' admirable 8,introduction to ! ,/udy ( ,v>iable ,/>s10 p4 #abb1 et seq4 ,"s modifications are describ$ 9 ,h4 ,a4 ,vol4 #bc4 ,^! direct comp>ison photomet]s give results subject to "s annoy+ small corrections1 b a va/ am\nt ( valuable "w has be5 done ) !m 9 ! ,h>v>d ,photometry4 ,! v>i\s =ms ( photomet] us+ an artificial /> z int]m$i>y 9 ! comp>ison ( real />s diff] *iefly 9 ! me?od ( v>y+ ! light 9 a det]m9ate measure4 ,r ! be/ kn[n is ! ,zlln] in/rum5t %[n 9 diagram 9 ,fig4 #adi4 ,"h ,a is ! eye end ( ! ma9 telescope tube4 ,across x at an angle ( #de is ?r[n a piece ( plane p>allel glass ,b :i* s]ves to reflect to ! focus ! beam f d[n ! side tube1 ,c1 =m+ ! artificial />4 ,illu/ration3 ,,fig4 #adi4--,zlln] ,photomet] ,diagram4 ,at ! end ( ?is tube is a small hole or m (t5 a diaphragm p]=at$ ) sev]al v small holes any ( :i* c be br\ght into ! axis ( ! tube4 ,beyond at ,d1 is ! s\rce ( light1 orig9ally a lamp flame1 n[ g5]ally a small inc&esc5t lamp1 ) a gr\nd glass disc or surface uni=mly to diffuse ! light4 ,)9 ! tube ,c lie ?ree ,nicol prisms n1 n#a1 n#b4 ,( ^! n1 is fix$ ) respect to ! mirror ,b & =ms ! analys]1 :i* n#a & n#b turn toge!r =m+ ! pol>iz+ sy/em4 ,betwe5 n#a & n#b is a qu>tz plate e cut p]p5dicul> to ! cry/al axis4 ,! color ( ! light transmitt$ by su* a plate 9 pol>iz$ light v>ies ?r\gh a wide range4 ,by turn+ ! ,nicol n#b !re=e1 ! color ( ! beam :i* =ms ! artificial /> c be made to mat* ! real /> "u exam9ation1 & !n by turn+ ! :ole sy/em n#b1 ,e1 n#a1 read+ ! rotation on ! divid$ circle at ,f1 ! real /> c be mat*$ 9 int5sity by ! artificial "o4 ,illu/ration3 ,,fig4 #aej4--,w$ge ,photomet]4 ,?is is view$ via ! l5s ,g & two t9y po9ts ( light appe> 9 ! field ( ! ocul> due respectively to reflection f ! front & back ( ! mirror ,b1 ! latt] slightly fa9t] ?an ! =m]4 ,alongside or betwe5 ^! ! real /> image c be br\ght = a comp>ison1 & by turn+ ! pol>iz] ?r\gh an angle ! images c be equaliz$ ) ! real image4 ,!n a simil> comp>ison is made ) a ref]5ce />4 ,if ,a be ! brightness ( ! =m] & ,b ( ! latt] !n ,a_/,b s9_/s9 ,! ,zlln] photomet] 0 at fir/ set up z an alt-azimu? in/rum5t ) a small objective & rotation 9 altitude ab\t ! axis ,c4 ,s9ce ! g5]al use ( electric lamps in/ead ( ! inconv5i5t flame x is (t5 fitt$ to ! eye end ( an equatorial4 ,ano!r v useful in/rum5t is ! mod]n w$ge photomet]1 closely resembl+ ! ,zlln] 9 "s respects b ) a v diff]5t me?od ( v>y+ ! light2 devis$ by ! late ,pr(essor ,e4 ,c4 ,pick]+4 ,x is %[n some:at 9 diagram 9 ,fig4 #aej4 ,"h z be=e ,o is ! eye end ( ! tube1 ,b ! plane p>allel reflector1 ,c ! side tube1 ,l ! s\rce ( light ,d ! diaphragm & ,a ! l5s =m+ ! artificial /> by project+ ! hole 9 ! diaphragm4 ,9 actual practice ! diamet] ( su* hole is #a_/ajj in* or less4 ,illu/ration3 ,,fig4 #aea4--,simple ,pol>iz+ ,photomet]4 ,! light v>y+ device ,w is a 8photographic w$ge0 set 9 a frame :i* is graduat$ on ! edge & mov$ 9 front ( ! ap]ture by a rack & p9ion at ,r4 ,!re are beside color$ & %ade glasses = use z occasion requires4 ,! 8photographic w$ge0 is m]ely a /rip ( f9e gra9$ photographic plate giv5 an ev5ly graduat$ exposure f end to end1 develop$1 & seal$ "u a cov] glass4 ,its absorption is p]man5t1 non-selective z to color1 & x c be made to %ade (f f a b>ely p]ceptible d5sity to any requir$ opacity4 ,sometimes a w$ge ( neutral t9t$ glass is us$ 9 its /ead4 ,be=e us+ su* a 8w$ge photomet]0 ! w$ge mu/ be accurately calibrat$ by obs]vation ( real or artificial />s ( kn[n diff]5ce 9 brightness4 ,?is is a task dem&+ mu* c>e & is well describ$1 toge!r ) ! :ole in/rum5t by ,p>khur/ 7,ap4 ,j4 #ac1 #bdi74 ,! great difficulty ) all in/rum5ts ( ?is g5]al type is ! =mation ( an artificial /> ! image ( :i* %all v closely resemble ! image ( ! real /> 9 appe>ance & color4 ,obvi\sly ei!r ! real or artificial />1 or bo?1 may be v>i$ 9 int5sity by w$ge or ,nicols1 & a v s]viceable modification ( ! ,zlln] in/rum5t1 ) ?is 9 m9d 0 rec5tly describ$ by ,%ook 7,pop4 ,a/4 #bg1 #eie7 & is %[n 9 diagram 9 ,fig4 #aea4 ,"h ,a is ! tube :i* fits ! ord9>y eyepiece sleeve4 ,e is a side tube into :i* is fitt$ ! ext5sion ,d ) a fitt+ ,h at its out] end into :i* sets ! lamp tube ,g4 ,?is c>ries on a base plug ,f a small fla% light bulb run by a c\ple ( dry cells4 ,at ,o is plac$ a little brass diaphragm p]=at$ ) a m9ute hole4 ,betwe5 ?is & ! lamp is a disc ( diffus+ glass or pap]4 ,a ,nicol prism is set a little ahead ( ,o1 & a l5s ,l focusses ! p]=ation at ! pr9cipal focus ( ! telescope aft] reflection f ! diagonal glass ,m1 z 9 ! prec$+ examples4 ,i is an ord9>y eyepiece ov] :i* is a rotatable ,nicol ,n ) a position circle ,k4 ,at ,p is a ?ird ,nicol 9 ! pa? ( ! rays f ! real />1 !reby increas+ ! conv5i5t range ( ! in/rum5t4 ,! orig9al pap] gives ! details ( con/ruction z well z ! me?ods ( work+4 ,obvi\sly ! same g5]al arrangem5t c\ld be us$ = a w$ge photomet] us+ ! w$ge on ei!r real or artificial /> or bo?4 ,! ?ird type ( visual photomet] dep5ds on r$uc+ ! light ( ! /> obs]v$ until x j disappe>s4 ,?is plan 0 ext5sively employ$ by ,pr(essor ,prit*>d ( ,ox=d "s #dj ye>s ago4 ,he us$ a slid+ w$ge ( d>k glass1 c>efully calibrat$1 & comp>$ two />s by not+ ! po9t on ! w$ge at :i* ea* 0 ext+ui%$4 ,a photographic w$ge may be us$ 9 exactly ! same way4 ,ano!r device to ! same end dep5ds on r$uc+ ! ap]ture ( ! telescope by a 8cat's eye10 an iris diaphragm1 or simil> means until ! /> is no long] visible or j disappe>+4 ,! great objection to su* me?ods is ! extremely v>iable s5sitivity ( ! eye "u v>y+ /imulus ( light4 ,! mo/ t c be said = ! ext9ction photomet] is t 9 skillful & exp]i5c$ h&s l ,prit*>d's x has sometimes giv5 mu* m consi/5t read+s ?an w\ld be expect$4 ,x is n[ & !n v conv5i5t = quick approximation b by no c\rtesy c x be consid]$ an in/rum5t ( precision ei!r 9 a/ronomical or o!r photometry4#bf #bf ,! g5]al ord] ( precision atta9$ by a/ronomical photomet]s is %[n 9 ! discov]y1 photographically1 by ,h]tzsprung 9 #aiaa1 t ,pol>is1 us$ z a /&>d magnitude = _m ye>s1 is actually a v>iable4 ,its p]iod is v ne> to f\r days1 its photographic amplitude #j4ag & its visual amplitude ab\t #j4a1 i4e41 a v>iation ( #e p] c5t 9 ! light 0 subm]g$ 9 ! obs]vational unc]ta9ties1 al?\gh once kn[n x 0 trac$ out 9 ! accumulat$ data )\t great difficulty4 ,! photomet] "q %\ld n be clos$ )\t ref]r+ ! read] to ! me?ods ( physical photometry z develop$ by ,/ebb9s1 ,gu?nick & o!rs4 ,! fir/ ( ^! dep5ds on ! use ( ! sel5ium cell 9 :i* ! electrical resi/ance falls on exposure ( ! sel5ium to light4 ,! device is n "o adapt$ to casual use1 & requires v c>eful nurs+ to give ! be/ results1 b ^! are ( an ord] ( precision beyond any?+ yet rea*$ ) an a/ronomical visual photomet]4 ,sett+s come d[n to v>iations ( some?+ l #b p] c5t1 & v>iations 9 /ell> light entirely escap+ previ\s me?ods become obvi\s4 ,! photoelectric cell dep5ds on ! l[]+ ( ! app>5t electric resi/ance ( a lay] ( r>ifi$ in]t gas betwe5 a plat9um grid & an electrode ( metallic potassium or o!r alkali metal :5 light falls on t electrode4 ,! rate ( transmission ( electricity is v exactly proportional to ! illum9ation1 & c be be/ measur$ by a v s5sitive electromet]4 ,! results are extraord9>ily consi/5t1 & ! !oretical 8probable error0 is v small1 ?\gh "h1 z else:]e1 8probable error0 is a r mean+less t]m apt to lead to a false presumption ( exactness4 ,aga9 ! app>atus is some:at intricate & delicate1 b gives a precision ( work+ if any?+ a little bett] ?an t ( ! sel5ium cell1 q c]ta9ly bel[ #a p] c5t4 ,nei!r in/rum5t con/itutes an atta*m5t to ! ord9>y telescope ( mode/ size :i* c be successfully us$ = ord9>y photometry1 & bo? require r$uction ( results to ! basis ( visual effect4#bg ,b bo? (f] great promise 9 detect+ m9ute v>iations ( light & h done admirable "w4 ,= a good fundam5tal description ( ! sel5ium cell photomet] see ,/ebb9s1 ,ap4 ,j4 #cb1 #ahe & = ! photoelectric me?od see ,gu?nick ,a4 ,n4 #aif1 #ceg also ,a4 ,f4 & ,f4 ,a4 ,l9demann1 ,m4 ,n4 #ci1 #cdc4 ,! volume by ,miss ,furness already ref]r$ to gives "s int]e/+ details ( bo?4 #bg ,su* app>atus is ess5tially appurt5ant to l>ge in/rum5ts only1 say ( n less ?an #ab ap]ture & pref]ably mu* m4 ,! eye is enorm\sly m s5sitive z a detector ( radiant en]gy ?an any device ( human contrivance1 & ?us small telescopes c be well us$ = visual photometry1 ! bigg] in/rum5ts hav+ !n m]ely ! advantage ( rea*+ fa9t] />s4 ,,*apt] ,,ix ,,! ,,c>e ,,& ,,te/+ ,,( ,,telescopes ,a ^w at ! />t conc]n+ ! *oice & pur*ase ( telescopes4 ,! "q ( refractors vs4 reflectors has be5 already consid]$4 ,! outcome ( ! case dep5ds on h[ mu* & h[ (t5 y are likely to use ! in/rum5t1 & j :at y want x =4 ,= casual obs]vations & occasional use--all t mo/ busy buy]s ( telescopes c expect--! refractor has a decid$ advantage 9 conv5i5ce4 ,if "o has leisure = frequ5t obs]vations1 & p>ticul>ly if he c give 8 telescope a p]man5t m\nt1 & is go+ 9 = s]i\s "w1 he w d well n to dismiss ! idea ( a reflector )\t due delib]ation4 ,9 any case x is good policy to procure an in/rum5t f "o ( ! be/ mak]s4 ,& if y d n buy directly ( ! actual mak] x is be/ to deal ) 8 accr$it$ ag5ts4 ,9 o!r words avoid telescopes casually pick$ up 9 ! optical trade unless y *ance to h facilities = ?or\gh te/+ "u compet5t guidance be=e pur*ase4 ,no bett] telescopes are made ?an c be _h f ! be/ ,am]ican mak]s4 ,a few ,briti% & ,g]man mak]s are q 9 ! same class4 ,s few high grade ,fr5* telescopes rea* ?is c\ntry z to cause a r common1 b actually unju/1#bh belief t !re are none4 #bh ,e4 g41 ! beautiful a/rographic & o!r objectives turn$ out by ! bro!rs ,h5ry4 ,if economy mu/ be en=c$ x is mu* wis] to try to pick up a us$ in/rum5t ( fir/ class manufacture ?an to *ance a new "o at a l[ price4 ,n[ & !n a mak] ( v ord9>y repute may turn out a good in/rum5t1 b ! fact is "o to be prov$--n assum$4 ,age & use d n s]i\sly det]iorate a telescope if x has be5 giv5 prop] c>e4 ,"s ( ,fraunh(]'s are /ill do+ good s]vice aft] a c5tury1 & occasionally an in/rum5t f "o ( ! great mak]s comes into ! m>ket at a real b>ga94 ,x may drift back to ! mak] = resale1 or turn up at any optician's %op1 & 9 any case is bett] wor? look+ at ?an an equally *eap new telescope4 ,! condition ( ! tube & /& cuts little figure if !y are me*anically 9 good %ape4 ,mo/ ( ! old] high grade in/rum5ts 7 ( brass1 beautifully f9i%$ & lacqu]$1 & no?+ looks worse aft] h>d usage4 ,x is ess5tial t ! fitt+ ( ! p>ts %\ld be accurate & t ! focuss+ rack %\ld "w ) ! utmo/ smoo?ness4 ,a fault j "h1 h[ev]1 c be rem$i$ at small co/4 ,! m\nt1 :atev] its "*1 %\ld be likewise smoo? work+ & )\t a trace ( %ak9ess1 unless "o figures on ?r[+ x away4 ,z to ! objective1 x dem&s v c>eful exam9ation be=e a real te/ ( its optical qualities4 ,! objective ) its cell %\ld be tak5 out & closely scrut9iz$ 9 a /rong light aft] ! sup]ficial du/ has be5 remov$ ) a camel's hair bru% or by wip+ v g5tly ) ! s(t ,japanese 8l5s pap]0 us$ by opticians4 ,"o is likely to f9d pl5ty to look at2 spots1 f+] m>ks1 obvi\s scrat*es1 & :at is worse a network ( sup]ficial scrat*es1 or a surface ) pat*es look+ l v f9e pitt+4 ,^! la/ two defects imply ! ne$ ( repoli%+ ! affect$ surface1 :i* means also m or less refigur+4 ,ord9>y br[ni% spots & f+] m>ks c usually be remov$ ) little tr\ble4 ,! layman1 s to speak1 is (t5 w>n$ nev] to remove ! cell f a telescope b he might z well le>n ! simpl] adju/m5ts fir/ z la/4 ,9 tak+ (f a cell ! ma9 ?+ is to see :at "o is ab\t & to proce$ 9 an ord]ly mann]4 ,if ! :ole cell unscrews1 z (t5 is ! case 9 small in/rum5ts1 ! only precaution requir$ is to put a p5cil m>k on ! cell & its seat s t x c be screw$ back to :]e x />t$4 ,if z is m usual ! cell fits on ) ?ree pairs ( screws1 "o ( ea* pair w =m an abutm5t aga9/ :i* its mate pulls ! cell4 ,a p5cil m>k locat+ ! position ( ! head ( ea* ( ! pull+ screws enables "o to back !m out & replace !m )\t %ift+ ! cell4 ,! fir/ inspection w g5]ally tell :e!r ! objective is wor? fur!r tr\ble or n4 ,if all surfaces save ! front are 9 good condition x may pay to s5d ! objective to ! mak] = repoli%+4 ,if m ?an "o surface is 9 bad %ape rework+ h>dly pays unless ! l5s c be _h = a nom9al figure4 ,9 buy+ a us$ in/rum5t f its orig9al s\rce ^! precautions are ne$less z ! mak] c be tru/$ to /& back ( 8 own & to put x 9 fir/ class condition4 ,h[ev]1 grant$ t ! objective /&s well ! inspection = sup]ficial defects1 x %\ld !n be giv5 a real te/ = figure & color correction1 be>+ 9 m9d t objectives1 ev5 f fir/ class mak]s1 may n[ & !n %[ slightly faulty corrections1 :ile ^? f comp>atively unkn[n s\rces may n[ & !n turn out well4 ,9 ?is matt] ( necess>y te/+ old & new glasses are q on all f\rs save t "o may safely tru/ ! mak] ) a well e>n$ reputation to make "r any imp]fections4 ,cleans+ o!r ?an du/+ (f & cauti\sly wip+ ) damp & !n dry l5s pap] requires removal ( ! l5ses f _! cell :i* dem&s real c>e4 ,) a promis+ look+ objective1 old or new1 ! fir/ te/ to be appli$ is ! artificial />--artificial r ?an natural s9ce ! =m] /ays put & c be us$ by day or by night4 ,= day use ! 8/>0 is m]ely ! bright reflection ( ! sun f a %>ply curv$ surface--! %\ld] ( a small r\nd bottle1 a sph]ical flask silv]$ on ! inside1 a small silv]$ ball su* z is us$ = ,*ri/mas tree decoration1 a bicycle ball1 or a glass 8alley0 de> to ! he>t ( ! small boy4 ,! object1 :atev] x is1 %\ld be set up 9 ! sun aga9/ a d>k backgr\nd di/ant say #dj or #ej times ! focal l5g? ( ! objective to be te/$4 ,! writ] r likes a silv]$ ball cem5t$ to a big %eet ( black c>dbo>d4 ,at night a p9 hole say #a_/cb in* or less 9 diamet] ?r\gh c>dbo>d or bett]1 t9foil1 ) a flame1 or bett] a gas fill$ inc&esc5t lamp beh9d x1 answ]s well4 ,! latt] requires r c>eful adju/m5t t ! project$ area ( ! closely coil$ little filam5t may prop]ly fill ! p9hole j 9 front ( x4 ,n[ if "o sets up ! telescope & focusses x approximately ) a l[ p[] ! /> c be accurately c5t]$ 9 ! field4 ,!n if ! eyepiece is remov$1 ! tube rack$ 9 a bit1 & ! eye br\ght into ! focus ( ! objective1 "o c inspect ! objective = /ri4 ,if ^! are abs5t ! field w be uni=mly bright all ov]4 ,n infrequ5tly h[ev] "o w see a field l ,fig4 #aeb or ,fig4 #aec4 ,! =m] is ! appe>ance ( a #d in* objective t ! au?or rec5tly got 8 eye ^u4 ,! latt] %[s typical /ri ( ! ord9>y sort4 ,an objective ( glass z bad z %[n 9 ,fig4 #aeb gives no hope ( a/ronomical usefulness1 & %\ld be relegat$ to ! por* ( a sea%ore cottage4 ,figure #aec may repres5t a condition practically h>mless ?\gh undesirable4 ,! next /ep is a really critical exam9ation ( ! focal image4 ,us+ a mod]ately high p[] ocul>1 magnify+ say #ej to ! in* ( ap]ture1 ! /> %\ld be br\ght to ! %>pe/ focus possible & ! image closely exam9$4 ,if ! objective is good & 9 adju/m5t ?is image %\ld be a v small spot ( light1 p]fectly r\nd1 s(t5+ v slightly 9 its brilliancy t[>d ! edge1 & surr\nd$ by two or ?ree ?91 %>p1 r+s ( light1 exactly circul> & ) well def9$ d>k spaces sep>at+ !m4 ,illu/ration3 ,,fig4 #aeb4--;,a ,bad ,case ( ,/ri4 ,illu/ration3 ,,fig4 #aec4--,ord9>y ,/ri4 ,(t5 f ! trembl+ ( ! air ! r+s w seem %aky & brok51 b /ill well c5t]$ on ! />-disc4 ,! g5]al appe>ance is t %[n 9 ,fig4 #aed4bi #bi ,?is & sev]al ( ! subsequ5t figures are tak5 f q ! be/ acc\nt ( te/+ objectives3 8,on ! ,adju/m5t & ,te/+ ( ,telescope ,objectives40 ,t4 ,cooke ,sons1 ,york1 #ahia1 a little bro*ure unhappily long s9ce out ( pr9t4 ,a new edition is j n[1 #aibb1 ann\nc$4 ,illu/ration3 ,,fig4 #aed4--;,a ,fir/ ,class ,/> ,image4 ,in/ead1 sev]al v diff]5t appe>ances may turn up4 ,fir/1 ! bright diffraction r+s may be visible only on "o side ( ! c5tral disc1 :i* may itself be drawn out 9 ! same direction4 ,second1 ! be/ image obta9able may be fairly %>p b angul> or irregul> in/ead ( r\nd or oval & p]haps ) a hazy fl>e on "o side4 ,?ird1 x may be impossible to get a really %>p focus any:]e1 ! image be+ a m]e blob ( light ) no?+ def9ite ab\t x4 ,"o %\ld be v sure t ! eyepiece is clean & )\t fault be=e proce$+ fur!r4 ,z to ! fir/ po9t a bit ( l5s pap] made into a t9y swab on a sliv] ( s(t wood w be ( s]vice1 & ! surfaces %\ld be inspect$ ) a pocket l5s 9 a good light to make sure t ! clean+ has be5 ?or\gh4 ,turn+ ! ocul> r\nd w %[ :e!r any app>5t defects ( ! image turn ) x4 ,9 ! fir/ case m5tion$ ! next /ep is to rack ! ocul> g5tly out :5 ! /> image w exp& into a m or less conc5tric s]ies ( bright int]f]5ce r+s sep>at$ by d>k spaces1 half a doz5 or s result+ f a r small movem5t out ( focus4 ,if ^! r+s are out ( r\nd & ecc5tric l ,fig4 #aee "o has a cle> case ( failure ( ! objective to be squ>e ) ! tube1 s t ! ocul> looks at ! image askew4 ,illu/ration3 ,,fig4 #aee4--;,effect ( ,objective ,askew4 ,9 ! ord9>y =ms ( objective ?is means t ! side ( ! objective t[>d ! bright] & less exp&$ "p ( ! r+ sy/em is too ne> ! ocul>4 ,?is c be rem$i$ by pu%+ t side ( ! objective outw>ds a trifle4 ,eas+ (f ! pull+ screw on t side & slightly tight5+ ! abutm5t screw makes ! ne$$ correction1 :i* c be less5$ if ov] done at ! fir/ trial1 until ! r+ sy/em is accurately c5t]$4 ,x is a r fussy job b n at all difficult if "o rememb]s to proce$ cauti\sly & to use ! screw driv] g5tly4 ,illu/ration3 ,,fig4 #aef4--;,effect ( ,flaws 9 ,objective4 ,9 ! second case1 rack+ out ! ocul> a little gives a r+ sy/em :i* exagg]ates j ! defects ( ! image4 ,! faults may be due to me*anical /ra9 ( ! objective 9 its cell1 :i* is easily cur$1 or to /ra9s or flaws 9 ! glass itself1 :i* are irrem$iable4 ,!re=e "o %\ld1 ) ! plane ( ! objective horizontal1 loos5 ! reta9+ r+ t holds ! l5ses1 )\t di/urb+ !m1 & !n set x back 9 g5tle contact & try ! out ( focus r+s once m4 ,if !re is no m>k$ improvem5t ! fault lies 9 ! glass & no m "t %\ld be wa/$ on t p>ticul> objective4 ,fig4 #aef is a typical example ( ?is fault4 ,9 deal+ ) case ?ree x is well to give ! l5s a *ance by reliev+ x ( any su* me*anical /ra9s1 = n[ & !n !y w app>5tly utt]ly ru9 ! def9ition1 b ! prognosis is v bad unless ! objective has be5 mo/ brutally mi%&l$4 ,9 any case failure to give a %>ply def9$ focus 9 a v def9ite plane is a w>n+ t ! l5s 7or mirror7 is r bad4 ,9 te/+ a reflector "s pa9s mu/ be tak5 at ! />t ) bo? ! ma9 & ! second>y mirror4 ,us+ an artificial /> z be=e1 "o %\ld focus & look %>ply to ! symmetry ( ! image1 tak+ c>e to leave ! in/rum5t 9 obs]v+ position & scre5$ f ! sun = an h\r or two be=e te/+4 ,reflectors are mu* m s5sitive to temp]ature ?an refractors & take long] to settle d[n to /ability ( figure4 ,) a well m\nt$ telescope ( ei!r sort a /> at fair altitude on a f9e night gives ev5 bett] te/+ conditions ?an an artificial />1 7,pol>is is good 9 nor!rn latitudes7 b "o may h a long wait4 ,if ! reflector is ( good figure & well adju/$1 ! /> image1 9 focus or out1 has q ! same appe>ance z 9 a refractor except t ) a bright /> 9 focus "o sees a ?9 %>p cross ( light c5t]$ on ! image1 r fa9t b p]fectly di/9ct4 ,?is is due to ! diffraction effect ( ! f\r ?9 /rips t support ! small mirror1 & fades z ! /> is put out ( focus4 ,! r+s !n appe> z usual1 b also a black disc due to ! %ad[+ ( ! small mirror4 ,fig4 #aeg %[s ! extra-focal image ( a real or artificial /> :5 ! mirror is well c5t]$1 & ! /> 9 ! middle ( ! field4 ,!re only are ! r+s r\nd & conc5tric ) ! mirror spot4 ,! r+s g out ( r\nd & ! spot out ( c5t] = v small dep>ture f ! middle ( ! field :5 ! mirror is ( l>ge relative ap]ture--,f_/#e or ,f_/#f4 ,illu/ration3 ,,fig4 #aeg4--;,extra-focal ,image f ,reflector4 ,if ! /> image %[s fl>e or oval out-(-focus r+s :5 c5tral ( ! field1 "o or bo? mirrors probably ne$ adju/m5t4 ,be=e lay+ ! tr\ble to imp]fect figure1 ! mirrors %\ld be adju/$1 ! small "o fir/ z ! mo/ likely s\rce ( tr\ble4 ,! side ( ! mirror t[>d :i* ! fl>e or ! exp&$ side ( ! r+ sy/em projects %\ld be slightly pu%$ away f ! ocul>4 7,note t ow+ to ! reflection ?is movem5t is ! rev]se ( t requir$ ) a refractor47 ,if ! lack ( symmetry p]si/s "o may z well get d[n to fir/ pr9ciples & c5t] ! mirrors at once4 ,p]haps ! easie/ plan is to prep>e a disc ( :ite c>dbo>d exactly ! size ( ! mirror ) conc5tric circles laid out ^u x & an eigh? in* hole 9 ! c5t]4 ,tak+ out ! ocul> & putt+ a half in* /op 9 its place "o c /& back1 l9+ up ! /op ) ! draw tube1 & see :e!r ! small mirror looks p]fectly r\nd & is conc5tric ) ! reflect$ circles4 ,if n1 a t\* ( ! adju/+ screws w be ne$$4 ,!n ) a f9e po9t$ bru% dot ! c5t] ( ! mirror itself ?r\gh ! hole1 ) :ite pa9t4 ,!n1 remov+ ! c>d1 "o w see ?is dot accurately c5t]$ 9 ! small mirror if ! l>ge "o is 9 adju/m5t1 & x rema9s z a p]man5t ref]5ce po9t4 ,if ! dot be ecc5tric x c be treat$ z be=e1 b by ! adju/+ screws ( ! l>ge mirror4 ,! f9al adju/m5t c !n be made by gett+ a slightly extra-focal /> image fairly 9 ! c5t] ( ! field ) a r high p[] & mak+ ! sy/em conc5tric z be=e describ$4 ,?is s\nds a bit complicat$ b x really is n4 ,if ! l>ge mirror is n 9 place1 its c\nt] cell may well be c5t]$ & levell$ by help ( a plumb l9e f ! c5t] ( ! small mirror & a /eel squ>e1 z a />t+ po9t1 ! small mirror hav+ be5 c5t]$ z ne>ly z may be by measurem5t4#cj #cj ,sometimes ) ev] s c>eful c5t]+ ! r+ sy/em 9 ! middle ( ! field is /ill ecc5tric ) respect to ! small mirror1 %[+ t ! axis ( ! p>abola is n p]p5dicul> to ! g5]al face ( ! mirror4 ,?is c usually be rem$i$ by ! adju/+ screws ( ! ma9 mirror z describ$1 b n[ & !n x is necess>y actually to move ov] ! small mirror into ! real optical axis4 ,drap] 7loc4 cit47 gives "s exp]i5ces ( ?is sort4 ,s mu* = ! g5]al adju/m5t ( ! objective or mirror4 ,its actual quality is %[n only on c>eful exam9ation4 ,z a />t+ po9t "o may take ! extra-focal sy/em ( r+s giv5 by an objective or mirror aft] prop] c5t]+4 ,if ! sph]ical ab]ration has ?or\ghly remov$ ! appe>ance ( ! r+s :5 exp&$ s t six or eight are visible %\ld be l ,fig4 #aeh4 ,! c5t] %\ld be a %>ply def9$ bright po9t & surr\nd+ x1 & exactly conc5tric1 %\ld be ! int]f]5ce r+s1 truly circul> & gradually increas+ 9 int5sity outw>ds1 ! la/ be+ v noticeably ! /ronge/4 ,"o c be/ make ! te/ :5 look+ ?r\gh a yell[ glass scre5 :i* removes ! some:at confus+ fl>e due to imp]fect a*romatism & makes ! appe>ances inside & outside focus closely simil>4 ,j inside or outside ( focus ! appe>ance %\ld be t ( ,fig4 #aei = a p]fectly correct$ objective or mirror4 ,illu/ration3 ,,fig4 #aeh4--;,correct ,extra-focal ,image4 ,sometimes an objective w be f\nd 9 :i* "o edge ( ! focuss$ /> image is notably r$ & ! opposite "o t9t$ ) gre5i% or blui%1 %[+ unsymmetrical color+1 /ill m obvi\s :5 ! image is put a little out ( focus4 ,?is means t ! optical c5t]s ( cr[n & fl9t are out ( l9e f c>eless edg+ ( ! l5ses or v bad fitt+4 ,! case is bad en\gh to ju/ify try+ ! only rem$y available outside ! optician's work%op--rotat+ "o l5s ^u ! o!r & ?us try+ ! pair 9 diff]5t relative azimu?s4 ,! initial positions ( ! pair mu/ be m>k$ pla9ly1 c>e mu/ be tak5 n to displace ! spac]s #abj ap>t (t5 f\nd at ! edges ( ! l5ses1 & ! v>i\s positions mu/ be tri$ 9 an ord]ly mann]4 ,"o n infrequ5tly f9ds a position 9 :i* ! fault is negligible or disappe>s altoge!r1 :i* po9t %\ld be at once m>k$ = ref]5ce4 ,illu/ration3 ,,fig4 #aei4--;,correct ,image ,j ,out ( ,focus4 ,9 case !re is uncorrect$ sph]ical ab]ration !re is dep>ture f regul> gradation ( brightness 9 ! r+s4 ,if !re is a 8%ort edge10 i4e41 sph]ical ab]ration1 s t rays f ! out] zone come to a focus too %ort1 ! edge r+ w look too /rong )9 focus1 & ! inn] r+s relatively weak2 ) ?is appe>ance rev]s$ outside focus4 ,a 8long edge0 i4e41 - sph]ical ab]ration1 %[s ! opposite condition1 edge r+s too /rong outside focus & too weak )94 ,bo? are r common faults4 ,! 8long edge0 effect is %[n 9 ,figs4 #afj & #afa1 z tak5 q close to focus4 ,x takes a r %>p eye & consid]able exp]i5ce to detect small am\nts ( sph]ical ab]ration2 p]haps ! be/ way ( judg+ is 9 quickly pass+ f j inside to j outside focus & back aga91 us+ a yell[ scre5 & wat*+ v closely = v>iations 9 brightness4 ,tru? to tell a small am\nt ( residual ab]ration1 l t ( ,fig4 #afj1 is n a s]i\s matt] z reg>ds actual p]=mance--x hurts ! telescopi/'s feel+s mu* m ?an ! quality ( 8 images4 ,illu/ration3 ,,fig4 #afj4--,sph]ical ,ab]ration ,j ,inside ,focus4 ,illu/ration3 ,,fig4 #afa4--,sph]ical ,ab]ration ,j ,outside ,focus4 ,a mu* grav] fault is zonal ab]ration1 :]e "s int]m$iate zone ( objective or mirror comes to a focus too long or too %ort1 g5]ally damag+ ! def9ition r s]i\sly1 dep5d+ on ! am\nt ( v>iation 9 focus ( ! faulty zone4 ,a typical case is %[n 9 ,fig4 #afb tak5 )9 focus4 ,"h two zones are abnormally /rong %[+1 j z 9 ! case ( simple sph]ical ab]ration1 too %ort focus4 ,outside ( focus ! int5sities w\ld *ange places1 ! out] & midway zones & c5t] be+ heavy1 & ! /rong zones ( ,fig4 #afb weak4 ,^! zonal ab]rations are easily detect$ & are r common bo? 9 objectives & mirrors1 ?\gh r>ely z conspicu\s z 9 ,fig4 #afb4 ,ano!r fail+ is ! appe>ance ( a/igmatism1 :i*1 broadly1 is due to a refract+ or reflect+ surface :i* is n a surface ( revolution & !re=e behaves diff]5tly = rays incid5t 9 diff]5t planes ar\nd its optical axis4 ,9 its commone/ =m ! surface reflects or refracts m /rongly along "o plane ?an along ano!r at "r angles to x4 ,h5ce ! two h diff]5t foci & !re is no po9t focus at all1 b two l9e foci at "r angles4 ,figs4 #afc & #afd illu/rate ?is fault1 ! =m] be+ tak5 inside & ! latt] outside focus1 "u fairly high p[]4 ,if a /> image is oval & ! major axis ( ?is oval has turn$ ?r\gh #ij :5 "o passes to ! o!r side ( focus1 a/igmatism is some:]e pres5t4 ,z m ?an half ( humanity is a/igmatic1 ?r\gh fault ( ! eye1 "o %\ld twi/ ! axis ( ! eyes "s #ij ar\nd ! axis ( ! telescope & look aga94 ,if ! axis ( ! oval has turn$ ) ! eyes a visit to ! oculi/ is 9 ord]4 ,if n1 x is wor? :ile rotat+ ! ocul>4 ,if ! oval does n turn ) x t p>ticul> telescope requires rework+ be=e x c be ( mu* use4 ,?is a/igmatism due to fault ( figure mu/ n be confus$ ) ! a/igmatic diff]5ce ( ! image surfaces ref]r$ to 9 ,*apt] ,,iv :i* is z]o on ! axis & n ( mat]ial importance 9 ord9>y telescopes4 ,a/igmatism ( figure on ! contr>y is bad ev]y:]e & always4 ,x %\ld be especially look$ out = 9 reflect+ surfaces1 curv$ or plane1 s9ce x is a common result ( flexure4 ,pass+ on n[ f ^! simple te/s = figure1 *romatic ab]ration has to be exam9$4 ,no?+ is bett] ?an an artificial /> =m$ by ! sun 9 daylight1 = ! prelim9>y inve/igation4 ,at night ,pol>is is advantage\s = ?is z = o!r te/s4 ,illu/ration3 ,,fig4 #afb4--;,a ,case ( ,zonal ,ab]ration4 ,illu/ration3 ,,fig4 #afc4--;,a/igmatism ,inside ,focus4 ,illu/ration3 ,,fig4 #afd4--;,a/igmatism ,outside ,focus4 ,! a*romatization curves1 ,fig4 #afc1 really tell ! :ole /ory ( :at is to be se54 ,:5 ! telescope is c>efully focuss$ = ! bright "p ( ! spectrum1 gett+ ! %>pe/ /> image atta9able1 ! c5tral disc1 small & clean1 %\ld be yell[i% :ite1 se5 "u a p[] ( #fj or #gj p] in* ( ap]ture4 ,b ! r$ & blue rays h a long] focus & h5ce rim ! image ) a n>r[ purpli% circle v>y+ slightly 9 hue accord+ to ! "* ( ! a*romatization4 ,pu%+ ! ocul> a little inside1 focus1 ! r$ some:at ov]balances ! blue & ! purple %ades t[>d ! r$4 ,pull+ out ! ocul> v slightly "o br+s ! deep r$ into focus z a m9ute c5tral r$ po9t1 j z ! image beg9s to exp& a little4 ,fur!r outside focus a blui% or purpli% fl>e fills ! c5t] ( ! field1 :ile ar\nd x lies a gre5i% circle due to ! rays f ! middle ( ! second>y spectrum exp&+ f _! %ort] focus4 ,9 an "u-correct$ objective ?is r$ po9t is bright] & ! fr+e ab\t ! image1 focuss$ or )9 focus1 is conspicu\sly r$di%4 ,heavy ov]correction gives a /rong blui% fr+e & ! r$ po9t is dull or abs5t4 ,) a l[ p[] ocul>1 unless x be giv5 a color correction ( its own1 any prop]ly correct$ objective w seem "u-correct$ z already expla9$4 ,! color correction c also be well exam9$ by us+ an ocul> spectroscope l ,fig4 #adj1 ) ! cyl9drical l5s remov$4 ,exam9+ ! focuss$ /> image ?us1 ! spectrum is a n>r[ l9e = ! middle color ( ! second>y spectrum1 wid5+ equally at ,f & ,b1 & exp&+ into a sort ( bru% at ! violet end4 ,conv]sely1 :5 mov$ outside focus until ! wid? is r$uc$ to a n>r[ l9e at ,f & ,b1 ! wid5+ t[>d ! yell[ & gre5 %[s v cle>ly ! nature & ext5t ( ! second>y spectrum4 ,9 ?is way too1 ! actual foci = ! sev]al colors c easily be measur$4 ,! exact nature ( ! color correction is some:at a matt] ( ta/e & ( ! uses = :i* ! telescope is design$1 b mo/ obs]v]s agree 9 ! desirability ( ! ,b-,f correction commonly us$ z be/ balanc+ ! errors ( eye & ocul>4 ,) reflectors1 a*romatic or ev5 ov]-correct$ ocul>s are desirable4 ,! ph5om5a 9 te/+ a telescope = color v>y ) ! class ( /> obs]v$--! sol> type is a good av]age4 ,try+ a telescope on ,lyr emphasizes unduly ! blue phases1 :ile ,orionis w\ld ov]do ! r$4 ,! simple te/s on /> discs 9 & out ( focus "h describ$ are ample = all ord9>y purposes1 & a glass :i* passes !m well is beyond "q an admirably figur$ "o4 ,! te/s are n h[ev] quantitative1 & x takes an exp]i5c$ eye to pick out quickly m9or errors1 :i* are some:at irregul>4 ,"o sometimes f9ds ! r+ sy/em excell5t b a sort ( haze 9 ! field1 mak+ ! contra/s poor--bad poli% or dirt1 b figure good4 ,a te/ f\nd v useful by con/ructors or ^? ) laboratory facilities is ! knife edge te/1 work$ out *iefly by ,f\cault & widely us$ 9 exam9+ specula4 ,x consi/s 9 pr9ciple ( sett+ up ! mirror s z to br+ ! rays to ! %>pe/ possible focus4 ,= in/ance 9 a sph]ical mirror a lamp %9+ ?r\gh a p9 hole is plac$ 9 ! c5tre ( curvature1 & ! reflect$ image is br\ght j alongside x :]e x c be inspect$ by eye or eyepiece4 ,9 ,fig4 #afe all ! rays :i* emanate f ! p9hole b & fall on ! mirror a are br\ght q exactly to focus at c4 ,! eye plac$ close to c w see1 if ! mirror surface is p]fect1 a uni=m disc ( light f ! mirror4 ,illu/ration3 ,,fig4 #afe4--,! ,pr9ciple ( ! ,f\cault ,te/4 ,if n[ a knife edge l d1 say a safety razor blade1 be v gradually pu%$ ?r\gh ! focus ! light w be cut (f 9 a p]fectly uni=m mann]--no zone or local spot go+ fir/4 ,if "s error 9 ! surface at any po9t causes ! reflect$ ray to miss ! focus & cross ahead ( or beh9d x z 9 ! ray bef1 !n ! knife edge w cat* x fir/ or la/ z ! case may be1 & ! spot e w be fir/ d>k5$ or rema9 bright aft] ! light else:]e is ext+ui%$4 ,illu/ration3 ,,fig4 #aff4--;,f\cault ,te/ ( ,p>abolic ,mirror4 ,"o may ?us explore ! surface piecemeal & detect n only zones b slight v>iations 9 ! same zone ) great precision4 ,9 case ( a p>abolic mirror z 9 ,fig4 #aff ! te/ is made at ! focus by aid ( ! auxili>y plane mirror1 & a diagonal z %[n1 ! p9hole & knife edge be+ arrang$ q z be=e4 ,a v good description ( ! practical use ( ! knife edge te/ may be f\nd 9 ! pap]s ( ,dr4 ,drap] & ,mr4 ,rit*ey already cit$4 ,x is also appli$ to refractors1 9 :i* case mono*romatic light _h bett] be us$1 & enables ! exp]im5t] to detect ev5 ! almo/ inf9itesimal m>k+s sometimes left by ! poli%+ tool1 to say no?+ ( slight v>iations 9 local figure :i* are cont9ually lo/ 9 ! g5]al illum9ation ab\t ! field :5 "o uses ! /> te/ 9 ! ord9>y mann]4 ,! set-up = ! knife edge exp]im5ts %\ld be v /eady & smoo? work+ to secure precise results1 & x !re=e is n g5]ally us$ save 9 ! te*nique ( figur+ mirrors1 :]e x is invaluable4 ,) micromet] motions on ! knife edge1 crosswise & longitud9ally1 "o c make a v exact diagnosis ( errors ( figure or flexure4 ,a /ill m delicate me?od ( exam9+ ! p]fection ( figur+ is f\nd 9 ! ,h>tmann te/4 7,zeit4 fur ,in/k41 #aijd1 #aiji74 ,?is is ess5tially a photographic te/1 comp>+ ! effect ( ! individual zones ( ! objective inside & outside ( focus4 ,n only are ! effects ( ! zones comp>$ b also ! effects ( diff]5t p>ts ( ! same zone1 s t any lack ( symmetry 9 p]=mance c be at once f\nd & measur$4 ,! ,h>tmann te/ is %[n diagrammatically 9 ,fig4 #afg4 ,! objective is set up = obs]v+ a natural or artificial />4 ,j 9 front ( x is plac$ an opaque scre5 p]=at$ ) holes1 z %[n 9 section by ,fig4 #afg1 :]e ,a is ! p]=at$ scre54 ,! diamet]s ( ! holes are ab\t #a_/bj ! diamet] ( ! objective z ! te/ is g5]ally appli$1 & !re are usually f\r holes #ij ap>t = ea* zone4 ,& su* holes are n all 9 "o l9e1 b are di/ribut$ symmetrically ab\t ! scre51 c>e be+ tak5 t ea* zone %all be repres5t$ by holes sep>at$ radially & also tang5tially1 correspond+ to ! pairs ( elem5ts 9 ! two a/igmatic image surfaces1 an arrangem5t :i* enables ! a/igmatism z well z figure to be inve/igat$4 ,illu/ration3 ,,fig4 #afg4--,! ,pr9ciple ( ! ,h>tmann ,te/4 ,! arrangem5t ( holes actually f\nd useful is %[n 9 ,h>tmann's orig9al pap]s1 & also 9 a v important pap] by ,plaskett 7,ap4 ,j4 #be #aie7 :i* conta9s ! be/ acc\nt 9 ,engli% ( ,h>tmann's me?ods & _! application4 ,n[ ea* hole 9 ! scre5 transmits a p5cil ( light ?r\gh ! objective at ! correspond+ po9t1 & ea* p5cil comes to a focus & !n div]ges1 ! foci be+ di/ribut$ some:]e 9 ! vic9ity ( :at "o may reg>d z ! pr9cipal focus1 ,b4 ,= in/ance 9 ,fig4 #afg are %[n five pairs ( ap]tures a1 a1 b1 b1 etc41 9 five diff]5t zones4 ,n[ if a photographic plate be expos$ a few in*es inside focus z at ,c ea* p5cil f an ap]ture 9 ! scre5 w be repres5t$ by a dot on ! photograph1 at su* di/ance f ! axis & f ! correspond+ dot on ! o!r side ( ! axis z ! respective incl9ations ( ! p5cils ( light may det]m9e4 ,simil>ly a plate expos$ at approximately equal di/ance on ! o!r side ( ! g5]al focus1 z at ,d1 w %[ a patt]n ( dots due to ! di/ribution ( ! sev]al rays at a po9t beyond focus4 ,n[ if all ! p5cils f ! sev]al ap]tures met at a common focus 9 ,b1 ! two patt]ns on ! plates ,c & ,d w\ld be exactly alike & = equal di/ance away f focus ( exactly ! same size4 ,9 g5]al ! patt]ns w n exactly correspond1 & ! diff]5ces measur$ ) ! micromet] %[ j h[ mu* any ray 9 "q has dep>t$ f meet+ at an exact common focus ) its fell[s4 ,= in/ance 9 ! cut x w be obs]v$ t ! rays e & a focus b>ely beyond ,c & by ! "t !y rea* ,d are well spread ap>t4 ,! relative di/ance ( ! dots ^u ^! correspond+ plates1 ) ! di/ance betwe5 ! plates1 %[s exactly at :at po9t betwe5 ,c & ,d ^! p>ticul> rays actually did cross & come to a focus4 ,det]m9+ ?is is m]ely a matt] ( measur+ up simil> triangles1 = ! pa? ( ! rays is /raight4 ,simil>ly inspection w %[ t ! rays d & d meet a little %ort ( ,b1 & measurem5t ( _! respective records on ! plates ,c & ,d w\ld %[ ! exi/5ce ( a zone int]m$iate 9 focus betwe5 ! focus ( e1e & ! g5]al focus at ,b4 ,! exact dep>ture ( ?is zone f correct focus c !re=e be at once measur$4 ,a little fur!r exam9ation discloses ! fact t ! out] zone repres5t$ by ! rays a1b1 & a1b has n q ! same focus at ! two extremities ( ! same diamet] ( ! objective4 ,9 o!r words ! l5s is a little bit flatt] at "o end ( ?is diamet] ?an x is at ! o!r1 s t ! rays "h h consid]ably long] focus ?an !y %\ld1 a fault by no means unkn[n al?\gh =tunately n v common4 ,x w be se5 t ! v>iations betwe5 ! two scre5 patt]ns on ,c & ,d1 toge!r ) ! diff]5ce betwe5 !m1 give accurately ! p]=mance ( ea* po9t ( ! objective repres5t$ by an ap]ture 9 ! scre54 ,& simil> inve/igations by sub/antially ! same me?od may be ext5d$ to ! a/igmatic v>iations1 to ! g5]al color correction1 & to ! diff]5ce 9 ! ab]rations = ! sev]al colors4 ,! orig9al pap]s cit$ %\ld be consult$ = ! details ( apply+ ?is v precise & int]e/+ te/4 ,x gives an invaluable record ( ! detail$ corrections ( an objective1 & :ile x is "o ) :i* ! ord9>y obs]v] has little conc]n !re are times :5 no?+ else c give ) equal precision ! necess>y record ( p]=mance4 ,!re are div]s o!r te/s us$ = "o purpose or ano!r 9 exam9+ objectives & mirrors1 b ^? "h describ$ are ample = ne>ly all practical purposes1 & inde$ ! fir/ two commonly disclose all t x is necess>y to "k4 ,n[ & !n "o has to deal ) an objective :i* is unmitigat$ly dirty4 ,x c be giv5 a casual prelim9>y clean+ 9 ! way already m5tion$1 b sometimes ev5 ?is w n leave x 9 condition = te/+4 ,!n "o mu/ get d[n to ! bottom ( ?+s & make a ?or\gh job ( x4 ,! *ief po9t to rememb] 9 und]tak+ ?is is t ! ?+ :i* "o is clean+ is glass1 & v easy to scrat* if "o rubs du/ into x1 b q easy to clean if "o is c>eful4 ,! second ?+ to be rememb]$ is t once clean$ x mu/ be replac$ z x 0 be=e & n 9 "s o!r mann]4 ,! possessor ( a dirty objective is g5]ally advis$ to take x to ! mak] or "s reliable optician4 ,if ! mak] is h&y1 or an optician ( l>ge exp]i5ce 9 deal+ ) telescope objectives is available1 ! advice is good1 b !re is no difficulty :atev] 9 clean+ an objective ) ! ex]cise ( t ord9>y c>e :i* ! us] ( a telescope may be reasonably expect$ to possess4 ,x is a fussy job1 b n difficult1 & ! be/ advice z to h[ to clean a telescope objective is to 8tub0 x1 lit]ally1 if beyond ! /age :]e ! sup]ficial wip+ describ$ is suffici5t4 ,to g ab\t ! task "o fir/ sets d[n ! objective 9 its cell on a horizontal surface & removes ! screws t hold 9 ! reta9+ r+1 or unscrews ! r+ itself z ! case may be4 ,?is leaves ! cell & objective ) ! latt] upp]mo/ & free to be tak5 out4 ,prep>e on a table a pad ( any?+ s(t1 a little small] ?an ! objective1 topp+ ! pad ) s(t & clean old clo?2 !n1 rais+ up ! cell at an edge1 slip ! two ?umbs "u x & lay ! f+]s lightly on ! out] l5s ( ! objective1 !n inv]t ! :ole affair ^u ! pad & lift (f ! cell1 leav+ ! objective on its s(t b$4 ,be=e any?+ else is done ! edge ( ! objective %\ld be m>k$ ) a h>d lead p5cil on ! edge ( bo? ! compon5t l5ses1 mak+ two well def9$ v's ) _! po9ts t\*+4 ,also1 if1 z usual1 !re are ?ree small sep>ators betwe5 ! edges ( ! fl9t & cr[n l5ses1 m>k ! position ( ea* ( ^! #a1 #b1 #c1 ) ! same p5cil4 ,=m+ ano!r conv5i5t pad ( some?+ s(t1 lift (f ! upp] l5s1 take out ! ?ree sep>ators & lay !m 9 ord] on a %eet ( pap] )\t turn+ !m upside d[n4 ,m>k alongside ea*1 ! s]ial numb] d5ot+ its position4 ,!n :5 ^! spac]s1 if 9 good condition1 are put back1 !y w g back 9 ! same place rightside up1 & ! objective itself w g back into place un*ang$4 ,n[ h at h& a wood5 or fibre tub or bas9 :i* has be5 ?or\ghly wa%$ out ) soap & wat] & wip$ dry4 ,half fill x ) wat] slightly lukew>m & ) a good mild toilet soap1 %av+ soap = example1 ) clean h&s & v s(t clean clo?1 g at "o ( ! l5ses & give x a ?or\gh wa%+4 ,aft] ?is x %\ld be r9s$ v ?or\ghly & wip$ dry4 ,z to mat]ial = wip+1 ! ma9 ?+ is t x mu/ be s(t & free f du/ t w scrat*4 ,old h&k]*iefs s]ve a good turn4 ,dr4 ,bra%e> ye>s ago 9 describ+ ?is process recomm5d$ *eese clo?4 ,! pres5t day mat]ial t goes "u ?is "n is f> f be+ z s(t at ! />t z x ought to be1 & only ! be/ quality ( x %\ld be us$1 & !n only aft] v ?or\gh soak+1 r9s+ & dry+4 ,! v s(t t[els us$ = clean+ cut glass1 if wa%$ ?or\ghly clean & kept free f du/1 answ] p]fectly well4 ,! *eese clo? has ! advantage ( be+ comp>atively *eap s t x c be ?r[n away aft] use4 ,:atev] ! clo?1 x %\ld be kept1 aft] ?or\gh wa%+ & dry+1 9 a clos$ j>4 ,r9s+ ! l5s ?or\ghly & wip+ x clean & dry is ! ma9 second /age ( cleans+4 ,x sometimes w be f\nd to be badly spott$ 9 a way :i* ?is wa%+ w n remove4 ,sometimes ! spott+ w yield to alcohol c>efully rubb$ on ) s(t absorb5t cotton or a bun* ( l5s pap]4 ,if alcohol fails ! condition ( ! surface is su* z to ju/ify try+ m /r5u\s means4 ,nitric acid ( mod]ate /r5g? rubb$ on ) a swab ( absorb5t cotton w sometimes cle> up ! spott+4 ,if ?is treatm5t be us$ x %\ld be foll[$ up ) a #aj p] c5t solution ( pure cau/ic pota% or mod]ately /rong c4p4 ammonia & !n by v ?or\gh r9s+4 ,glass w /& )\t risk cauti\s application ( bo? acid & alkali1 b ! =m] bett] ?an ! latt]4 ,!n a f9al r9s+ & dry+ is 9 ord]4 ,_m op]ators use a f9al wa%+ ) alcohol ( at lea/ #ij p] c5t /r5g? :i* is all[$ to evaporate ) little or no wip+4 ,alcohol d5atur$ ) me?yl alcohol s]ves well if /rong en\gh b bew>e d5atur$ alcohol ( unkn[n composition4 ,o!rs h us$ petroleum naph?a & ?+s ( t sort4 ,at ! pres5t "t ^! comm]cial petroleum products are extremely unc]ta9 9 quality1 l gasol9e1 be+ obta9$1 ,heav5 kn[s h[1 f ! break+ d[n ( heavi] petroleum products4 ,if pure petroleum e!r c be obta9$ x answ]s q z well z alcohol1 b unless ! volatile fluid is pure x may leave /reaks4 ,ord9>ily nei!r has to be us$1 z aft] ! prop] wip+ ! glass comes p]fectly clean4 ,?is done ! glass c be replac$ on ! pad :5ce x came & its mate put ?r\gh ! same process4 ,fl9t glass is m liable to spot ?an ! cr[n1 b ! cr[n is by no means immune aga9/ ! det]ioration ( ! surface1 p]haps incipi5t devitrification1 & dur+ ! w> _m objectives 8w5t bl9d0 f unexpla9$ action ( ?is "*4 ,z a rule ! soap & wat] treatm5t appli$ ) c>e leaves ev5 a pretty h>d look+ specim5 ( objective 9 fairly good condition except = ! scrat*es :i* previ\s us]s h put ^u x4 ,!n if ! spac+ pieces1 usually ( t9foil1 are n torn or corrod$ !y c be put back into place1 ! "o l5s sup]impos$ ^u ! o!r1 & ! pair put back into ! cell by dropp+ x g5tly ov] !m & re-inv]t+ ! :ole1 tak+ c>e ?is "t to h s(t clo? or l5s pap] "u ! f+]s4 ,!n ! reta9+ r+ c be put into place aga9 & ! objective is ready = te/+ or use z ! case may be4 ,if ! spac]s are corrod$ or damag$ x may be necess>y to replace !m ) v ?9 t9foil cut ! same size & %ape1 leav+ h[ev] a little extra l5g? to turn d[n ov] ! edge ( ! l[] l5s4 ,!y are fa/5$ 9 place on ! extreme edge only by ! m]e/ t\* ( mucilage1 %ellac or ,canada balsam1 :i*ev] comes to h&4 ,! "o important ?+ is t ! spac]s %\ld be entirely free ( ! /icky mat]ial :]e !y lap ov] ! edge ( ! l5s to p]=m ! sep>ation4 ,?is lap is g5]ally n ov] #a_/af ( an in*1 n en\gh to %[ at ! outside ( ! objective :5 x is 9 its cell4 ,:5 ! upp] l5s is lightly press$ d[n into place1 aft] ! gum or %ellac is dry1 all ! project+ portion c be trimm$ away ) a %>p p5-knife leav+ simply ! spac]s 9 ! appo9t$ places f :i* ! orig9al ones 7 remov$4 ,"s little space has be5 giv5 to ?is matt] ( clean+ objectives1 z 9 _m situations objectives accumulate dirt r rapidly & x is highly desirable = ! us] to le>n h[ to p]=m ! simple b c>eful task ( cleans+ !m4 ,9 ord9>y use1 :5 dirt beyond ! rea* ( m]e du/+ ) a camel's hair bru% has /uck itself to ! ext]ior ( an objective1 a succession ( tufts ( absorb5t cotton or wads ( l5s pap] at fir/ damp5$ ) pure wat] or alcohol & !n foll[$ lightly1 aft] ! visible dirt has be5 g5tly mopp$ up1 by c>eful wip+ ) ! same mat]ials1 w keep ! ext]ior surface 9 good condition1 ! process be+ j t sugge/$ 9 ! beg9n+ ( ?is *apt] z ! ord9>y clean+ up prep>atory to a ?or\gh exam9ation4 ,! ma9 ?+ to be avoid$ 9 ! c>e ( a telescope1 aside f r\gh usage g5]ally1 is gett+ ! objective wet & !n lett+ x take its *ances ( dry+4 ,9 _m climates dew is a v s]i\s enemy & ! cu/om>y dew cap ?ree or f\r diamet]s long1 bright on ! outside & black5$ )91 is ( v great s]vice 9 less5+ ! deposit ( dew ^u ! glass4 ,also ! dew cap keeps out mu* /ray light t might o!rwise d mis*ief by bright5+ ! g5]al field4 ,9 fact its function z a light-trap is v important especially if x is mat]ially l>g] 9 diamet] ?an ! objective & provid$ ) /ops4 ,! f9d] %\ld be simil>ly protect$1 o!rwise x w my/]i\sly g bl9d 9 ! middle ( an ev5+'s "w due to a heavy deposit ( moi/ure on ! objective4 ,! effect is an onset ( dimness & bad def9ition :i* is altoge!r obnoxi\s4 ,z reg>ds ! metal p>ts ( a telescope !y %\ld be treat$ l ! metal p>ts ( any o!r ma*9e1 :ile ! mov+ p>ts require f "t to "t a little t\* ( sp]m or simil> oil l e o!r surface :]e friction may occur4 ,! old fa%ion$ highly poli%$ & lacqu]$ brass tube 0 practically impossible to keep look+ respectably well provid$ x 0 really us$ to any consid]able ext5t4 ,ab\t ! mo/ t c\ld be done to x 0 du/+ :5 du/y1 & cauti\sly & promptly wip+ (f any cond5s$ moi/ure4 ,! m mod]n lacqu]$ tubes require v little c>e & if !y get 9 really bad condition c be relacqu]$ )\t mu* exp5se or difficulty4 ,wood5 tubes1 occasionally f\nd 9 old in/rum5ts1 dem& ! treatm5t :i* is accord$ to o!r highly f9i%$ wood5 ?+s1 occasional rubb+ ) oil or furniture poli% accord+ to ! "* ( ! orig9al surface4 ,pa9t$ tubes may occasionally require a fre% coat1 :i* x does n require great skill to adm9i/]4 ,if ! surface ( wood5 tripods comes to be 9 bad %ape x ne$s ! oil or poli% :i* w\ld be accord$ to o!r well f9i%$ wood5 articles4 ,m\nt+s are usually pa9t$ or lacqu]$ & ei!r surface c be r5ew$ ev5tually at no great tr\ble4 ,bright p>ts may be lightly t\*$ ) oil z an ord9>y ru/ prev5tive4 ,reflect+ telescopes are consid]ably m tr\blesome to keep 9 ord] ?an refractors ow+ to ! t5d] nature ( ! silv]$ surface4 ,x may rema9 9 good condition ) fairly /eady use = sev]al ye>s or x may g bad 9 a few mon?s or a few weeks4 ,! latt] is n an unusual figure 9 telescopes us$ ab\t a city :]e smoke is pl5tiful4 ,! ma9 ?+ is to prev5t ! deposit ( dew on ! mirror1 or gett+ x wet 9 any o!r way1 = 9 dry+ (f ! drops almo/ inv>iably leave spots4 ,_m s*emes h be5 propos$ = ! prev5tion ( injury to ! mirror surface4 ,a close fitt+ metal cov]1 employ$ :5ev] ! mirror is n 9 use1 has giv5 good results 9 _m places4 ,:]e conditions are extreme ?is is sometimes l9$ ) a lay] ( dry absorb5t cotton com+ fairly d[n ^u ! mirror surface1 & if ?is muffl] is dry1 clean1 & a little w>m] ?an ! mirror :5 put on1 x seems to be fairly effective4 ,pref]ably ! mirror %\ld be kept1 :5 n 9 use1 at a little high] temp]ature ?an ! surr\nd+ air s t dew w n t5d to deposit ^u x4 ,z to actual protective measures ! only ?+ t seems to be really effici5t is a v ?9 coat+ ( lacqu]1 fir/ tri$ by ,p]ot at ! ,p>is ,obs]vatory4 ,! au?or "s t5 ye>s s9ce took up ! problem 9 protect+ "s laboratory mirrors aga9/ fumes & moi/ure & f\nd t ! highe/ grade ( :ite lacqu]1 su* z is us$ = ! coat+ ( f9e silv]w>e 9 ! trade1 answ]$ admirably if dilut$ ) six or eight volumes ( ! ?9n] sold ) su* comm]cial lacqu]s4 ,x is be/ to ?9 ! lacqu] to ! requisite am\nt & !n filt]4 ,if n[ a lib]al am\nt ( ! mixture is p\r$ ^u ! mirror surface aft] c>eful du/+1 swi%$ quickly ar\nd1 & ! mirror is !n imm$iately turn$ up on edge to dra9 & dry1 a v ?9 lay] ( lacqu] w be left ^u x1 only a fraction ( a wave l5g? ?ick1 s t x %[s broad areas ( int]f]5ce colors4 ,treat$ 9 ?is way & kept dry ! coat+ w protect ! brilliancy ( ! silv] = a good _m mon?s ev5 "u r unfavorable circum/ances4 ,aft] try+ out ! s*eme r ?or\ghly ! treatm5t 0 appli$ to ! #bd in* reflector ( ! ,h>v>d ,obs]vatory & has be5 9 use ev] s9ce4 ,! au?or appli$ ! fir/ coat+ 9 ! spr+ ( #aiac1 & s9ce t "t x has only be5 necess>y to resilv] p]haps once 9 six mon?s z aga9/ ab\t z _m weeks previ\sly4 ,! lacqu] us$ 9 ?is case 0 ! s-call$ 8,la/9a0 lacqu] made by ! ,egyptian ,lacqu] ,company ( ,new ,york1 b !re are d\btless o!rs ( simil> grade 9 ! m>ket4 ,x is a collodion lacqu] & 9 rec5t ye>s x has prov$ desirable to use z a ?9n] /raight comm]cial amylacetate r ?an ! ?9n] usually provid$ ) ! lacqu]1 p]haps ow+ to ! fact t difficulty ( obta9+ mat]ials dur+ ! w> may h caus$1 z 9 s _m o!r cases1 sub/itutions :i*1 :ile p]fectly good = ! orig9al purpose did n answ] s well "u ! extreme conditions requir$ 9 pres]v+ telescope mirrors4 ,! lacqu] coat+ :5 ?9n$ to ! ext5t "h recomm5d$ does n app>5tly 9 any way det]iorate ! def9ition z "s ye>s ( regul> "w at ,h>v>d h %[n4 ,"s exp]im5t]s h1 h[ev]1 f\nd difficulty1 q c]ta9ly ow+ to us+ too ?ick a lacqu]4 ,! endurance ( a lacqu] coat+ :]e ! mirror is kept free f moi/ure1 & its p[] to hold ! orig9al brilliancy ( ! surface is v extraord9>y4 ,! writ] took out & te/$ "o laboratory mirror coat$ sev5 ye>s be=e1 & kept 9 a dry place1 & f\nd ! reflect+ p[] /ill a little above 4#gj1 despite ! fact t ! coat+ 0 s dry z to be almo/ p[d]y :5 t\*$ ) a tuft ( cotton4 ,at ! />t ! mirror _h se5 "s little use unprotect$ & its reflection coeffici5t 0 probably ar\nd 4#hj4 ,if ! silv] coat+ is ?ick z x c be conv5i5tly made1 on a well coat$ mirror1 ! coat ( lacqu]1 :5 t>ni% has begun1 c be wa%$ (f ) amylacetate & tufts ( cotton until ! surface is practically cle> ( x1 & ! silv] itself repoli%$ by ! ord9>y me?od & relacqu]$4 ,!re are _m silv]+ processes 9 use & :i* "o %\ld be *os5 = re-silv]+ a mirror1 big or little1 is q l>gely a matt] ( individual ta/e1 & m p>ticul>ly exp]i5ce4 ,! two mo/ us$ 9 ?is c\ntry are ^? ( ,dr4 ,bra%e> & ,mr4 ,lund91 head ( ! ,alvan ,cl>k ,corporation1 & bo? h be5 ?or\ghly tri$ out by ^! exp]i5c$ mak]s ( big mirrors4 ,! two processes diff] 9 sev]al important p>ticul>s b bo? seem to "w v successfully4 ,! fundam5tal ?+ 9 us+ ei!r ( !m is t ! glass surface to be silv]$ %\ld be *emically clean4 ,! old silv]1 if a mirror is be+ resilv]$1 is remov$ ) /rong nitric acid :i* is v ?or\ghly r9s$ (f aft] e trace ( silv] has be5 remov$4 ,sometimes a second treatm5t ) nitric acid may advantage\sly foll[ ! fir/ ) m r9s+4 ,! acid %\ld be foll[$ by a #aj p] c5t solution ( c4p4 cau/ic pota% 7"s op]ators use c4p4 ammonia z easi] to cle> away7 r9s$ (f ) ! utmo/ ?or\ghness4 ,on g5]al pr9ciples ! la/ r9s+ %\ld be ) di/ill$ wat] & ! glass surface %\ld n be all[$ to dry betwe5 ?is r9s+ & />t+ ! silv]+ process1 b ! :ole mirror %\ld be kept "u wat] until ! "t = silv]+4 ,9 ,dr4 ,bra%e>'s process ! foll[+ two solutions are made up2 fir/ ! r$uc+ solution z foll[s3 ,rock c&y1 #bj p>ts by weight4 ,/rong nitric acid 7spec4 gr4 #a4bb71 #a "p4 ,alcohol1 #bj p>ts4 ,di/ill$ wat]1 #bjj p>ts4 ,?is improves by keep+ & if ?is prep>ation has to be hurri$ ! acid1 sug> & di/ill$ wat] %\ld be boil$ toge!r & !n ! alcohol add$ aft] ! solution is cool$4 ,second1 make up ! silv]+ solution 9 ?ree di/9ct portions2 fir/ ! silv] solution prop] z foll[s3 #a4 #b p>ts silv] nitrate4 #bj p>ts di/ill$ wat]4 ,second1 ! alkali solution z foll[s3 #b4 #a p>ts c4p4 cau/ic pota%4 #bj p>ts di/ill$ wat]4 ,?ird1 ! res]ve silv] solution z foll[s3 #c4 "p silv] nitrate4 #af p>ts di/ill$ wat]4 ,! work+ solution ( silv] is !n prep>$ ?us3 ,gradually add to ! silv] solution ,no4 #a ! /ronge/ ammonia1 sl[ly & ) con/ant /irr+4 ,at fir/ ! solution w turn d>k br[n & !n x w gradually cle> up4 ,ammonia %\ld be add$ only j to ! po9t necess>y to cle> ! solution4 ,!n add ,no4 #b1 ! alkali solution4 ,aga9 ! mixture w turn d>k br[n & mu/ be cauti\sly cle>$ once m ) ammonia until x is /raw color$ b cle> ( precipitate4 ,f9ally add ,no4 #c1 ! res]ve solution1 v cauti\sly ) /irr+ until ! solution gr[s d>k] & beg9s to %[ traces ( susp5d$ matt] :i* w n /ir out4 ,!n filt] ! :ole ?r\gh absorb5t cotton to free x ( precipitate & x is ready = use4 ,"o is !n ready = ! actual silv]+4 ,n[ !re are two ways ( work+ ! process1 ) ! mirror face up1 or face d[n4 ,! =m] is advantage\s 9 all[+ bett] inspection ( ! surface z x =ms1 & also x p]mits ! mirror ( a telescope to be silv]$ )\t remov+ x f ! cell1 z 0 9 fact done habitually 9 case ( ! big reflector ( ! ,alleghany ,obs]vatory :]e ! conditions 7 su* z to dem& re-silv]+ once a mon?4 ,! solution 0 kept 9 motion dur+ ! process by rock+ ! telescope z a :ole4 ,:5 silv]+ face up ! mirror is made to =m ! bottom ( ! silv]+ vessel1 be+ fitt$ ) a wrapp+ ( /rong p>aff9$ or wax$ pap] or clo?1 w\nd sev]al times ar\nd ! rim ( ! mirror & c>ri$ up p]haps half ! ?ickness ( ! mirror to =m a reta9] = ! silv]+ solution4 ,?is b& is firmly ti$ ar\nd ! edge ( ! mirror mak+ a wat] tight jo9t4 ,rit*ey uses a copp] b& fitt$ to ! edge ( ! mirror & drawn tight by screws1 & f9i%es mak+ tight ) p>aff9 & a w>m iron4 ,9 silv]+ face d[n ! mirror is susp5d$ a little di/ance above ! bottom ( a %all[ di%1 pref]ably ( e>!n w>e1 conta9+ ! solution4 ,v>i\s means are us$ = support+ x4 ,?us cleats across ! back cem5t$ on ) h>d optician's pit* answ] well = small mirrors1 & sometimes special provision is made = hold+ ! mirror by ! extreme edge 9 clamps4 ,silv]+ face d[n is 9 "s respects less conv5i5t b does free ! op]ator f ! v s]i\s tr\ble ( ! heavy s$im5t :i* is deposit$ f ! r /rong silv] solution4 ,?is is ! ess5tial difficulty ( ! ,bra%e> process 9 silv]+ face up4 ,! tr\ble may be rem$i$ by v g5tle swabb+ ( ! surface "u ! liquid ) absorb5t cotton1 f ! "t :5 ! silv] coat+ beg9s fairly to =m until x is complet$4 ,! ,bra%e> process is mo/ successfully work$ at a temp]ature betwe5 #fe & #gj ,f4 & "s exp]i5ce is requir$ to det]m9e ! exact proportion ( ! r$uc+ solution to be add$ to ! silv]+ solution4 ,rit*ey advises su* quantity ( ! r$uc+ solution z conta9s ( sug> "o-half ! total weight ( ! silv] nitrate us$4 ,! total am\nt ( solution aft] mix+ %\ld cov] ! mirror ab\t an in* deep4 ,too mu* increases ! tr\ble f s$im5t & fails to give a clean coat+4 ,! requisite quantity ( r$uc+ solution is p\r$ into ! silv]+ solution & !n imm$iately1 if ! mirror is face up1 fairly ^u x1 )\t dra9+ x ( ! wat] "u :i* x has be5 /&+4 ,if silv]+ face d[n ! face w h be5 imm]s$ 9 a ?9 lay] ( di/ill$ wat] & ! mix$ solutions are p\r$ into ! di%4 ,9 ei!r case ! solution is rock$ & kept mov+ pretty ?or\ghly until ! process is complet$ :i* w take ab\t five m9utes4 ,if silv]+ is cont9u$ too long !re is likelihood ( an inf]ior :iti% out] surface :i* w n poli% well1 b %ort ( ?is po9t ! ?ick] ! coat ! bett]1 s9ce a ?ick coat /&s reburni%+ :]e a ?9 "o does n & moreov] ! ?9 "o may be ?9 en\gh to transmit "s valuable light4 ,:5 ! silv]+ is done ! solution %\ld be rapidly p\r$ (f1 ! edg+ remov$ or ! mirror lift$ out ( ! solution1 r9s$ (f fir/ ) tap wat] & !n ) di/ill$1 & swabb$ g5tly to cle> ! rema9+ s$im5t4 ,!n ! mirror c be set up on edge to dry4 ,a f9al fl[+ ) alcohol & ! use ( a fan ha/5s ! process4 ,9 ,lund9's me?od ! initial clean+ process is ! same b aft] ! nitric acid has be5 ?or\ghly r9s$ (f ! surface is g5tly b ?or\ghly rubb$ ) a saturat$ solution ( t9 *loride1 appli$ ) a wad ( absorb5t cotton4 ,aft] ! c>eful rubb+ ! t9 *loride solution mu/ be wa%$ (f ) ! utmo/ ?or\ghness1 pref]ably ) mod]ately w>m wat]4 ,x is j z important to get (f ! t9 *loride completely1 z x is to clean completely ! mirror surface by its use4 ,o!rwise /reaks may be left :]e ! silv]+ w n take well4 ,:5 ! job has be5 prop]ly done "o c wet ! :ole surface ) a film ( wat] & x w /ay wet ev5 :5 ! surface is slightly tilt$4 ,z 9 ! ,bra%e> process ! mirror mu/ be kept cov]$ ) wat]4 ,mr4 ,lund9 always silv]s l>ge mirrors face up1 & =ms ! di% by wrapp+ ar\nd ! edge ( ! mirror a /rip ( b&age clo? soak$ 9 melt$ beeswax & smoo!d (f by pull+ x :ile /ill hot betwe5 metal rods to secure ev5 di/ribution ( ! wax s z to make a wat] tight jo9t4 ,?is rim ( clo? is ti$ firmly ar\nd ! edge ( ! mirror & ! /r+s !n wet to draw !m /ill tight]4 ,mean:ile ! wat] %\ld cov] ! mirror by ( an in* or m4 ,x is to be not$ t 9 ! ,lund9 process ord9>y wat] is usually f\nd j z effici5t z di/ill$ wat]1 b x is h>dly safe to assume t su* is ! case1 )\t try+ x out on a sample ( glass4 ,!re are !n prep>$ two solutions1 a silv] solution1 #b4af p>ts silv] nitrate 7see ,k+1 ,pop4 ,a/ #cj1 #ic7 #ajj p>ts wat]4 & a r$uc+ solution1 #d p>ts ,m]ck's =maldehyde #bj p>ts wat]4 ,?is latt] quantity is us$ = ea* #ajj p>ts ( ! above silv] solution1 & ! :ole quantity made up is det]m9$ by ! am\nt ( liquid necess>y to cov] ! mirror z j describ$4 ,! silv] solution is cauti\sly & completely cle>$ up by /rong ammonia z 9 ! ,bra%e> process4 ,! silv] & r$uc+ solutions are !n mix$1 ! wat] cov]+ ! mirror p\r$ quickly (f1 & ! silv]+ solution imm$iately p\r$ on4 ,! mirror %\ld !n be g5tly rock$ & ! silv] coat+ c>efully wat*$ z x =ms4 ,z ! op]ation is complet$ some:at co>se black gra9s ( s$im5t w =m & :5 ^! beg9 to be 9 evid5ce ! solution %\ld be p\r$ (f1 ! mirror r9s$ 9 runn+ wat]1 ! edg+ remov$ :ile ! mirror is /ill r9s+ & f9ally ! s$im5t v g5tly swabb$ (f ) wet absorb5t cotton4 ,!n ! mirror c be set up to dry4 ,! ,lund9 process uses a consid]ably weak] silv] solution ?an ! ,bra%e> process1 is a good deal m cleanly :ile 9 action1 & is by exp]i5c$ work]s said to p]=m be/ at a mat]ially l[] temp]ature ?an ! ,bra%e> process1 ) ! mirror1 h[ev]1 always slightly w>m] ?an ! solution4 ,"s work]s h _h good results by omitt+ ! t9 *loride solution & clean+ up ! surface by ! m ord9>y me?ods4 ,9 ! ,lund9 process ! solution is suffici5tly cle> = ! d5sity acquir$ by ! silv] coat+ to be r\ghly judg$ by hold+ an inc&esc5t lamp "u ! mirror4 ,a good coat+ %\ld %[ at mo/ only ! fa9te/ possible outl9e ( ! filam5t1 ev5 ( a gas fill$ lamp4 ,:i*ev] process ( silv]+ is employ$1 & bo? "w well1 ! f9al burni%+ ( ! mirror aft] x is ?or\ghly dry is p]=m$ 9 ! same way1 />t+ by ty+ up a v s(t ball ( absorb5t cotton 9 ! s(te/ ( *amois sk94 ,?is burni%] is us$ at fir/ )\t any addition1 simply to smoo? & cond5se ! film by go+ ov] x ) quick1 %ort1 & g5tle circul> /rokes until ! entire surface has be5 ?or\ghly clean$ & beg9s to %[ a t5d5cy to take poli%4 ,!n a v little ( ! f9e/ optical r\ge %\ld be put on to ! same1 or bett] ano!r1 rubb]1 & ! mirror gone /eadily ov] 9 a simil> way until x comes to a brilliant poli%4 ,a good deal ( c>e %\ld be tak5 9 p]=m+ ?is op]ation to avoid ! settl+ ( du/ ^u ! surface s9ce scrat*es w inevitably result4 ,great pa9s %\ld also be tak5 n to take any *ance ( brea?+ on ! mirror or 9 any o!r way gett+ ! surface 9 ! slighte/ degree damp4 ,o!rwise x w n come to a dec5t poli%4 ,num]\s o!r directions = silv]+ w be f\nd 9 ! lit]ature1 & all ( !m h be5 successfully work$ at "o "t or ano!r4 ,! fundam5tal basis ( ! :ole process is less 9 ! p>ticul> =mula us$ ?an 9 ! mo/ scrupul\s c>e 9 clean+ ! mirror & keep+ x clean until ! silv]+ is complet$4 ,also a good bit ( exp]i5ce is requir$ to enable "o to p]=m ! op]ation s z to obta9 a uni=m & d5se deposit4 ,,*apt] ,x ,,sett+ ,,up ,,& ,,h\s+ ,,! ,,telescope ,9 reg>d to gett+ a telescope into action & giv+ x suitable protection1 two entirely diff]5t situations pres5t !mselves4 ,! fir/ relates to portable in/rum5ts or ^? on tempor>y m\nts1 ! second to in/rum5ts ( position4 ,z respects ! two1 ! =m] ord9>ily implies g5]al use = obs]vational purposes1 ! latt] at lea/ ! possibility ( measurem5ts ( precision1 & a m\nt usually fitt$ ) circles & ) a driv+ clock4 ,portable telescopes may h ei!r alt-azimu? or equatorial m\nt+1 :ile ^? p]man5tly set up are n[ q univ]sally equatorials4 ,portable telescopes are commonly small1 rang+ f ab\t #b in*es to ab\t #e in*es 9 ap]ture4 ,! =m] is ! smalle/ t c fairly be consid]$ = cele/ial obs]vations4 ,if ?or\ghly good & well m\nt$ ev5 ?is is capable ( real usefulness1 :ile ! five in* telescope if built & equipp$ 9 ! usual way1 is q ! heavie/ t c be rat$ z portable1 & des]ves a fix$ m\nt4 ,sett+ up an alt-azimu? is ! simple/ possible matt]4 ,if on a regul> tripod x is m]ely tak5 out & ! tripod r\ghly levell$ s t ! axis 9 azimu? is approximately v]tical4 ,n[ & !n "o sets x delib]ately askew s t x may be possible to pass quickly betwe5 two objects at some:at diff]5t altitudes by sw++ on ! azimu? axis4 ,if "o is deal+ ) a table tripod l ,fig4 #fi x %\ld m]ely be set on any level & solid support t may be at h&1 ! ma9 ?+ be+ to get x plac$ s t "o may look ?r\gh x conv5i5tly4 ,?is is a grave problem 9 ! case ( all small refractors1 :i* pres5t _! ocul>s 9 e sort ( unrea*able & uncom=table position4 ,( c\rse a diagonal eyepiece promises a way out ( ! difficulty1 b ) small ap]tures "o hesitates to lose ! light1 & (t5 some?+ ( def9ition1 & ! obs]v] mu/ pretty ne>ly /& on 8 head to use ! f9d]4 ,) well adju/$ circles1 su* are commonly f\nd on a fix$ m\nt1 location ( objects is easy4 ,on a portable set-up p]haps ! easie/ rem$y is a pair ( well align$ co>se sights ne> ! objective end ( ! tube & !re=e )9 rea* :5 x is po9t$ z5i?-w>d4 ,! writ] has f\nd a l[1 armless1 *eap spl9t rock]1 su* z is sold = piazza use1 invaluable "u ^! pa9ful circum/ances1 & c cordially recomm5d x4 ,ev5 bett] is an obs]v+ box & a flat cu%ion4 ,! box is m]ely a cov]less affair ( any smoo? in* /uff firmly nail$ or screw$ toge!r1 & ( ?ree unequal dim5sions1 giv+ ?ree available heights on :i* to sit or /&4 ,! dim5sions orig9ally sugge/$ by ,*amb]s 7,h&book ( ,a/ronomy1 ,,ii1 #bae7 7 #ba #ab #ae in*es1 b ! writ] f9ds #ah #aj #ad in*es a bett] comb9ation4 ,! fact is t ! ord9>y /ock telescope tripod is r too high = sitt+1 & too l[ = /&+1 com=tably4 ,a some:at /ubby tripod is advantage\s bo? 9 po9t ( /ead9ess & 9 accessibility ( ! eyepiece :5 "o is obs]v+ )9 #cj ( ! z5i?1 :]e ! see+ is at its be/2 & a sitt+ position gives a mu* great] range ( conv5i5t upw>d vision ?an a /&+ "o4 ,:5 an equatorial m\nt is 9 use "o faces ! "q ( adju/m5t 9 its broade/ aspect4 ,aga9 two totally diff]5t situations arise 9 us+ ! telescope4 ,fir/ is ! ord9>y c\rse ( visual obs]vation = all g5]al purposes1 9 :i* no precise measurem5ts ( position or dim5sions are involv$4 ,"h exact foll[+ is n necess>y1 a clock drive is conv5i5t r ?an at all indisp5sable1 & ev5 circles "o may get along )\t at ! co/ ( a little "t4 ,su* is ! usual situation ) portable equatorials4 ,"o does n !n ne$ to adju/ !m to ! pole ) extreme precision1 b m]ely well en\gh to insure easy foll[+2 o!rwise "o is h>dly bett] (f ?an ) an alt-azimu?4 ,9 a totally diff]5t class falls ! in/rum5t ) :i* "o und]takes regul> micrometric "w1 or ent]s ^u an ext5d$ spectroscopic program or ! use ( precise photometric app>atus1 to say no?+ ( photography4 ,9 su* cases a p]man5t m\nt is almo/ imp]ative1 ! adju/m5ts mu/ be made ) all ! exactitude practicable1 "o f9ds great ne$ ( circles1 & ! lack ( a clock drive is a s]i\s h&icap or worse4 ,moreov] 9 ?is latt] case "o usually has1 & ne$s1 "s sort ( timepiece regulat$ to sid]eal "t1 )\t :i* a "r asc5sion circle is ( v little use4 ,9 broad t]ms1 !n1 "o has to deal1 fir/2 ) a telescope on a portable m\nt1 ) or )\t position circles1 g5]ally lack+ bo? sid]eal clock & driv+ clock1 & locat$ :]e conv5i5ce dictates2 second1 ) a telescope on a fix$ m\nt 9 a p]man5t location1 commonly ) circles & clock1 & ) "s sort ( p]man5t h\s+4 ,let u suppose !n t "o is equipp$ ) a #e in* in/rum5t l ,fig4 #afh1 hav+ ei!r ! tripod m\nt1 or ! fix$ pill> m\nt %[n alongside x2 h[ %all x be set up1 &1 if on ! fix$ m\nt1 h[ %elt]$8 ,9 gett+ an equatorial into action ! fundam5tal ?+ is to place ! optical axis ( ! telescope exactly p>allel to ! pol> axis ( ! m\nt & to po9t ! latt] z ne>ly z possible at ! cele/ial pole4 ,! conv5tional adju/m5ts ( an equatorial telescope are z foll[s3 #a4 ,adju/ pol> axis to altitude ( pole4 #b4 ,adju/ index ( decl9ation circle4 #c4 ,adju/ pol> axis to ! m]idian4 #d4 ,adju/ optical axis p]p5dicul> to decl9ation axis4 #e4 ,adju/ decl9ation axis p]p5dicul> to pol> axis4 #f4 ,adju/ index ( "r asc5sion circle1 & #g4 ,adju/ optical axis ( f9d] p>allel to t ( telescope4 ,n[ let u take ! simple/ & commone/ case1 ! adju/m5t ( a portable equatorial on a tripod m\nt1 :5 ! in/rum5t has a f9d] b nei!r circles nor driv+ clock4 ,adju/m5ts #b & #f automatically drop out ( sight1 #e vani%es = lack ( any means to make ! adju/m5t1 & on a m\nt made ) high precision1 l ! "o be=e u1 #d is negligible = any purpose to :i* our in/rum5t is applicable4 ,adju/m5ts #a1 #c & #g are left & ^! %\ld be p]=m$ 9 ! ord] #g1 #a1 #c1 = sake ( simplicity4 ,to beg9 ) ! f9d] has cross-wires 9 ! focus ( its eyepiece1 & ! next /ep is to provide ! telescope itself ) simil> cross-wires4 ,^! c readily be made1 if n provid$1 by cutt+ out a disc ( c>dbo>d to fit snugly ei!r ! spr+ coll> j 9 front ( a positive eyepiece or ! eyepiece itself at ! diaphragm1 if an ord9>y ,huyg5ian4 ,rule two diametral l9es on ! circle /ruck = cutt+ ! c>dbo>d1 cross+ at ! c5t]1 cut out ! c5tral ap]ture1 & !n v c>efully /ret* ov] x1 guid$ by ! diametral l9es1 two v f9e ?reads or wires made fa/ ) wax or %ellac4 ,illu/ration3 ,,fig4 #afh4--;,cl>k #e-;in* ) ,tripod & ,pi]4 ,n[ po9t+ ! telescope at ! mo/ di/ant well def9$ object 9 view1 rotate ! spr+ coll> or ocul>1 :51 if ! cross+ ( ! ?reads is c5tral1 _! int]section %\ld /ay on ! object4 ,if n %ift a ?read cauti\sly until ! error is correct$4 ,keep+ ! int]section set on ! object by clamp+ ! tube1 "o turns att5tion to ! f9d]4 ,ei!r ! :ole tube is adju/able 9 its supports or ! cross-wires are capable ( adju/m5t by screws j 9 front ( ! eyepiece4 ,9 ei!r case f9d] tube or cross-wires %\ld be %ift$ until ! latt] be> squ>ely ^u ! object :i* is 9 l9e ) ! cross ?reads ( ! ma9 telescope4 ,!n ! adju/+ screws %\ld be tight5$1 & ! f9d] is 9 correct alignm5t4 ,z to adju/m5ts #a & #c1 9 default ( circles ! ord9>y a/ronomical me?ods are n available1 b a pretty close approximation c be made by levell+4 ,a good ma*9i/'s level is q s5sitive & reliable4 ,! writ] has "o pick$ out ( /ock at a h>dw>e %op t is pla9ly s5sitive to #b ( arc1 al?\gh ! :ole affair is b f\r in*es long4 ,mo/ m\nts l ! "o ( ,fig4 #afh h a m>k rul$ on ! support ( ! pol> axis & a latitude scale on "o ( ! *eek pieces4 ,adju/m5t ( ! pol> axis to ! correct altitude is !n made by plac+ ! level on ! decl9ation axis1 or any o!r conv5i5t place1 br++ x to a level1 & !n adju/+ ! tripod until ! equatorial head c be revolv$ )\t di/urb+ ?is level4 ,!n set ! pol> axis to ! correct latitude & adju/m5t numb] #a is complete = ! purpose 9 h&4 ,lack+ a latitude scale1 x is good judgm5t to m>k out ! latitude by ! help ( ! level & a pap] protractor4 ,to d ?is level ! pol> axis to ! horizontal1 level ! telescope tube also1 & clamp x 9 decl9ation to ma9ta9 x p>allel4 ,!n fix ! protractor to a bit ( wood ti$ or screw$ to ! telescope support1 drop a ?9 ?read plumb l9e f a p9 driv5 into ! wood1 ! decl9ation axis be+ /ill clamp$1 note ! protractor read+1 & !n raise ! pol> axis by ! am\nt ( ! latitude4 ,next1 ) a knife blade scrat* a conspicu\s ref]5ce l9e on ! sleeve ( ! pol> axis & its support s t :5 ! equatorial head is aga9 levell$ c>efully y c set approximately to ! latitude at once4 ,n[ comes adju/m5t #c1 ! alignm5t ( ! pol> axis to ! m]idian4 ,"o c get x approximately by sett+ ! telescope tube r\ghly p>allel ) ! pol> axis &1 sight+ along x1 %ift+ ! equatorial head 9 azimu? until ! tube po9ts to ! pole />4 ,!n sev]al me?ods ( bett]+ ! adju/m5t are available4 ,at ! pres5t date ,pol>is is q ne>ly #a #jg f ! true pole & describes a circle ( t radius ab\t x e #bd h\rs4 ,to get ! correct place ( ! pole ) ref]5ce to ,pol>is "o mu/ h at lea/ an approximate k ( ! place ( t /> 9 its little orbit1 te*nically its h\r-angle4 ,) a little k ( ! />s ?is c be told (f 9 ! skies almo/ z easily z "o reckons "t on a clock4 ,fig4 #afi is1 9 fact1 ! face ( ! cosmic clock1 ) a huge sweep+ h\r h& t he :o runs may read4 ,illu/ration3 ,,fig4 #afi4--,! ,cosmic ,clock4 ,x is a clock 9 "s respects curi\s2 x has a tw5ty-f\r h\r face l "s clocks & wat*es design$ = ,cont95tal railway "t2 ! h\r h& revolves backw>d1 78c\nt]-clockwise07 & x /&s 9 ! v]tical n at noon1 b at #a4bj ,/> ,"t4 ,! two />s :i* m>k ! tip & ! rev]se end ( ! h\r h& are delta ,cassiope & zeta ,urs ,majoris respectively4 ,! fir/ is ! /> t m>ks ! b5d 9 ! back ( ! great 8*air10 ! second 7,miz>71 ! /> :i* is next to ! end ( ! 8dipp]0 h&le4 ,"o or ! o!r is above ! horizon any:]e 9 ! nor!rn hemisph]e4 ,fur!r1 ! l9e jo9+ ^! two />s passes almo/ exactly ?r\gh ! cele/ial pole1 & also v ne>ly ?r\gh ,pol>is1 :i* lies betwe5 ! pole & ,cassiope4 ,consequ5tly if y want to "k ! h\r-angle ( ,pol>is j glance at ! clock & note :]e on ! face ,cassiope /&s1 betwe5 ! v]tical :i* is ,,xxiv o'clock1 & ! horizontal1 :i* is ,,vi 7ea/7 or ,,xviii 7we/7 o'clock4 ,y c readily e/imate its position to ! ne>e/ half h\r1 & kn[+ t ! great h\r h& is v]tical 7 ,cassiope up7 at ,ih #bjm or 7 ,urs ,majoris up7 at ,,xiiih #bjm1 y c make a fairly close e/imate ( ! sid]eal "t4 ,a little exp]i5ce enables "o to make excell5t use ( ! clock 9 locat+ cele/ial objects1 & k ( ! approximate h\r angle ( ,pol>is ?us obs]v$ c be turn$ to imm$iate use 9 mak+ adju/m5t #c4 ,to ?is end slip into ! plane ( ! f9d] cross wires a circul> /op ( metal or pap] hav+ a radius ( approximately #a #ae :i* means a diamet] ( #j4eb in* p] foot ( focal l5g?4 ,!n1 leav+ ! telescope clamp$ 9 decl9ation z x 0 aft] adju/m5t #a1 turn x 9 azimu? across ! pole until ! pole /> ent]s ! field :i*1 if ! f9d] inv]ts x w d on ! o!r side ( ! c5t]2 i4e4 if x /&s at ,,iv to ! nak$ eye x w ent] ! field app>5tly f ! ,,xvi o'clock qu>t]4 ,:5 j com=tably inside ! field1 ! axis ( ! telescope is po9t+ sub/antially at ! pole4 ,x is bett] to get ,pol>is 9 view be=e slipp+ 9 ! /op & if x is cle>ly com+ 9 too high or too l[ %ift ! altitude ( ! pol> axis a trifle to correct ! error4 ,?is approximate sett+ c be made ev5 ) ! smalle/ f9d] & on any night wor? an attempt at obs]vation4 ,) a f9d] ( an in* or m ap]ture a v quick & q accurate sett+ to ! m]idian c be made by ! use ( ,fig4 #agj1 :i* is a *>t ( all />s ( #h mag4 or bright] )9 #a #cj ( ! pole4 ,!re are only ?ree />s besides ,pol>is at all conspicu\s 9 ?is region1 "o q close to ,pol>is1 ! o!r two =m+ ) x ! triangle m>k$ on ! *>t4 ,^! two are1 to ! left1 a /> ( magnitude #f4d designat$ ,b4 ,d4 #hh #aab1 & to ! "r "o ( magnitude #g4j1 ,b4 ,d4 #hi #ac4 ,! position ( ! pole = ! re/ ( ! c5tury is m>k$ on ! v]tical arr[ & ) ! />s 9 ! field ( ! f9d] "o c set ! cross wires on ! pole1 ! in/rum5t rema9+ clamp$ 9 decl9ation1 )9 a v few m9utes ( arc1 q closely en\gh = any ord9>y use ( a portable m\nt4 ,all ?is c\ld be done ev5 bett] ) ! telescope itself1 b x is v r>e to f9d an eyepiece ) suffici5t field4 ,illu/ration3 ,,fig4 #agj4--,! ,pole among ! ,/>s4 ,at all ev5ts ! effect ( any error likely to be made 9 ^! adju/m5ts is n s]i\s = ! purpose 9 h&1 s9ce if "o makes an error ( a m9ute ( arc 9 ! sett+ ! result+ displacem5t ( a /> 9 ! field w ev5 9 ! mo/ unfavorable case rea* ?is full am\nt only aft] #f h\rs foll[+4 ,i4e4 ) any giv5 eyepiece an error ( adju/m5t equal to ! radius ( ! field w /ill p]mit foll[+ a /> = an h\r or two be=e x drifts inconv5i5tly wide ( ! c5t]4 ,consid]able space has be5 devot$ to ^! easy approximations 9 sett+ up1 s9ce ! directions commonly giv5 require circles & (t5 a clock drive4 ,9 "s cases "o has to set up a portable equatorial :]e f necessity = cle> sky space1 ,pol>is is n visible4 ,! be/ plan !n is to set up ) great c>e :]e ,pol>is c be se51 pay+ especial att5tion to ! levell+4 ,!n e/abli% two m]idian m>ks on /akes at a conv5i5t di/ance by turn+ ! telescope #ahj on its decl9ation axis & sight+ ?r\gh x 9 bo? directions4 ,n[ ) a surveyor's tape transf] ! m]idian l9e ,ea/ or ,we/ z ! case may be until x c be us$ :]e !re is cle> sky room4 ,few obs]v]s ne> a city c get good sky room1 f ! int]f]5ce ( h\ses1 trees or blaz+ /reet lamps1 & ! telescope mu/ (t5 be mov$ f "o site to ano!r to rea* diff]5t fields4 ,9 su* case x is wise to take ! v fir/ /ep t[>d giv+ ! telescope a local habitation by e/abli%+ a def9ite placem5t = ! tripod4 ,to ?is end ! ?ree legs %\ld be firmly l9k$ toge!r by *a9s t w n /ret*--leg directly to leg1 & n to a common junction4 ,!n see to x t ea* leg has a /rong & mod]ately %>p metal po9t1 &1 ! ?ree po9ts ( support be+ ?us def9itely fix$1 e/abli% ! old reliable po9t-slot-plane be>+ z foll[s3 ,lay out at ! site 7or sites7 giv+ ! desir$ cle> view1 a circle scrat*$ on ! gr\nd ( su* size t ! ?ree legs ( y\r tripod may re/ approximately on its p]iph]y4 ,!n lay out on ! circle ?ree po9ts #abj ap>t4 ,at ea* po9t s9k a %ort po/ #ab to #ah in*es long & ( any conv5i5t diamet]1 well t>r$1 & firmly set ) ! top levell$ (f q closely horizontal4 ,to ! top ( ea* bolt a squ>e or r\nd ( brass or iron ab\t half an in* ?ick4 ,! :ole arrangem5t is indicat$ 9 diagram 9 ,fig4 #aga4 ,9 a s9k a conical depression su* z is made by drill+ ne>ly ?r\gh ) a #a in* twi/ drill4 ,! angle "h %\ld be a little broad] ?an ! po9t on ! tripod leg4 ,9 b h plan$ a ,v %ap$ groove ( equally broad angle set ) its axis po9t+ to ! conical hole 9 a4 ,leave ! surface ( c a horizontal plane4 ,n[ if y set a tripod leg 9 a1 ano!r 9 ! slot at b & ! ?ird on c1 ! tripod w come 9 e in/ance to ! same level & ori5tation4 ,s1 if y set up y\r equatorial c>efully 9 ! fir/ place & leave ! head clamp$ 9 azimu?1 y c take x 9 & replace x at any "t /ill 9 adju/m5t z exact z at ! />t4 ,& if x is necess>y to %ift f "o location to ano!r y c d x )\t delay /ill hold+ accurate adju/m5t ( ! pol> axis to ! pole1 & avoid+ ! ne$ ( readju/m5t4 ,9 case ! in/rum5t has a decl9ation circle ! orig9al set-up becomes ev5 simpl]4 ,"o has only to level ! tripod1 ei!r ) or )\t ! equatorial head 9 place1 & !n to set ! pol> axis ei!r v]tical or horizontal1 levell+ ! tube ) x ei!r by plac+ ! level across ! objective cell p]p5dicul> to ! decl9ation axis1 or lay+ x along ! tube :5 horizontal4 ,illu/ration3 ,,fig4 #aga4--;,a ,p]man5t ,foo?old = ! ,tripod4 ,!n1 read+ ! decl9ation circle1 "o c set (f ! co-atitude or latitude z ! case may be &1 leav+ ! telescope clamp$ 9 decl9ation1 l[] or raise ! pol> axis until ! tube levels to ! horizontal4 ,:5 ! m\nt does n p]mit wide adju/m5t & has no latitude scale "o is driv5 to lay+ out a latitude templet &1 plac+ a /raight edge "u ! equatorial head1 or susp5d+ a plumb l9e f ! axis itself1 sett+ x me*anically to latitude4 ,n[ suppose we are deal+ ) ! same in/rum5t1 b are plann+ to plant x p]man5tly 9 position on its pill> m\nt4 ,x is n[ wor? :ile to make ! adju/m5ts q exactly1 & to sp5d "s "t ab\t x4 ,! pill> is commonly assembl$ by well set bolts on a brick or concrete pi]4 ,! prelim9>y /eps are z already describ$4 ,! pill> is levell$ across ! top1 ! equatorial head1 :i* turns ^u x 9 azimu?1 is levell$ z be=e1 ! adju/m5t be+ made by metal w$ges "u ! pill> or by levell+ screws 9 ! m\nt if !re are any4 ,!n ! latitude is set (f by ! scale1 or by ! decl9ation circle1 & ! pol> axis turn$ to ! approximate m]idian z already describ$4 ,!re is likely to be an out/&+ error ( a few m9utes ( arc :i* %\ld 9 a p]man5t m\nt be r$uc$ z f> z practicable4 ,at ! />t adju/ ! decl9ation ( ! optical axis ( ! telescope to t ( ! pol> axis4 ,?is is done 9 ! mann] sugge/$ by ,fig4 #agb4 ,"h p is ! pol> axis & d ! decl9ation axis4 ,n[ if "o sights1 us+ ! cross wires1 ?r\gh ! telescope a /> ne> ! m]idian1 i4e41 "o t is *ang+ 9 decl9ation q sl[ly1 />t+ f ! position ,a ) ! telescope ,e4 ( ! pol> axes1 & turns x ov] #ahj into ! position ,b1 ,w4 ( ! pol> axis1 ! prolongation ( ! l9e ( sight1 b1 w fall bel[ a1 :5 z "h ! telescope po9ts too high 9 ! ,a position4 ,illu/ration3 ,,fig4 #agb4--;,align+ ! ,optical ,axis4 ,9 o!r words ! app>5t altitude ( ! /> w *ange by twice ! angle betwe5 ,a & p4 ,read bo? altitudes on ! decl9ation circle & split ! diff]5ce ) ! sl[ motion z precisely z ! graduation ( ! decl9ation circle p]mits4 ,! telescope w probably n n[ po9t exactly at ! />1 b z ! tube is swung f ! ,a to ! ,b position & back ! visible />s w describe arcs ( circles :i* %\ld be ne>ly conc5tric ) ! field z def9$ by ! /op 9 ! eyepiece4 ,if n1 a v slight t\* on ! decl9ation sl[ motion "o way or ! o!r w make !m d s to a suffici5t exactness1 especially if a r high p[] eyepiece is us$4 ,! optical axis ( ! telescope is n[ p>allel to ! pol> axis1 b ! latt] may be slightly out ( position 9 spite ( ! prelim9>y adju/m5t4 ,n[ rev]t+ to ! pol> field ( ,fig4 #agj1 sw+ f position ,a to ,b & back aga91 correct+ any rema9+ ecc5tricity ( ! /> arcs ar\nd ! pole by cauti\s %ift+ ( ! pol> axis1 leav+ ! telescope clamp$ 9 decl9ation4 ,! fir/ c5t]+ is ar\nd ! pole ( ! in/rum5t1 ! second ar\nd ! cele/ial pole by help ( a half doz5 small />s )9 a half degree on bo? sides ( x1 magnitudes #i & #aj1 easily visible 9 a #c0 or #d0 telescope1 us+ ! l>g] field ( ! f9d] = ! co>se adju/m5t4 ,if ! divid$ circles read to s+le m9utes or clos]1 :i* !y g5]ally d n on in/rum5ts ( mod]ate size1 "o c use ! read+s to set ! pol> axis & ! decl9ation circle1 & to make ! o!r adju/m5ts z well4 ,9 default ( ?is help1 ! decl9ation circle adju/m5t may be set to read #ij :5 ! optical axis is br\ght p>allel to ! pol> axis1 & aft] ! adju/m5t ( ! latt] is complete1 ! ,r4 ,a4 circle c be set by sw++ up ! telescope 9 ! m]idian & wat*+ = ! transit ( any /> ( kn[n ,r4 ,a4 ov] ! c5tral cross wire1 at :i* mom5t ! circle %\ld be clamp$ to ! ,r4 ,a4 ?us def9$4 ,two possible adju/m5ts are left1 ! p]p5dicul>ity ( ! pol> & decl9ation axes1 & t ( ! optical axis to ! decl9ation axis4 ,z a rule !re is no provision = ei!r ( ^!1 :i* are suppos$ to h be5 c>ri$ out by ! mak]4 ,! latt] adju/m5t if ( any mom5t w disclose itself z a lat]al wobble 9 try+ to complete ! adju/m5t ( optical axis to pol> axis4 ,x c be rem$i$ by a l9] ( t9foil or ev5 pap] "u "o end ( ! tube's be>+ on its cradle4 ,adju/m5t ( ! =m] is /rictly a job = ! mak]4 ,= details ( ! rigor\s adju/m5ts on ! l>g] in/rum5ts ! read] w d well to consult ,loomis' ,practical ,a/ronomy page #bh & foll[+4#ca ,! adju/m5ts "h consid]$ are ^? :i* c be effectively made )\t driv+ clock1 f9ely divid$ circles1 or exact k ( sid]eal "t4 ,! fir/ & la/ ( ^! auxili>ies1 h[ev]1 prop]ly belong ) an in/rum5t z l>ge z ,fig4 #afh1 on a fix$ m\nt4 #ca ,see also two valuable pap]s by ,sir ,h[>d ,grubb1 ,! ,obs]vatory1 ,vol4 ,,vii1 pp4 #i1 #dc4 ,also 9 ,j\r4 ,roy4 ,a/4 ,soc4 ,canada1 ,dec41 #aiba1 ,jan4 #aibb4 ,!re are sev]al r elegant me?ods ( adju/+ ! pol> axis to ! pole :i* dep5d on ! use ( special graticules 9 ! eyepiece1 or on auxili>y devices appli$ to ! telescope1 ! g5]al pr9ciple be+ automatically to provide = sett+ (f ! di/ance betwe5 ,pol>is & ! pole at ! prop] h\r angle4 ,a beautifully simple "o is t ( ,g]ri% 7,pop4 ,a/4 #bi1 #bhc74 ,! simple plan "h outl9$ w g5]ally1 h[ev]1 prove suffici5t = ord9>y purposes & :]e high precision is necess>y "o has to turn to ! m conv5tional a/ronomical me?ods4 ,if "o gives 8 telescope a p]man5t foot+ su* z is %[n 9 ,fig4 #aga adju/m5t has r>ely to be repeat$4 ,) a pill> m\nt su* z we h j n[ be5 consid]+ ! in/rum5t itself c be tak5 9 doors & replac$ ) v slight risk ( di/urb+ its sett+1 b "s provision mu/ be made = %elt]+ ! m\nt4 ,a t>paul9 is sometimes recomm5d$ & inde$ answ]s well1 p>ticul>ly if a bag ( rubb] %eet+ is drawn loosely ov] ! m\nt fir/4 ,bett] /ill is a box cov] ( copp] or galvaniz$ iron set ov] ! m\nt & closely fitt+ well d[n ov] a base clamp$ to ! pill> ) a gasket to close ! jo9t4 ,b ! fact is :5 "o is deal+ ) a f9e in/rum5t l ,fig4 #afh ( z mu* z #e in*es ap]ture1 ! "q ( a p]man5t h\s+ 7call x obs]vatory if y l7 at once comes up & w n d[n4 ,x is ( c\rse always m conv5i5t to h ! telescope p]man5tly 9 place & ready = action4 ,"s obs]v]s feel t work+ conditions are bett] ) ! telescope 9 ! op51 b mo/ pref] a %elt] f ! w9d1 ev5 if b p>tial1 & ! protection ( a cov]+1 h[ev] slight1 9 sev]e wea!r4 ,9 ! la/ resort ! "q is ma9ly "o ( climate4 ,:]e nights1 o!rwise ( ! be/ see+ quality1 are g5]ally w9dless or ) breezes s slight t ! tube does n quiv] a telescope 9 ! op51 h[ev] protect$ betwe5 times1 works p]fectly well4 ,9 o!r regions ! cle>e/ nights are apt to be ^? ( a /eady g5tle w9d produc+ great uni=mity ( conditions at ! exp5se ( occasional vibration ( ! in/rum5t & ( discom=t to ! obs]v]4 ,h5ce "o f9ds all sorts ( practice1 v>i$ too1 by ! inevitable "q ( exp5se4 ,! simple/ possible h\s+ is to provide = ! fix$ in/rum5t a moveable cov] :i* c be lift$ or slid q out ( ! way leav+ ! telescope 9 ! op5 air1 expos$ to w9d1 b free f ! di/urb+ air curr5ts t play ar\nd ! op5+ ( a dome4 ,%elt]s ( ?is *eap & simple sort h be5 long 9 use bo? = small & l>ge in/rum5ts4 ,illu/ration3 ,,fig4 #agc4--,! ,simple/ ( ,telescope ,h\s+s4 ,= example sev]al small a/rographic in/rum5ts 9 ! ,h>v>d equipm5t are m\nt$ z %[n 9 ,fig4 #agc4 ,"h are two =k m\nts1 ea* on a %ort pi]1 & cov]$ 9 by galvaniz$ iron hoods made 9 two p>ts1 a v]tical door :i* sw+s d[n1 z 9 ! cam]a ( ! =egr\nd1 & ! hood prop]1 h+$ to ! base plate & free to sw+ d[n :5 ! re> door is unlock$ & op5$4 ,a little to ! re> is a simil> a/rographic cam]a ) ! hood clos$4 ,x is all v simple1 *eap1 & effective = an in/rum5t n exce$+ say two or ?ree feet 9 focal l5g?4 ,a v simil> s*eme has be5 successfully tri$ on reflectors z %[n 9 ,fig4 #agd4 ,! in/rum5t %[n is a ,br[n+ equatorial ( #h in*es ap]ture4 ,! cov] is arrang$ to op5 aft] ! mann] ( ,fig4 #agc & ! plan prov$ v effective1 pres]v+ mu* great] uni=mity ( conditions & h5ce p]mitt+ bett] def9ition ?an 9 case ( a simil> in/rum5t pe]+ ?r\gh ! op5 %utt] ( a dome4 ,su* a contrivance gets unwieldly 9 case ( a refractor on acc\nt ( ! m consid]able height ( ! pi] & ! l5g? ( ! tube itself4 ,b a modification ( x may be made to s]ve exce$+ly well 9 climates :]e work+ 9 ! op5 is advantage\s4 ,a good example is ! equatorial ( ! ,h>v>d ,obs]vatory /ation at ,m&eville1 ,jamaica1 :i* has be5 ?us h\s$ = "s tw5ty ye>s1 z %[n 9 ,fig4 #age4 ,?is #aa in* refractor1 us$ ma9ly on planet>y detail1 is locat$ alongside ! pol> telescope ( #ab in*es ap]ture & #ace feet #d in*es focal l5g? us$ = mak+ a photographic atlas ( ! moon & on o!r special problems4 ,! h\s+1 j big en\gh to take 9 ! equatorial ) ! tube turn$ l[1 op5s on ! s\? side & !n c be roll$ nor?w>d on its track1 into ! position %[n1 :]e x is well cle> ( ! in/rum5t1 :i* is !n ready = use4 ,illu/ration3 ,,fig4 #agd4--;,cov] = ,small ,reflector4 ,! climate ( ,jamaica1 albeit extremely damp1 af=ds rem>kably good see+ dur+ a l>ge "p ( ! ye>1 & p]mits use ( ! telescope q 9 ! op5 )\t inconv5i5ce to ! obs]v]4 ,! success ( ?is & all simil> h\s+ plans dep5ds on ! local climate m ?an on any?+ else--*iefly on w9d dur+ ! h\rs ( good see+4 ,an in/rum5t q uncov]$ suff]s f gu/s f> m ?an "o h\s$ "u a dome1 :i* is really ! sum ( ! :ole matt]1 save t a dome to a slight ext5t does %elt] ! obs]v] 9 extremely cold wea!r4 ,ev5 v l>ge reflectors c be h\s$ 9 simil> fa%ion if suitably m\nt$4 ,= example 9 ,fig4 #agf is %[n ! #cf in* ap]ture reflector ( ! late ,dr4 ,common1 :i* 0 fitt$ ) an op5 =k equatorial m\nt+4 ,"h ! telescope itself1 ) its %ort pi] & =k$ pol> axis1 is %[n 9 dott$ l9es4 ,illu/ration3 ,,fig4 #age4--,slid+ ,h\s+ = #aa-;in* ,refractor4 ,built ab\t x is a comb9$ h\s+ & obs]v+ /& rotatable on :eels ,t ab\t a circul> track ,r4 ,! h\s+ consi/s ( l[ corrugat$ metal sides & ends1 "h %[n p>tly brok5 away1 ( dim5sions j com=tably suffici5t to take 9 ! telescope :5 ! h\s+ is rotat$ to ! nor? & s\? position1 & ! tube turn$ d[n ne>ly flat s\?w>d4 ,a well brac$ track ,,ww ext5ds back along ! top ( ! side h\s+ & well to ! re>4 ,on ?is track rolls ! ro( ( ! h\s+ ,x1,x1,x1 ) a %elt] door at ! front end4 ,illu/ration3 ,,fig4 #agf4--,slid+ ,h\s+ = a ,big ,reflector4 ,! memb]s ,u con/itute a fram+ :i* supports at once ! h\s+ & ! obs]v+ plat=m1 to :i* access is _h by a ladd]1 ,z1 provid$ ) a c\nt]balanc$ obs]v+ seat4 ,! in/rum5t is put into action by cle>+ ! door at ! end ( ! ro(1 runn+ ! ro( back to ! position %[n 9 ! dott$ l9es1 rais+ ! tube1 & !n revolv+ ! :ole h\s+ into :atev] position is necess>y to p]mit ! prop] sett+ ( ! tube4 ,illu/ration3 ,,fig4 #agg4--,slid+ ,ro( ,obs]vatory4 ,?is arrangem5t work$ well b 0 f\nd a bit tr\blesome ow+ to w9d & wea!r4 ,) a skeleton tube & 9 a favorable climate ! plan w\ld succe$ admirably provid+ an excell5t %elt] = a l>ge telescope at v l[ co/4 ,s9ce a =k m\nt all[s ! tube to lie flat1 su* an in/rum5t1 up to say #h or #aj in*es ap]ture c be excell5tly protect$ by cov]s fitt+ snugly ^u a base & light en\gh to lift (f z a :ole4 ,! successful use ( all ^! %elt]s h[ev] dep5ds on climatic conditions4 ,!y require circum/ances all[+ obs]vation 9 ! op51 z ) tripod m\nts1 & af=d no protection f w9d or cold4 ,complete protection = ! obs]v] _c be _h1 except by "s ( ! devices %[n 9 ,*apt] ,v1 b conditions c be improv$ by p]man5t placem5t 9 an obs]vatory1 simple or elaborate1 z ! build] may wi%4 ,! ^w obs]vatory may s\nd =midable1 b a mode/ "o c be provid$ at less exp5se ?an a g>age = ! humble/ motor c>4 ,! *ief diff]5ce 9 ! economic situation is t n ev5 ! mo/ d]id$ c> c be pick$ up & c>ri$ into ! back hall = %elt]1 & x really ought n to be left out 9 ! wea!r4 ,! next /age ( evolution is ! telescope h\se ) a slid+ ro( 9 "o or m sections--ord9>ily two4 ,9 ?is case ! build+ itself is a simple squ>e /ructure l>ge en\gh to accommodate ! in/rum5t ) maneuv]+ room ar\nd x4 ,! side walls are c>ri$ m]ely high en\gh to give cle>ance to ! tube :5 turn$ ne>ly flat & to give head room to ! obs]v]4 ,! ro( laps ) a close jo9t 9 ! middle & ea* half rolls on a track support$ beyond ! ends ( ! build+ by an out-rigg] arrang$ 9 any conv5i5t mann]4 ,:5 ! telescope is 9 use ! ro( sections are displac$ en\gh to give an ample cle> space = obs]v+1 (t5 wide op5 z %[n 9 ,fig4 #agg1 :i* is ! h\se ( ! #af in* ,metcalf photographic d\blet at ! ,h>v>d ,obs]vatory4 ,?is in/rum5t is 9 an op5 =k m\nt l t %[n 9 ,fig4 #aci4 ,! slid+ ro( type is on ! :ole ! simple/ /ructure t c be reg>d$ z an obs]vatory 9 ! s5se ( giv+ "s %elt] to ! obs]v] z well z ! in/rum5t4 ,x gives ample sky room = practical purposes ev5 to an in/rum5t ) a =k m\nt1 s9ce 9 mo/ localities ! see+ )9 #cj or s ( ! horizon is decid$ly bad4 ,if view ne>] ! horizon is ne$$ x c readily be secur$ by build+ up ! pi] a bit4 ,numb]less modifications ( ! slid+ ro( type w sugge/ !mselves on a little /udy4 ,"o r int]e/+ "o is us$ 9 ! h\s+ ( ! #bd in* reflector ( ! ,h>v>d ,obs]vatory1 #aa feet #c in*es 9 focal l5g?1 ! same ( :i* ! drive 9 its orig9al dome is %[n 9 ,fig4 #aci4 ,z n[ arrang$ ! l[] "p ( ! obs]vatory rema9s :ile ! upp] works are q simil> 9 pr9ciple to ! h\s+ ( ,dr4 ,common's #c foot reflector ( ,fig4 #agf4 ,! cov] op5 is %[n 9 ,fig4 #agh4 ,x w be se5 t on ! nor? side ( ! obs]vatory !re is an out-rigg] on :i* ! top h\s+ slides cle> ( ! l[ revolv+ turret :i* gives access to ! ocul> fitt+ us$ g5]ally to c>ry ! plate hold]1 & ! eyepiece = foll[+ :5 requir$4 ,! tube _c be br\ght to ! horizontal1 b x easily comm&s all ! sky-space t c advantage\sly be us$ 9 ?is situation1 & ! protection giv5 ! telescope :5 n 9 use is v complete4 ,to close ! obs]vatory ! tube is br\ght nor? & s\? & turn$ l[ & ! slid+ ro( is !n run back into its fix$ position4 ,! turret is v easily turn$ by h&4 ,illu/ration3 ,,fig4 #agh4--,turret ,h\s+ ( ! #bd-;in* ,h>v>d ,reflector4 ,( c\rse = /eady "w ) ! maximum %elt] = obs]v] obta9able )\t turn+ to highly special types ( h\s+1 ! famili> dome is ! a/ronom]'s ma9 reliance4 ,x is 9 ! l>g] sizes usually fram$ 9 /eel & cov]$ ) wood1 ext]nally %ea!d 9 copp] or /eel4 ,sometimes 9 small] domes felt cov]$ ) rubb]oid s]ves a good purpose1 & pa9t$ canvas is n[ & !n us$1 ) wood5 fram+4 ,b ev5 ! smalle/ dome ( conv5tional con/ruction is heavy & r exp5sive1 & = home tal5t (f]s _m difficulties1 especially ) respect to ! %utt] & %utt] op5+4 ,a hemisph]e is nei!r easy to frame nor to cov]1 & ! curv$ slid+ %utt] is especially tr\blesome4 ,illu/ration3 ,,fig4 #agi4--,! ,orig9al 8,romsey0 ,obs]vatory4 ,h5ce = small obs]vatories o!r =ms ( revolv+ ro( are desirable1 & q ! easie/ & *eape/ contrivance is t embodi$ 9 ! 8,romsey0 type ( obs]vatory1 devis$ half a c5tury ago by t accompli%$ amateur ! ,rev4 ,e4 ,l4 ,b]?on1 vic> ( ,romsey4 ,! feature ( 8 con/ruction is an unsymmetrical peak 9 ! revolv+ ro( :i* p]mits ! ord9>y %utt] to be replac$ by a h+$ %utt] l ! skylight 9 a ro(1 expos+ ! sky beyond ! z5i? :5 op51 & clos+ d[n ov] a coam+ to =m a wat] tight jo9t4 ,b]?on's orig9al description ( 8 obs]vatory1 :i* accommodat$ a #i in* reflector1 may be f\nd 9 ,vol4 #ad ( ! ,engli% ,me*anic & ,_w ( ,sci5ce :5ce ,fig4 #agi is tak54 ,9 ?is plate ,fig4 #a %[s ! complete elevation & ,fig4 #b ! gr\nd plan1 ea* to a scale ( a eigh? ( an in* to ! foot4 ,9 ! plan1 ,a1,a1 are ! ma9 joi/s1 ,p ! pi] = ! telescope1 ,t t = ! transit1 & ,c ! clock4 ,figs4 #c1 #d1 & #e are ( details4 ,9 ! la/ nam$ ,a is a raft]1 b ! base r+1 c ! plate1 d "o ( ! sa% roll]s c>ry+ ! ro(1 & e a lat]al guide roll] hold+ ! ro( 9 place4 ,! /ructure c readily be built )\t ! transit %elt]1 & 9 fact n[-a-days mo/ obs]v]s f9d x easi] to pick up _! "t by wireless4 ,! ma9 be>+ r+ is cut out ( ord9>y in* bo>d1 9 t5 or a doz51 or m1 sections accord+ to conv5i5ce1 done 9 duplicate1 jo9ts lapp+1 & put v firmly toge!r ) screws set up h>d4 ,sometimes #c lay]s are ?us us$4 ,! ro( 9 ! orig9al 8,romsey0 obs]vatory 0 ( pa9t$ canvas1 b rubb]oid or galvaniz$ iron l9$ ) ro(+ pap] answ]s well4 ,! %utt] c be made s+le or d\ble 9 wid?1 & c\nt]balanc$ if necess>y4 ,! fram+ may be ( po/s set 9 ! gr\nd z "h %[n1 or ) sills re/+ on a f\ndation1 & ! walls ( any con/ruction--mat*$ bo>ds ( any k9d1 cem5t on wire la?1 holl[ tile1 or concrete blocks4 ,*amb]s' ,h&book ( ,a/ronomy ,vol4 ,,ii conta9s q complete details ( ! 8,romsey0 type ( obs]vatory & is easi] to get at ?an ! orig9al description4 ,a v neat adaptation ( ! plan is %[n 9 ,fig4 #ahj1 ( :i* a description may be f\nd 9 ,popul> ,a/ronomy #bh1 #ahc4 ,?is obs]vatory 0 ab\t #i feet 9 diamet]1 to h\se a #d in* telescope1 & 0 provid$ ) a r\gh concrete f\ndation on :i* 0 built a circul> wall #f feet high ( holl[ glaz$ tile1 well levell$ on top4 ,to ?is 0 secur$ a r+ plate built up 9 two lay]s1 c>ry+ two circles ( wood5 /rips ) a c\ple ( in*es space betwe5 !m = a runway4 ,9 ?is ran #f two-in* truck ca/ors secur$ to a simil> r+ plate on :i* 0 built up ! frame ( ! 8dome0 arrang$ z %[n4 ,altoge!r a v neat & workmanlike affair1 9 ?is case built l>gely by ! own] b p]mitt+ con/ruction at v small exp5se almo/ any:]e4 ,ano!r int]e/+ modification ( ! same g5]al plan 9 ! same volume j cit$ is %[n 9 ,fig4 #aha4 ,?is is also = a #d in* refractor & ! dome prop] is b #h feet #d in*es 9 diamet]4 ,l ! prec$+ /ructure ! f\ndation is ( concrete b ! walls are fram$ 9 spruce & %ea!d 9 mat*$ bo>ds ) a 8beav]-bo>d0 l9+4 ,illu/ration3 ,,fig4 #ahj4--;,a ,m ,sub/antial 8,romsey0 ,type4 ,! r+ plate is ?ree-ply1 #ab sections to ! lay]1 & its mate on :i* ! dome is assembl$ is simil>ly =m$1 ?\gh left ) ! figure ( a dodecagon to mat* ! dome4 ,! weight is c>ri$ on f\r rubb] tir$ truck roll]s1 & !re are lat]al guide roll]s on ! plan ( ^? 9 ,fig4 #agi4 ,! dome itself h[ev]1 is :olly ( galvaniz$ iron1 9 #ab gores jo9$ ) /&+ seams1 turn$1 rivet$1 & sold]$4 ,!re is a %ort %utt] at ! z5i? slid+ back ^u a frame1 :ile ! ma9 %utt] is remov$ f ! outside by h&les4 ,illu/ration3 ,,fig4 #aha4--;,detail ( ,light ,metal ,dome = ,small ,obs]vatory4 ,obs]vatories ( ! ,romsey or alli$ types c be erect$ at v mod]ate co/1 v>y+ consid]ably f place to place1 b runn+ at pres5t say f #bjj to #fjj1 & big en\gh to %elt] refractors ( #d to #f in*es ap]ture4 ,! revolv+ ro(s w range f #i to #ab feet 9 diamet]4 ,if reflectors are 9 use1 ^? ( ab\t d\ble ^! ap]tures c be accommodat$ s9ce ! reflector is ord9>ily mu* ! %ort] = equal ap]ture4 ,! slid+ ro(1 n to say ! slid+ %elt]1 =ms ( h\s+ co/ some:at less1 dep5d+ on ! con/ruction adopt$4 ,go+ to brick may d\ble ! figures quot$1 b su* solidity is g5]ally q ne$less1 ?\gh x is highly desirable t ! cov] ( a valuable in/rum5t %\ld be fire-pro( & n easily brok5 op54 ,! /eal+ ( objectives & accessories is n unkn[n1 & v&alism is a risk n to be =gott54 ,b to ev5 ! matt] up1 h\s+ a telescope is r an easy ?+ to accompli%1 & z a matt] ( fact = ! price ( a v mode/ motor c> "o c bo? buy & h\se an in/rum5t big en\gh to be ( g5u9e s]vice4 ,,*apt] ,,xi ,,see+ ,,& ,,magnification ,few ?+s are m g5]ally disappo9t+ ?an "o's fir/ glimpse ( ! ,heav5s ?r\gh a telescope4 ,! novice is f$ up ) maps ( ,m>s z a great disc full ( intricate m>k+s1 & he g5]ally sees a little wriggl+ ball ( light ) no m visible detail ?an an egg4 ,x is almo/ impossible to believe t1 at a fair opposition1 ,m>s "u ! p[] ( ev5 ! smalle/ a/ronomical telescope really looks z big z ! full moon4 ,aga91 "o looks at a d\ble /> to see n two brilliant little discs respl5d5t 9 color1 b an indet]m9ate flick] void ( %ape & hue4 ,! fact is1 t mo/ ( ! "t ov] mo/ ( ! _w see+ conditions are bad1 s t ! telescope does n h a fair *ance1 & on ! :ole ! bigg] ! telescope ! worse ! *ance4 ,"o fam\s ,engli% a/ronom]1 possess$ ( a f9e refractor t w\ld be reckon$ l>ge ev5 n[-a-days1 av]r$ t he _h se5 b "o fir/ class night 9 fifte5 ye>s pa/4 ,! case is really mu* less bad ?an ?is implies1 = ev5 9 r unfavorable climates _m a night1 at "s o'clock or o!r1 w furni% an h\r or two ( pretty good see+1 :ile n[ & !n1 )\t any app>5t connection ) ! previ\s /ate ( ! wea!r1 a night w turn up :5 ! pictures 9 ! popul> a/ronomies come true1 ! />s %r9k to /eady po9ts set 9 clean cut r+s1 & no available p[] seems too high4 ,"o c get a good idea ( ! true inw>dness ( bad see+ by try+ to read a newspap] ?r\gh an op]a glass across a hot /ove4 ,if ! actual movem5ts 9 ! atmosph]e c\ld be made visible !y w\ld pres5t a /range sc5e ( turbul5ce--ru%+ curr5ts tak+ devi\s c\rses up & ar\nd ob/acles1 sl[ly mov+ :irlpools1 upw>d slants su* z gulls hug on ! qu>t] ( a l9]1 great d[nw>d ru%es dread$ by ! aviator1 & ov] x all incessant ripples 9 e direction4 ,& movem5ts ( air are usually associat$ ) *anges ( temp]ature1 z ov] ! /ove1 v>y+ ! refraction & contort+ ! rays t come f a di/ant /> until ! image is q ru9$4 ,! condition = excell5ce ( def9ition is t ! atmosph]e ?r\gh :i* we see %all be homog5e\s1 :atev] its temp]ature1 humidity1 or g5]al tr5d ( movem5t4 ,irregul> refraction is ! ?+ to be fe>$1 p>ticul>ly if ! v>iations are sudd5 & frequ5t4 ,h5ce ! common tr\bles ne> ! gr\nd & ab\t build+s1 especially :]e !re are ro(s & *imneys to radiate heat--ev5 9 & ab\t an obs]vatory dome4 ,pr(essor ,w4 ,h4 ,pick]+1 :o has _h a v>i$ exp]i5ce 9 climatic idiosyncrasies1 gives ! ,nor!rn ,atlantic seabo>d ! bad prem95ce ( hav+ ! wor/ obs]v+ conditions ( any region )9 8 k4 ,! au?or *e]fully concurs1 yet n[ & !n1 q (t5 aft] midnight1 ! air /eadies &1 if ! o!r conditions are good1 def9ition becomes fairly respectable1 sometimes ev5 excell5t4 ,temp]ature & humidity z su*1 seem to make little diff]5ce1 & a /eady breeze unless x %akes ! in/rum5t is relatively h>mless4 ,h5ce we f9d ! mo/ admirable def9ition 9 situations z widely diff]5t z ! ,h>v>d /ation at ,m&eville1 ,jamaica2 ,flag/aff1 ,arizona #gjjj feet up & sn[ b\nd 9 w9t]2 ,italy1 & ,egypt4 ,! fir/ nam$ is w>m & ) v heavy ra9fall & dew1 ! second dry ) r l>ge seasonal v>iation ( temp]ature1 & ! o!rs temp]ate & hot respectively4 ,p]haps ! mo/ /rik+ evid5ce ( ! importance ( uni=mity 0 not$ by ,ev]%$ at an ,indian /ation :]e good conditions imm$iately foll[$ ! flood+ ( ! rice fields ) its t5d5cy to /abilize ! temp]ature4 ,m\nta9 /ations may be good z at ,flag/aff1 ,mt4 ,hamilton1 or ,mt4 ,wilson1 or v bad z ,pike's ,peak prov$ to be1 probably ow+ to local conditions4 ,9 fact mu* ( ! tr\ble comes f ne>by s\rces1 atmosph]ic waves & ripples r ?an l>ge movem5ts1 ripples inde$ (t5 small comp>$ ) ! ap]ture ( ! telescope & sometimes 9 or n f> outside ( ! tube itself4 ,aside f ^! difficulties1 !re are /ill o!rs :i* h to d ) ! transp>5cy ( ! atmosph]e ) respect to its susp5d$ matt]4 ,?is does n affect ! def9ition z su*1 b x cuts d[n ! light to a degree t may int]f]e s]i\sly ) ! obs]vation ( fa9t />s & nebul4 ,! smoke ne> a city aggravates ! situation1 b 9 p>ticul> x dep5ds on g5]al wea!r conditions :i* may be p]si/5t or m]ely tempor>y4 ,(t5 see+ conditions may be admirable save = ?is lack ( transp>5cy 9 ! atmosph]e1 s t /udy ( ! moon1 ( planet>y m>k+s & ev5 ( d\ble />s1 n too fa9t1 may g on q unimp$$4 ,! actual loss ( light may rea* h[ev] a magnitude or m1 :ile ! sky is q cl\dless & )\t a trace ( fog or noticeable haz9ess by day4 ,!re h be5 a good _m nights ! pa/ ye> 7#aiba7 :5 ,alcor 7#hj ,urs ,majoris7 ! t9y neighbor ( ,miz>1 v ne>ly ( ! #d? magnitude1 has be5 b>ely or n at all visible :ile ! see+ o!rwise 0 respectably good4 ,ord9>ily />s ( #fm %\ld be visible 9 a really cle> night1 & 9 a brilliant w9t] sky 9 ! temp]ate zones1 or 9 ! cle> air ( ! tropics1 a good _m eyes w d bett] ?an ?is1 rea*+ #fm4#e or ev5 #gm1 occasionally a bit m4 ,! relation ( air waves & su* l irregul>ities to telescopic vision 0 r ?or\ghly inve/igat$ by ,d\glass m ?an tw5ty ye>s ago 7,pop4 ,a/4 #f1 #aic7 ) v int]e/+ results4 ,9 sub/ance1 f c>eful obs]vation ) telescopes f #d in*es up to #bd in*es ap]ture1 he f\nd t ! real tr\ble came f :at "o may call ripples1 di/urbances f say #d in*es wave l5g? d[n to in* or less4 ,long waves are r>e & relatively unimportant s9ce _! g5]al effect is to cause %ift+ ( ! image z a :ole r ?an ! de/ruction ( detail :i* accompanies ! %ort] waves4 ,?is rippl+ ( ! air is probably associat$ ) ! contact displacem5ts 9 air curr5ts su* z on a big scale become visible 9 cl\d =ms4 ,cle>ly ripples1 m>k$ z !y are by diff]5ce ( refraction1 locat$ 9 front ( a telescope objective1 produce diff]5t focal l5g?s = diff]5t p>ts ( ! objective & r5d] a clean & /able image q out ( ! "q4 ,9 r\gh t]ms ,d\glass f\nd t waves ( great] l5g? ?an half ! ap]ture did n mat]ially det]iorate ! image1 al?\gh !y did %ift x z a :ole1 :ile waves ( l5g? less ?an "o ?ird ! ap]ture did s]i\s mis*ief to ! def9ition1 ! great] z ! ripples 7 %ort]1 & ! image itself m m9ute 9 dim5sion or detail4 ,h5ce !re are times :5 decreas+ ! ap]ture ( an objective by a /op improves ! see+ consid]ably by increas+ ! relative l5g? ( ! air waves4 ,su* is 9 fact f\nd to be ! case 9 practical obs]v+1 especially :5 ! see+ ) a l>ge ap]ture is decid$ly poor4 ,9 o!r words "o may (t5 ga9 m by increas$ /ead9ess ?an he loses by less5$ 8resolv+ p[]10 ! result dep5d+ some:at on ! class ( obs]vation :i* *ances to be "u way4 ,& ?is br+s u1 willy-nilly1 to ! some:at ab/ruse matt] ( resolv+ p[]1 dep5d+ fundam5tally ^u ! !ory ( diffraction ( light1 & practically ^u a good _m o!r ?+s t modify ! "* ( ! diffraction patt]n1 or ! actual visibility ( its elem5ts4 ,:5 light %9es ?r\gh a hole or a slit ! light waves are b5t at ! m>g9s & ! sev]al sets1 ev5tually ov]lapp+1 int]f]e ) ea* o!r s z to produce a patt]n ( bright & d>k elem5ts dep5d+ on ! size & %ape ( ! ap]ture1 & di/ribut$ ab\t a c5tral bright image ( t ap]ture4 ,"o gets ! effect well 9 look+ ?r\gh an op5 umbrella at a di/ant /reet light4 ,! out] images ( ! patt]n are fa9t] & fa9t] z !y get away f ! c5tral image4 ,)\t burd5+ ! read] = ! mom5t ) details to be consid]$ pres5tly1 ! effect 9 telescopic vision is t a /> ( real angul> diamet] q negligible1 p]haps #j4jja ( arc1 is repres5t$ by an image "u p]fect conditions l ,fig4 #aed1 ( q p]ceptible diamet]1 surr\nd$ by a sy/em ( r+s1 fa9t b cle>-cut1 dim9i%+ 9 int5sity outw>ds4 ,:5 ! see+ is bad no r+s are visible & ! c5tral disc is a m]e bright blur sev]al times l>g] ?an x ought to be4 ,! v>y+ appe>ance ( ! /> image is a v good index ( ! quality ( ! see+1 s t1 hav+ a cle> indication ( ?is appe>ance1 two a/ronom]s 9 diff]5t p>ts ( ! _w c ga9 a def9ite idea ( ea* o!r's relative see+ conditions4 ,to ?is end a /&>d scale ( see+1 due l>gely to ! ef=ts ( ,pr(4 ,w4 ,h4 ,pick]+1 has come into r common use4 7,h4 ,a4 #fa #bi74 ,x is z foll[s1 bas$ on obs]vations ) a #e in* telescope4 ,,/&>d ,,scale ,,( ,,see+ #a4 ,image usually ab\t twice ! diamet] ( ! ?ird r+4 #b4 ,image occasionally twice ! diamet] ( ! ?ird r+4 #c4 ,image ( ab\t ! same diamet] z ! ?ird r+1 & bright] at ! c5tre4 #d4 ,disc (t5 visible1 arcs 7( r+s7 sometimes se5 on bright] />s4 #e4 ,disc always visible1 arcs frequ5tly se5 on bright] />s4 #f4 ,disc always visible1 %ort arcs con/antly se54 #g4 ,disc sometimes %>ply def9$1 7a7 long arcs4 7b7 ,r+s complete4 #h4 ,disc always %>ply def9$1 7a7 long arcs4 7b7 ,r+s complete all 9 motion4 #i4 ,r+s1 7a7 ,inn] r+ /ation>y1 7b7 ,out] r+s mom5t>ily /ation>y4 #aj4 ,r+s all /ation>y1 7a7 ,detail betwe5 ! r+s sometimes mov+4 7b7 ,no detail betwe5 ! r+s4 ,! fir/ ?ree scale numb]s indicate v bad see+2 ! next two1 poor2 ! next two1 good2 & ! la/ ?ree1 excell5t4 ,"o c get "s idea ( ! extreme badness ( scale divisions #a1 #b1 #c1 9 realiz+ t ! ?ird bright diffraction r+ is ne>ly #d times ! diamet] ( ! prop] />-disc4 ,x mu/ be not$ t = a giv5 condition ( atmosph]e ! see+ ) a l>ge in/rum5t ranks l[] on ! scale ?an ) a small "o1 s9ce z already expla9$ ! usual air ripples are ( dim5sions t might affect a #e in* ap]ture imp]ceptibly & a #ae in* ap]ture v s]i\sly4 ,d\glass 7loc4 cit47 made a c>eful comp>ison ( see+ conditions = ap]tures up to #bd in*es & f\nd a sy/ematic diff]5ce ( #b or #c scale numb]s betwe5 #d or #f in*es ap]ture1 & #ah or #bd in*es4 ,) ! smalle/ ap]ture ! image %[$ m]ely bodily motion due to air waves t produc$ s]i\s injury to ! image 9 ! l>ge ap]tures1 z might be expect$4 ,!re is likewise a great diff]5ce 9 ! av]age quality ( see+ z betwe5 />s ne> ! z5i? & ^? t[>d ! horizon1 due aga9 to ! great] opportunity = atmosph]ic di/urbances 9 ! latt] case4 ,pick]+'s exp]im5ts 7loc4 cit47 %[ a diff]5ce ( ne>ly #c scale divisions betwe5 say #bj & #gj elevation4 ,?is diff]5ce1 :i* is important1 is well %[n 9 ,fig4 #ahb1 tak5 f 8 report4 ,! ?ree l[] curves 7 f ,cambridge obs]vations1 ! o!rs obta9$ at v>i\s ,jamaica /ations4 ,!y cle>ly %[ ! sy/ematic regional diff]5ces1 z well z ! rapid fall+ (f 9 def9ition bel[ altitude #dj1 :i* po9ts ! importance ( mak+ provision = com=table obs]v+ above ?is altitude4 ,illu/ration3 ,,fig4 #ahb4--,v>iation ( ,see+ ) ,altitude4 ,illu/ration3 ,,fig4 #ahc4--;,airy's ,diffraction ,patt]n4 ,! relation ( ! diffraction patt]n z disclos$ 9 ! mom5ts ( be/ see+ to its !oretical =m is a v int]e/+ "o4 ,! diffraction ?r\gh a !oretically p]fect objective 0 work$ out _m ye>s ago by ,sir ,george ,airy :o calculat$ ! exact di/ribution ( ! light 9 ! c5tral disc & ! surr\nd+ r+s4 ,?is is %[n f ! c5tre outw>ds 9 ,fig4 #ahc1 9 :i* ! ord9ates ( ! curve repres5t relative int5sities :ile ! absciss repres5t to an arbitr>y scale ! di/ances f ! axis4 ,x w be at once notic$ t ! /> image1 brilliant at its c5tre1 s9ks1 fir/ rapidly & !n m sl[ly1 to a m9imum & !n v gradually rises to ! maximum ( ! fir/ bright r+1 !n z sl[ly s9ks aga9 to increase = ! second r+ & s on4 ,illu/ration3 ,,fig4 #ahd4--;,diffraction ,solid = a ,/>4 ,= unity brightness 9 ! c5tre ( ! /> disc ! maximum brightness ( ! fir/ r+ is #j4jag1 ( ! second #j4jjd & ! ?ird #j4jjaf4 ,! r+s are equidi/ant & ! /> disc has a radius sub/antially equal to ! di/ance betwe5 r+s4 ,"o's vision does n foll[ d[n to z]o ! int5sities ( ! r+s or ( ! m>g9 ( ! disc1 s t ! latt] has an app>5t diamet] mat]ially less ?an ! diamet] to ! fir/ diffraction m9imum1 & ! r+s !mselves look %>p] & ?9n] ?an ! figure w\ld %[1 ev5 7 ! horizontal scale mu* dim9i%$4 ,! eye does n desc5d 9 ! pres5ce ( bright areas to its f9al ?re%old ( p]ception4 ,"o ga9s a some:at vivid idea ( ! situation by pass+ to ?ree dim5sions z 9 ,fig4 #ahd1 ! 8diffraction solid0 = a />1 a conception due to ,m4 ,&r 7,mem4 de l',acad4 de ,lyon #cj1 #di74 ,"h ! solid repres5ts 9 volume ! :ole light receiv$ & ! height tak5 at any po9t1 ! int5sity at t po9t4 ,a cross section at any po9t %[s ! app>5t diamet] ( ! disc1 its di/ance to ! apex ! rema9+ int5sity1 & ! volume above ! section ! rema9+ total light4 ,sub/antially #he ( ! total light belongs to ! c5tral cone1 = ! !oretical di/ribution4 ,grant+ t ! eye c di/+ui% f ! backgr\nd ( ! sky1 9 pres5ce ( a bright po9t1 only light above a c]ta9 int5sity1 "o readily sees :y ! discs ( fa9t />s look small1 & :y %ade glasses are sometimes useful 9 wip+ out ! m>g9al int5sities ( ! solid4 ,!re are physiological factors t alt] pr(\ndly ! appe>ance ( ! actual /> image1 despite ! fact t ! !oretical diffraction image = ! ap]ture is indep5d5t ( ! />'s magnitude4 ,practically ! g5]al r$uction ( illum9ation 9 ! fa9t] />s cuts d[n ! app>5t diamet]s ( _! discs1 & r$uces ! numb] ( r+s visible aga9/ ! backgr\nd ( ! sky4 ,! scale ( ! diffraction sy/em det]m9es ! resolv+ p[] ( ! telescope4 ,?is scale is giv5 9 ,airy's orig9al pap] 7,cambr4 ,phil4 ,trans4 #ahcd p4 #bhc71 f :i* ! angle to any maximum or m9imum 9 ! r+ sy/em is def9$ by s9 n_/,r 9 :i* is num]ically ! wave l5g? ( any light consid]$ & ,r is ! radius ( ! objective4 ,we !re=e see t ! r+ sy/em v>ies 9 dim5sion inv]sely ) ! ap]ture ( ! objective & directly ) ! wave l5g? consid]$4 ,h5ce ! bigg] ! objective ! small] ! disc & its surr\nd+ r+ sy/em2 & ! great] ! wave l5g?1 i4e4 ! r$d] ! light1 ! bigg] ! diffraction sy/em4 ,evid5tly !re %\ld be color 9 ! r+s b x v seldom %[s on acc\nt ( ! fa9tness ( ! illum9ation4 ,n[ ! factor n is = ! fir/ d>k r+ #j4fa1 & = ! fir/ bright r+ #j4ha1 z comput$ f ,airy's g5]al !ory1 & !re=e if we reckon t two />s w be se5 z sep>ate :5 ! c5tral disc ( "o falls on ! fir/ d>k r+ ( ! o!r ! angul> di/ance w be ,s9 #j4fa _/,r &1 tak+ at ! brighte/ "p ( ! spectrum i4e41 ab\t #efj 1 9 ! yell[ gre51 ) tak5 = s9 1 we c compute ?is assum$ sep>at+ p[] = any ap]ture4 ,?us #efj be+ v ne>ly #a_/de1ejj in*1 & assum+ a #e in* telescope1 ! in/rum5t %\ld on ?is basis %[ z d\ble two />s ^: c5tres are sep>at$ by #a4a ( arc4 ,9 actual fact "o c d some:at bett] ?an ?is1 %[+ t ! visible diamet] ( ! c5tral disc is 9 effect less ?an ! diamet] indicat$ by ! diffraction patt]n1 ow+ to ! reasons already /at$4 ,evid5tly ! brightness ( ! /> is a factor 9 ! situation s9ce if v bright ! disc ga9s app>5t size1 & :5 v fa9t !re is suffici5t difficulty 9 see+ "o />1 let alone a pair4 ,! mo/ ?or\gh inve/igation ( ?is matt] ( resolv+ p[] 0 made by ! ,rev4 ,w4 ,r4 ,dawes _m ye>s ago 7,mem4 ,r4,a4,s4 #ce1 #aeh74 ,8 /udy includ$ ye>s ( obs]vation ) telescopes ( diff]5t sizes1 & 8 f9al result 0 to e/abli% :at has s9ce be5 kn[n z 8,dawes' ,limit40 ,to sum up ,dawes' results he e/abli%$ ! fact t on ! av]age a "o in* ap]ture w\ld enable "o to sep>ate two #f? magnitude />s ! c5t]s ( :i* 7 sep>at$ by #d4ef4 ,or1 to g5]alize f ?is basis1 ! sep>at+ p[] ( any telescope is = v ne>ly equal />s1 mod]ately bright1 #d4ef_/;,a :]e ,a is ! ap]ture ( ! telescope 9 in*es4 ,_m ye>s ( exp]i5ce h emphasiz$ ! usefulness ( ?is approximate rule1 b t x is only approximate mu/ be c&idly admitt$4 ,x is a limit decid$ly "u t j assign$ on ! basis ( ! !ory ( diffraction = ! c5tral bright wave-l5g?s ( ! spectrum4 ,attempts h be5 made to squ>e ! two figures by assum+ 9 ! diffraction !ory a wave l5g? ( #a_/ee1jjj in*1 b ?is figure corresponds to a po9t well up into ! blue1 ( s l[ lum9osity t x is ( no importance :atev] 9 ! visual use ( a telescope4 ,! fact is t ! visibility ( two neighbor+ bright po9ts z di/9ct1 dep5ds on a complex ( physical & physiological factors1 ! exact relations ( :i* h nev] be5 unravell$4 ,to />t ) we h ! pr9ciples ( diffraction z j expla9$1 :i* def9e ! relation ( ! /ell> disc to ! c5t] ( ! fir/ d>k r+1 b we "k t "u no circum/ances c "o see ! disc out to ?is limit1 s9ce vision fails to take cognizance ( ! fa9t rim ( ! image4 ,! app>5t diamet] ( ! diffraction solid !re=e corresponds to a section tak5 "s di/ance above ! base1 ! exact po9t dep5d+ on ! s5sitiv5ess ( ! p>ticul> obs]v]'s eye1 ! actual brilliancy ( ! c5t] ( ! disc1 & ! correspond+ factors = ! neighbor+ />4 ,illu/ration3 ,,fig4 #ahe4--;,diffraction ,solid = a ,disc4 ,"u favorable circum/ances "o w\ld n g f> amiss 9 tak+ ! visible diamet] ( ! disc at ab\t half t reckon$ to ! c5t] ( ! fir/ d>k r+4 ,?is figure 9 fact corresponds to :at has be5 %[n to be )9 ! grasp ( a good obs]v] "u favorable conditions1 z we %all pres5tly see4 ,on ! o!r h&1 if ! />s are decid$ly bright !re is increase ( app>5t diamet] ( ! disc due to ! ph5om5on kn[n z irradiation1 ! spread+ ( light ab\t its true image on ! ret9a :i* corresponds q closely to ! halation produc$ by a bright spot on a photographic plate4 ,if1 on ! contr>y1 ! />s are v fa9t ! total am\nt ( light available is n suffici5t to make contra/ ov] & above ! backgr\nd suffici5t to disclose ! two po9ts z sep>ate1 :ile if ! pair is v unequal ! bright] "o w produce suffici5t gl>e to q ov]-p[] ! light f ! small] "o s t ! eye misses x entirely4 ,a /rik+ case ( ?is is f\nd 9 ! companion to ,sirius1 an extremely difficult object = ord9>y telescopes al?\gh ! di/ance to ! companion is ab\t #aj4f & its magnitude is #h4d1 mak+ a sup]latively easy d\ble = ! v smalle/ telescope save = ! ov]p[]+ effect ( ! light ( ! l>ge />4 ,ano!r notori\sly difficult object = small telescopes is ,cygni1 a beautiful d\ble ( :i* ! small] compon5t falls unpleasantly ne> ! fir/ diffraction maximum ( ! prim>y 9 :i* x is apt to be lo/4 8,dawes' ,limit0 is !re=e subject to _m qualify+ factors4 ,lewis1 9 ! pap]s already ref]r$ to 7,obs4 #cg1 #cgh7 did an admirable piece ( inve/igation 9 go+ ?r\gh ! d\ble /> "w ( ab\t two score tra9$ obs]v]s work+ ) telescopes all ! way f #d in*es to #cf in*es ap]ture4 ,f ?is accumulation ( data sev]al /rik+ facts /& out4 ,fir/ !re is great diff]5ce betwe5 individual obs]v]s work+ ) telescopes ( simil> ap]ture z respects _! agreem5t ) 8,dawes' ,limit10 %[+ ! effect ( v>iation 9 ! physiological factors z well z in/rum5tal ones4 ,second1 !re is also a v l>ge diff]5ce betwe5 ! facility ( obs]v+ equal bright pairs & equal fa9t pairs1 or unequal pairs ( any k9d1 aga9 emphasiz+ ! physiological z well z ! physical factors4 ,f9ally1 !re is mo/ unmi/akable diff]5ce betwe5 small & l>ge ap]tures 9 _! capacity to "w up to or pa/ ! /&>d ( 8,dawes',limit40 ,! small] telescopes are cle>ly ! m effici5t z w\ld be anticipat$ f ! facts j po9t$ out reg>d+ ! diff]5t effect ( ! ord9>y & inescapable atmosph]ic waves on small & l>ge in/rum5ts4 ,! big telescopes are unque/ionably z good optically speak+ z ! small ones b "u ! ord9>y work+ conditions1 ev5 z good z ^? a d\ble /> obs]v] seeks1 ! small] ap]ture by reason ( less di/urbance f atmosph]ic factors does relatively mu* ! bett] "w1 h[ev] good ! big in/rum5t may be "u exceptional conditions4 ,?is is admirably %[n by ! discussion ( ! beautiful "w ( ! late ,mr4 ,burnham1 ?an :om probably no bett] obs]v] ( d\bles has be5 kn[n to a/ronomy4 ,8 records ( discov]y ) telescopes ( #f1 #i4d1 #ab1 #ah & #cf in*es %[ ! relative ease ( work+ to ! !oretical limit ) in/rum5ts n s]i\sly upset by ord9>y atmosph]ic waves4 ,) ! #f in* ap]ture ,burnham rea*$ 9 ! av]age #j4ec ( ,dawes' limit1 q ne> ! r\gh figure j sugge/$1 & he also fell well inside ,dawes' limit ) ! #i4d in* in/rum5t4 ,) none ( ! o!rs did he rea* x & 9 fact fell %ort ( x by #ae to #fj4 ,all obs]vations be+ by ! same notably skill$ obs]v] & repres5t+ discov]ies ( d\bles1 s t no aid c\ld h be5 ga9$ by famili>ity1 ! issue becomes exce$+ly pla9 t size ) all its advantages 9 resolv+ p[] br+s s]i\s c\nt]vail+ limitations due to atmosph]e4 ,b a l>ge ap]ture has besides its possible sep>at+ p[] "o advantage t c n be disc\nt$ 9 8light grasp10 ! p[] ( disc]n+ fa9t objects4 ,?is is ! ?+ 9 :i* a small telescope necess>ily fails4 ,! 8light grasp0 ( ! telescope obvi\sly dep5ds *iefly on ! area ( ! objective1 & visually only 9 v m9or degree on ! absorption ( ! ?ick] glass 9 ! case ( a l>ge l5s4 ,accord+ to ! conv5tional scale ( /> magnitudes z n[ 9 univ]sal use1 />s are classifi$ 9 magnitudes :i* diff] f ea* o!r by a light ratio ( #b4eab4 a numb] ! log>i?m ( :i* is #j4d1 a relation sugge/$ by ,pogson "s =ty ye>s ago4 ,a second magnitude /> !re=e gives only ab\t #dj ( ! light ( a fir/ magnitude />1 :ile a ?ird magnitude /> gives aga9 a little less ?an #dj ( ! light ( a second magnitude /> & s on4 ,b d\bl+ ! ap]ture ( a telescope increases ! available area ( ! objective f\r times & s on1 ! 8light grasp0 be+ 9 proportion to ! squ>e ( ! ap]ture4 ,?us a #aj in* objective w take 9 & deliv] ne>ly #ajj times z mu* light z w\ld a #a in* ap]ture4 ,if "o foll[s ,pogson's scale d[n ! l9e he w f9d t ?is corresponds exactly to #e /ell> magnitudes1 s t if a #a in* ap]ture discloses1 z x readily does1 a #i? magnitude />1 a #aj in* ap]ture %\ld disclose a #ad? magnitude />4 ,su* is sub/antially 9 fact ! case1 & "o c !re=e readily tabulate ! m9imum visible = an ap]ture j z he c tabulate ! approximate resolv+ p[] by ref]5ce to ,dawes' limit4 ,fig4 #ahf %[s 9 graphic =m bo? ^! relations = ready ref]5ce1 ! v>iation ( resolv+ p[] ) ap]ture1 & t ( 8light grasp10 reckon$ 9 /ell> magnitudes4 ,x is h>dly necess>y to /ate t consid]able individual & obs]vational diff]5ces w be f\nd 9 ea* ( ^! cases1 9 ! latt] am\nt+ to n less ?an #j4e to #a4j magnitude ei!r way4 ,! scale is bas$ on ! #i? magnitude /> j be+ visible ) #a in* ap]ture1 :]eas 9 fact "u v>y+ conditions & ) v>i\s obs]v]s ! range may be f ! #h? to #aj? magnitude4 ,all ^! ?+s1 h[ev] conv5i5t1 mu/ be tak5 m]ely at _! true value z good work+ approximations4 ,ev5 ! diffraction !ory c be tak5 only z an approximation s9ce no optical surface is absolutely p]fect & 9 ! ord9>y refract+ telescope !re is a necess>y residual *romatic ab]ration beside :atev] may rema9 ( sph]ical errors4 ,illu/ration3 ,,fig4 #ahf4--,light-grasp & ,resolv+ ,p[]4 ,x is a fact !re=e1 z has be5 %[n by ,conrady 7,m4,n4 #gi #ege7 foll[+ up a di/+ui%$ inve/igation by ,"l ,rayleigh 7,sci4 ,pap]s #a #dae71 t a c]ta9 small am\nt ( ab]ration c be tol]at$ )\t mat]ial effect on ! def9ition1 :i* is v =tunate consid]+ t ! second>y spectrum repres5ts ab]rations ( ab\t #a_/b1jjj ( ! focal l5g?1 z we h already se54 ,! *ief effect ( ?is1 z ( v slight sph]ical ab]ration1 is m]ely to r$uce ! maximum int5sity ( ! c5tral disc ( ! diffraction patt]n & to produce a fa9t haze ab\t x :i* slightly illum9ates ! diffraction m9ima4 ,! visible diamet] ( ! disc & ! relative di/ribution ( int5sity 9 x is n h[ev] mat]ially *ang$ s t ! ma9 effect is a little loss & scatt]+ ( light4 ,) l>g] ab]rations ^! effects are m s]i\s b :]e ! *ange 9 l5g? ( optical pa? betwe5 ! ray proce$+ ?r\gh ! c5t] ( ! objective & t f ! m>g9 does n exce$ ! injury to ! def9ition is sub/antially negligible & virtually disappe>s :5 ! image is focuss$ = ! be/ def9ition1 ! loss ( maximum int5sity 9 ! /> disc am\nt+ to less ?an #bj4 ,ev5 twice ?is error is n a v s]i\s matt] & c be = ! mo/ "p comp5sat$ by a m9ute *ange ( focus z is v beautifully %[n 9 a pap] by ,buxton7,m4 ,n4 #ha1 #edg71 :i* %\ld be consult$ = detail ( ! v>iations to be effect$4 ,conrady f9ds a giv5 *ange dp 9 ! diff]5ce 9 l5g?s ( ! optical pa?s1 relat$ to ! equival5t l9e> *ange ( focus1 df1 z foll[s3-- df #h;dp7f_/,a7 ,a be+ ! ap]ture & f ! focal l5g?1 :i* indicates = telescopes ( ord9>y focal ratio a tol]ance ( ! ord] ( #j4ja in* be=e gett+ outside ! limit = v>iation ( pa?4 ,= in/rum5ts ( great] relative ap]ture ! precision ( focus & 9 g5]al ! requirem5ts = less5$ ab]ration are f> m sev]e1 proportional 9 fact to ! squ>e ( ?is ap]ture4 ,h5ce ! sev]e dem&s on a reflector = exact figure4 ,an in/rum5t work+ at ,f_/#e or ,f_/#f is extremely s5sitive to focus & dem&s great precision ( figure to fall )9 p]missible values1 say to 1 = dp4 ,fur!r1 ) a giv5 value ( dp & ! relation e/abli%$ by ! *romatic ab]ration1 i4e41 ab\t f_/#bjjj1 a relation is also det]m9$ betwe5 f & ,a1 requir$ to br+ ! ab]ration )9 limits4 ,! equation ?us f\nd is f #b4h;,a ,?is practically am\nts to ! common ,f_/#ae ratio = an ap]ture ( approximately #e in*es4 ,= small] ap]tures a great] ratio c be well us$1 = l>g]1 a relatively long] focus is indicat$1 ! p5alty be+ light spread into a halo ov] ! diffraction image & r$uc+ fa9t contra/s some:at s]i\sly4 ,?is is "o ( ! factors aside f atmosph]e1 int]f]+ ) ! full advantage ( l>ge ap]tures 9 refractors4 ,:ile z already not$ small am\nts ( sph]ical ab]ration may be to a c]ta9 ext5t focuss$ out1 ! sign ( df mu/ *ange ) ! sign ( ! residual ab]ration1 & a quick & c]ta9 te/ ( ! pres5ce ( sph]ical ab]ration is a v>iation 9 ! appe>ance ( ! image inside & outside focus4 ,to emphasize ! importance ( exact k ( exi/+ ab]rations note ,fig4 #ahg1 :i* %[s ! results ( ,h>tmann te/s on a typical gr\p ( ! _w's l>ge objectives4 ,all %[ traces ( residual zones1 b diff]+ greatly 9 magnitude & position z ! atta*$ scales %[4 ,! mo/ conspicu\s ab]rations are 9 ! big ,potsdam photographic refractor1 ! lea/ are 9 ! #bd in* ,l[ell refractor4 ,! =m] has s9ce be5 refigur$ by ,s*midt & revis$ data are n yet available2 ! latt] receiv$ its f9al figure f ! ,lund9s aft] ! la/ ( ! ,cl>ks _h pass$ on4 ,n[ a glance at ! curves %[s t ! bad zone ( ! ,potsdam glass 0 orig9ally ne> ! p]iph]y1 7,i71 h5ce bo? involv$ l>ge area &1 f ,conrady's equation1 s]i\sly enl>g$ df due to ! l>ge relative ap]ture at ! zone4 ,an ab]rant zone ne> ! axis z 9 ! /age 7,,iii7 ( ! ,potsdam objective or 9 ! ,ottawa #ae in* objective is mu* less h>mful = correspond+ reasons4 ,su* diff]5ces h a direct be>+ on ! use ( /ops1 s9ce ^! may d good 9 case ( p]iph]al ab]ration & h>m :5 ! faults are axial4 ,unless ! ab]rations are kn[n no g5]al conclusions c be drawn z to ! effect ( /ops4 ,ev5 9 ! ,l[ell telescope %[n z a :ole 9 ,fig4 #ahh1 ! late ,dr4 ,l[ell f\nd /ops to be useful 9 keep+ d[n atmosph]ic tr\bles & r$uc+ ! illum9ation al?\gh !y c\ld h _h no effect 9 relation to figure4 ,fig4 #ahh %[s at ! head ( ! tube a fitt+ = a big iris diaphragm1 controll$ f ! eye-end1 ! value ( :i* 0 well demon/rat$ by num]\s obs]v]s4 ,!re are1 too1 cases 9 :i* a small in/rum5t1 despite intr9sic lack ( resolv+ p[]1 may actually d bett] "w ?an a big "o4 ,su* are met 9 in/ances :]e extreme contra/ ( details is s\ght1 z has be5 well po9t$ out by ,nutt+ 7,ap4 ,j4 #dj1 #cc7 & ! situation disclos$ by him f9ds amplification 9 ! extraord9>y "w done by ,b>n>d ) a *eap lant]n l5s ( #a in* diamet] & #e in*es focus 7,pop4 ,a/41 #f1 #deb74 ,illu/ration3 ,,fig4 #ahg4--;,h>tmann ,te/s ( ,telescopes ,f ,h>tmann's ,measures4 ,! fact is t e task mu/ seek its own prop] in/rum5t4 ,& 9 any case ! int]pretation ( obs]v$ results is a matt] t passes f> beyond ! b\nds ( geometrical optics1 & involves physiological factors t are dom9ant 9 all visual problems4 ,) respect to ! visibility ( objects ! g5]al diffraction !ory aga9 comes into play4 ,= a bright l9e1 = example1 ! diffraction figure is no long] *iefly a cone l ,fig4 #ahc1 b a simil> long w$ge-%ap$ figure1 ) wave-l %\ld]s correspond+ to ! diffraction r+s4 ,! visibility ( su* a l9e dep5ds n only on ! di/ribution ( int5sity 9 ! !oretical w$ge b on ! s5sitiv5ess ( ! eye & ! nature ( ! backgr\nd & s =?1 j z 9 ! case ( a /> disc4 ,if ! eye is f its nature or /ate ( adaptation ke5 en\gh on detail b n p>ticul>ly s5sitive to slight diff]5ces ( int5sity1 ! l9e w v likely be se5 z if a section 7 made ( ! w$ge ne> its ?9 edge4 ,9 o!r words ! l9e w appe> ?9 & %>p z ! diffraction r+s ab\t a /> frequ5tly d4 ,) an eye v s5sitive to light & small diff]5ces ( contra/ ! appe>ance ( absolutely ! same ?+ may correspond to a section ?r\gh ! w$ge ne> its base1 9 o!r words to a broad /rip %ad+ (f some:at indi/9ctly at ! edges1 influ5c$ aga9 by irradiation & ! "* ( ! backgr\nd4 ,if !re be mu* detail simultane\sly visible ! diffraction patt]ns may be mix$ up 9 a mo/ intricate fa%ion & "o c readily see ! confusion :i* may exi/ 9 correlat+ ! "w ( v>i\s obs]v]s on ?+s l planet>y & lun> detail4 ,9 ! planet>y case ! total image is a complex ( illum9at$ areas ( diffraction at ! edges1 :i* may be repres5t$ z ! diffraction solid ( ,fig4 #ahe1 9 :i* ! dott$ l9es %[ :at may correspond fairly to ! real diamet] ( ! planet1 ! edge %ad+ (f 9 a way aga9 complicat$ by irradiation4 ,illu/ration3 ,,fig4 #ahh4--,! ,l[ell ,refractor ,fitt$ ) ,iris ,diaphragm4 ,fancy detail sup]impos$ on a disc ( ?is sort & "o has a vivid idea ( ! difficulty ( int]pret+ obs]vations4 ,x w\ld be an exce$+ly good ?+ if ev]yone :o uses 8 telescope _h ! advantage ( at lea/ a brief c\rse 9 microscopy1 :]eby he w\ld ga9 v mu* 9 ! practical und]/&+ ( resolv+ p[]1 see+ conditions1 & ! int]pretation ( ! image4 ,! pr9ciples reg>d+ ^! matt]s are 9 fact v mu* ! same ) ! two great in/rum5ts ( rese>*4 ,ap]ture1 l9e> 9 ! case ( ! telescope & ! s-call$ num]ical 9 ! case ( ! microscope1 be> precisely ! same relation to resolution1 ! m9imum resolvable detail be+ 9 ea* case directly proportional to ap]ture 9 ! s5ses "h employ$4 ,fur!r1 al?\gh ! turbul5ce ( int]v5+ atmosph]e does n int]f]e ) ! visibility ( microscopic detail1 a simil> di/urb+ factor does ent] 9 ! =m ( irregul> & misplac$ illum9ation4 ,x is a p]fectly easy matt] to make beautifully di/9ct detail q vani% f a microscopic image m]ely by mismanagem5t ( ! illum9ation1 j z un/eady atmosph]e w produce sub/antially ! same effect 9 ! telescopic image4 ,9 ! matt] ( magnification ! two cases run q p>allel1 & magnification pu%$ beyond :at is ju/ifi$ by ! resolv+ p[] ( ! in/rum5t does sub/antially little or no good4 ,x nei!r discloses new detail nor does x br+ out m %>ply detail :i* c be se5 at all ) a l[] p[]4 ,! microscopi/ e>ly le>ns to %un high p[] ocul>s1 bo? f _! be+ less com=table to "w )1 & f _! fail+ to add to ! effici5cy ( ! in/rum5t except 9 "s r>e cases ) objectives ( v high resolv+ p[]4 ,fur!rmore 9 ! int]pretation ( detail ! lessons to be le>n$ f ! two in/rum5ts are q ! same1 al?\gh "o belongs to ! inf9itely little & ! o!r to ! inf9itely great4 ,no?+ is m in/ructive 9 grasp+ ! relation betwe5 resolv+ p[]1 magnification1 & ! v]ity ( detail1 ?an ! /udy "u ! microscope ( "s well kn[n objects4 ,= example1 9 ,fig4 #ahi is %[n a r\gh sket* ( a common diatom1 ,navicula ,lyra4 ,! t9y silice\s valve appe>s ?us "u an objective ( slightly insuffici5t resolv+ p[]4 ,! g5]al =m ( ! object is cle>ly p]ceiv$1 z well z ! c5tral m>k+s1 /&+ boldly out 9 ! =m :i* sugge/s ! specific "n4 ,no trace ( any f9] detail appe>s & no am\nt ( dext]ity 9 arrang+ ! illum9ation or increase ( magnify+ p[] w %[ any m ?an "h appe>s1 ! draw+ be+ "o actually made ) ! cam]a lucida1 us+ an objective ( num]ical ap]ture j too small to resolve ! details ( ! diatoms on ?is p>ticul> slide4 ,figure #ahi;a %[s :at happ5s :51 ) ! same magnify+ p[]1 an objective ( slightly great] ap]ture is employ$4 ,"h ! :ole surface ( ! valve is m>k$ ) f9e /riations1 beautifully %>p & di/9ct l ! l9es ( a /eel engrav+4 ,!re is a complete *ange ( aspect wr\ght by an increase ( ab\t #bj 9 ! resolv+ p[]4 ,aga9 no?+ fur!r c be made out by an increase ( magnification1 ! only effect be+ to make ! outl9es a little hazi] & ! view !re=e some:at less satisfactory4 ,illu/ration3 ,,fig4 #ahi4--,! ,/ages ( ,resolution4 ,f9ally 9 ,fig4 #ahi;b we h aga9 ! same valve "u ! same magnify+ p[]1 b "h obta9$ f an objective ( num]ical ap]ture #fj above t us$ = ! ma9 figure4 ,! %>p /ri n[ %[ _! true "*4 ,!y _h _! orig9 9 l9es ( v cle>ly di/+ui%$ dots1 :i* are p]fectly di/9ct1 & are due to ! resolv+ p[] at la/ be+ suffici5t to %[ ! detail :i* previ\sly m]ely =m$ a %>p l9e> diffraction patt]n entirely incapable ( be+ resolv$ into any?+ else by ! eye1 h[ev] mu* x might be magnifi$4 ,"h "o has1 set out 9 unmi/akable t]ms1 ! same k9d ( diff]5ces :i* appe> 9 view+ cele/ial detail ?r\gh telescopes ( v>i\s ap]ture4 ,:at _c be se5 at all ) a l[ ap]ture may be se5 ) high] ones "u totally diff]5t aspects2 :ile 9 ea* case ! app>5t %>pness & cl>ity ( ! image is some:at extraord9>y4 ,fur!r 9 ,fig4 #ahi;b 9 us+ ! resolv+ p[] ( ! objective ( high num]ical ap]ture1 ! image may be q wreck$ by a little c>elessness 9 focuss+1 or by mismanagem5t ( light1 s t "o w\ld h>dly "k t ! valve _h m>k+s o!r ?an ^? se5 ) ! objectives ( l[] ap]ture1 & "u ^! circum/ances add$ magnification w\ld d m h>m ?an good4 ,9 precisely ! same way mismanagem5t ( ! illum9ation 9 ,fig4 #ahi;a w\ld cause ! /ri to vani% & ) ,navicula ,lyra1 z ) _m o!r diatoms1 ! resolution into /ri is a ?+ :i* (t5 dep5ds entirely on c>eful light+1 & ! detail fla%es into di/9ctness or vani%es ) a sudd5ness :i* is altoge!r surpris+4 ,= 8light+0 read 8atmosph]e10 & y h j ! sort ( conditions t exi/ 9 telescope vision4 ,) respect to magnify+ p[]s :at has already be5 said is suffici5t to indicate t on ! :ole ! l[e/ p[] :i* discloses to ! eye ! detail )9 ! rea* ( ! resolv+ p[] ( ! objective is ! mo/ satisfactory4 ,e increase above ?is magnifies all ! optical faults ( ! telescope & ! atmosph]ic difficulties z well1 beside decreas+ ! diamet] ( ! em]g5t p5cil :i* ent]s ! eye1 & !reby caus+ s]i\s loss ( acuity4 ,= ! eye l any o!r optical in/rum5t loses resolv+ p[] ) decrease ( effective ap]ture1 &1 besides1 a v n>r[ beam ent]+ x is subject to ! int]f]5ce ( entoptic defects1 su* z float+ motes & ! l1 to a s]i\s ext5t4 ,figure #aij %[s f ,cobb's exp]im5ts 7,am4 ,j\r4 ( ,physiol41 #ce1 #cce7 ! effect ( r$uction ( ocul> ap]ture ^u acuity4 ,! curve %[s v pla9ly t = em]g5t p5cils bel[ a millimet] 7#a_/be in*7 9 diamet]1 visual acuity falls (f almo/ 9 direct proportion to ! decreas+ ap]ture4 ,bel[ ?is figure !re c be only incid5tal ga9s1 su* z may be due to op5+ up d\ble />s & simultane\sly s dim9i%+ ! g5]al illum9ation z to r5d] ! m>g9s ( ! /> discs a little less conspicu\s4 ,an em]g5t p5cil ( ?is diamet] is n q suffici5t = ! av]age eye to utilize fully ! available resolv+ p[] & "s excess ( magnification ev5 ?\gh x actually dim9i%es visual acuity mat]ially1 may be ( "s s]vice4 ,illu/ration3 ,,fig4 #aij4--,resolv+ ,p[] ( ! ,eye4 ,increas$ acuity w ( c\rse be ga9$ = ! same magnification 9 us+ an objective ( great] diamet]1 to say no?+ ( increas$ resolv+ p[]1 at ! co/1 ( c\rse1 ( relatively great] atmosph]ic tr\bles4 ,to come d[n to figures z to ! resolv+ p[] ( ! eye1 (t5 repeat$ exp]im5ts h %[n t two po9ts (f]+ /rong contra/ ) ! backgr\nd c be not$ z sep>ate by ! normal eye :5 at an angul> sep>ation ( ab\t #c ( arc4 ,p1 z we h se51 diff] consid]ably 9 acuity s t n[ & !n individuals w consid]ably bett] ?is figure1 :ile o!rs1 f> less ke5 sight$1 may require a sep>ation ( #d or ev5 #e4 ,! pair ( d\ble />s #a1 #b1 ,lyr1 sep>at$ by #c #bg mags4 ne>ly #d & #e respectively1 c be se5 z sep>ate by ^? ( fairly ke5 vision1 :ile ,miz> & ,alcor1 #aa ap>t1 seem ?r[n wide to ne>ly e "o4 ,on ! o!r h& ! writ] has nev] kn[n anybody :o c\ld sep>ate ! two compon5ts ( ,a/]ope ( ! ,pleiades1 di/ant a scant #b b ( mags4 #f4e & #g4j only1 :ile ,pleione & ,atlas1 di/ance ab\t #e1 mags4 #f4e & #d1 are v easy4 ,assum+ = lib]ality t ! sep>ation con/ant is 9 ! neighborhood ( #e "o c readily e/imate ! magnification t = any telescope w take full advantage ( its resolv+ p[]4 ,z we h already se5 ?is resolv+ p[] is practically #d4ef_/;,a = equal />s mod]ately bright4 ,an objective ( #d4ef in*es ap]ture has a resolv+ con/ant ( #a & to develop ?is %\ld take a magnification ( say #cjj1 ab\t #fe to ! in* ( ap]ture1 requir+ a focal l5g? ( ocul> ab\t #j4bj to #j4be in* = telescopes ( normal relative ap]ture1 & pu%+ ! em]g5t p5cil d[n to little m ?an #j4jb in*1--r fur!r ?an is physiologically desirable4 ,except = ^! extreme /unts ( sep>ation1 half to two ?irds ?is p[] is pref]able & conditions "u :i* "o c advantage\sly g above ?is limit are v r>e inde$4 ,a ?or\ghly good objective or mirror w /& q #ajj magnification to ! in* )\t1 z ! microscopi/ w\ld say1 8break+ d[n ! image10 b 9 at lea/ n9e cases out ( t5 ! result w be decid$ly unsatisfactory4 ,z ! relative ap]ture ( ! in/rum5t increases1 o!r ?+s be+ equal1 "o is driv5 to ocul>s ( %ort] & %ort] focus to obta9 ! same magnification & soon gets into tr\ble4 ,v few ocul>s bel[ #j4bj in* 9 focus are made1 & su* are r>ely advisable1 al?\gh occasionally 9 use d[n to #j4ae in* or !reab\ts4 ,! usual ,f_/#ae ap]ture is a figure q probably z mu* due to ! undesirability ( extremely %ort focus ocul>s z to ! easi] corrections ( ! objective4 ,9 ! actual practice ( exp]i5c$ obs]v]s ! indications ( !ory are well borne out4 ,data ( ! habits ( _m obs]v]s ( d\ble />s are ( record & ! accompli%$ vet]an editor ( ,! ,obs]vatory1 ,mr4 ,t4 ,lewis1 took ! tr\ble 9 "o ( 8 admirable pap]s on 8,d\ble ,/> ,a/ronomy0 7,obs4 #cf1 #dbf7 to tabulate f ! orig9al s\rces ! practice ( a l>ge gr\p ( exp]ts4 ,! g5]al result 0 to %[ ! habitual use ) telescopes ( mod]ate size ( p[]s ar\nd #ej p] in* ( ap]ture1 n[ & !n on special occasions rais$ to ! neighborhood ( #gj p] in*4 ,b ! data %[$ unequivocally j :at has be5 already indicat$1 t l>ge ap]tures1 suff]+ sev]ely z !y g5]ally d f turbul5ce ( ! air1 w n ord9>ily /& _! due proportion ( magnification4 ,) ! refractors ( #bd in*es ap]ture & upw>ds ! records %[ t ev5 9 ?is d\ble /> "w1 :]e1 if any:]e1 high p[] c\nts1 ! g5]al practice ran 9 ! vic9ity ( #cj p] in* ( ap]ture4 ,analyz+ ! data m completely 9 ?is respect ,mr4 ,lewis f\nd t ! be/ practise ( ! skill$ obs]v]s /udi$ 0 approximately repres5t$ by ! empirical equation m #adj ,a ,( c\rse ! actual figures mu/ v>y ) ! conditions ( location & ! g5]al quality ( ! see+1 z well z ! "w 9 h&4 ,= o!r ?an d\ble /> "w ! t5d5cy w be g5]ally t[>d l[] p[]s4 ,! details :i* dep5d on %ade p]ception r ?an visual acuity are usually hurt r ?an help$ :5 magnifi$ beyond ! po9t at :i* !y are fairly resolv$1 q z 9 ! case ( ! microscope4 ,n[ & !n !y may be made m di/9ct by ! judici\s use ( %ade glasses4 ,q ap>t f ! matt] ( ! high p[]s :i* c advantage\sly be us$ on a telescope1 "o mu/ = c]ta9 purposes consid] ! l[e/ p[]s :i* are fairly applicable4 ,?is "q really turns on ! l>ge/ utilizable em]g5t p5cil f ! eye piece4 ,x us$ to be commonly /at$ t in* = ! em]g5t p5cil 0 ab\t a work+ maximum1 lead+ to a magnification ( #h p] in* ( ap]ture ( ! objective4 ,?is 9 view ( our pres5t k ( ! eye & its prop]ties is too l[ an e/imate ( pupill>y ap]ture4 ,x is a fact :i* has be5 well kn[n = m ?an a decade t 9 fa9t light1 :5 ! eye has become adapt$ to its situation1 ! pupil op5s up to two or ?ree times ?is diamet] & !re is no d\bt t a fif? or a f\r? ( an in* ap]ture c be well utiliz$1 provid$ ! eye is prop]ly d>k-adapt$4 ,= scrut9iz+ fa9t objects1 comet sweep+ & ! l1 "o %\ld !re=e h "o ocul> ( v wide field & magnify+ p[] ( #d or #e p] in* ( ap]ture1 ! ma9 po9t be+ to secure a field z wide is practicable4 ,"o may use = su* purposes ei!r a v wide field ,huygh5ian1 or1 if cross wires are to be us$1 a ,kelln] =m4 ,fifty degrees ( field is p]fectly practicable ) ei!r4 ,z reg>ds ! re/ ( ! eyepiece equipm5t ! obs]v] may well suit 8 own conv5i5ce & res\rces4 ,usually "o ocul> ( ab\t half ! maximum p[] provid$ w be f\nd extremely conv5i5t & p]haps (t5] us$ ?an ei!r ! high or l[ p[]4 ,ocul>s ( int]m$iate p[] & adapt$ = v>i\s purposes w g5]ally f9d _! way into any telescopic equipm5t4 ,& z a la/ ^w d n expect to improve bad conditions by magnify+4 ,if ! see+ is bad ) a l[ p[]1 cap ! telescope & await a bett] opportunity4 ,,app5dix ,,"w ,,= ,,! ,,telescope ,to make at fir/ h& ! acqua9tance ( ! cele/ial bodies is1 9 & ( itself1 wor? ! :ile1 z lead+ ! m9d to a new s5se ( ultimate values4 ,to tell ! tru? ! mod]n man on ! :ole kn[s ! ,heav5s less intimately ?an did 8 ance/ors4 ,he glances at 8 wri/-wat* to le>n ! h\r & at ! almanac to id5tify ! day4 ,! ris+ & sett+ ( ! con/ellations1 ! w&]+ ( ! planets among ! />s1 ! seasonal %ift+ ( ! sun's pa?--all ^! are a seal$ book to him1 & ! intricate my/]ies t lie 9 ! backgr\nd are q unsuspect$4 ,! telescope is ! lift] ( ! cosmic veil1 & ev5 = m]ely disclos+ ! spectacul> is a s\rce ( f>-rea*+ enlight5m5t4 ,b = ! s]i\s /ud5t x (f]s opportunities = ! g5u9e advancem5t ( human k t are h>d to und]e/imate4 ,x is true t ! great mod]n obs]vatories c ga!r in=mation on a scale t /agg]s ! private inve/igator4 ,b 9 ?is matt] =tune favors ! p]t9aci\s1 & ! obs]v] :o settles to a l9e ( delib]ate inve/igation & pati5tly foll[s x is likely to f9d 8 rew>d4 ,!re is s mu* )9 ! rea* ( p[]ful in/rum5ts only1 t ^! are 9 ! ma9 turn$ to _! own p>ticul> sph]es ( usefulness4 ,= mode/ equipm5t !re is /ill pl5ty ( "w to d4 ,! /udy ( v>iable />s (f]s a va/ field = exploration1 mo/ fruitful p]haps ) respect to ! irregul> & long-p]iod *anges ( :i* our own ,sun (f]s an example4 ,ev5 9 sol> /udy !re are transi5t ph5om5a ( sudd5 eruptions & ( swift *anges t escape ! eye ( ! spectro-heliograph1 & admirable "w c be done1 & has be5 done1 ) small telescopes 9 /udy+ ! spectra ( sun spots ,tempor>y />s visible to ! nak$ eye or to ! smalle/ in/rum5ts turn up e few ye>s & _! discov]y has usually fall5 to ! lot ( ! some:at r>e a/ronom]1 pr(essional or amateur1 :o kn[s ! field ( />s z he kn[s ! alphabet4 ,! la/ ?ree important nov fell to ! amateurs--two to ! same man4 ,comets are to be _h = ! seek+ by ! p]si/5t obs]v] ) an in/rum5t ( fair light-grasp & field2 "o di/+ui%$ amateur f\nd a pair )9 a few days1 act+ on ! !ory t small comets are really common & %\ld be look$ =--mo/ easily by "o :o kn[s 8 nebul1 x %\ld be add$4 ,& )9 our small planet>y sy/em lies labor suffici5t = g5]ations4 ,we "k little ev5 ab\t ! sup]ficial *>act]s ( ! planets1 /ill less ab\t _! real physical condition4 ,we are n ev5 sure ab\t ! rotation p]iods ( ,v5us & ,neptune4 ,! clue to _m ( ! my/]ies requires et]nal vigilance r ?an p[]ful equipm5t1 = ! appe>ance ( tempor>y *anges may tell ! :ole /ory4 ,! old g5]ation ( a/ronom]s :o believ$ 9 ! complete inviolability ( cele/ial ord] has be5 = ! mo/ "p ga!r$ to its fa!rs1 & we n[ realize t *ange is ! law ( ! univ]se4 ,)9 ! sol> sy/em !re are planet>y surfaces to be wat*$1 a/]oids to be scann$ = v>iability or *ange ( x1 meteor sw>ms to be correlat$ ) _! s\rces1 occultations to be m9utely exam9$1 & :5 "o runs %ort ( ^!1 our ne>e/ neighbor ! ,moon (f]s a wild & physically unkn[n c\ntry = exploration4 ,x is suspect$ ) good reason ( dynamic *anges1 to say no?+ ( ! possible la/ remnants ( organic life4 ,mu* ( ?is "w is well )9 ! useful range ( in/rum5ts ( ?ree to six in*es ap]ture4 ,! /rategy ( successful inve/igation is 9 turn+ att5tion ^u ^? ?+s :i* are )9 ! scope ( "o's equipm5t1 & select+ ^? :i* give promise ( yield+ to a well direct$ attack4 ,& to ?is end ef=ts correlat$ ) ^? ( o!rs are e>ne/ly to be advis$4 ,x is h>d to say too mu* ( ! usefulness ( direct$ en]gies l ^? ( ! ,v>iable ,/> ,association & simil> bodies4 ,!y n only organize activities to an important common end1 b /r5g!n ! morale ( ! individual obs]v]4 ,,index ,a ,abb1 ro( prism1 #afb ,ab]ration1 comp5sat$ by m9ute *ange ( focus1 #bff illum9ates ! diffraction m9ima1 #bfe relation det]m9es ( focus & ap]ture1 #bff ,a*romatic long relief ocul>1 #adf objective1 #gg ,a*romatism1 condition =1 #gh det]m9ation (1 #gh imp]fection (1 #hg ,adju/m5t :]e ,pol>is invisible1 #bce ,air waves1 l5g? (1 #bee ,alt-azimu? m\nt = reflector1 #ajb m\nts1 ) sl[ motions1 #ajb sett+ up an1 #bbh ,ana/igmats1 #hd ,anneal+1 patt]n ( /ra91 #fh ,a/igmatism1 #hd1 #bji ( figure1 #baj ,a/ronomy1 dawn ( popul>1 #ai ,b ,bacon1 ,rog]1 alleg$ description ( telescopes1 #f ,b>l[ l5s1 #aeb 8,b5t10 objective1 #hf ,b9ocul>1 #b advantage (1 exagg]at$1 #aea = /rictly a/ronomical use1 #aeb telescopes = a/ronomical use1 #afc ,c ,cam\flage1 9 optical pat5ts1 #ig ,cassegra91 design = reflect+ telescope1 #bb ,cassegra91 sculptor & f\nd] ( /atues1 #bb ,cell1 tak+ (f f a telescope1 #bjb ,*romatic ab]ration1 #aa1 #gf inve/igation (1 #baj correction1 diff]5ces 91 #ia error ( ! eye1 #ij ,clairault's condition1 #ha two cem5t$ =ms =1 #ha ,cl>ks1 portable equatorial m\nt+1 #aji t]re/rial prismatic eyepiece1 #aeh ,clock1 ! cosmic1 #bcc ,clock drive1 #aaj1 #agd ,clock me*anism1 regulat+ rate ( motor1 #agi ,codd+ton l5s1 #acg ,clo/at con/ructions1 #abf t[] telescopes1 #abg ,color correction1 commonly us$1 #baa exam9$ by spectroscope1 #baa ( ! great mak]s1 #ij ,coma-free1 condition comb9$ ) ,clairault's1 #hc ,comet seek]1 ,c>ol9e ,h]s*el's #aah seek]s ) triple objective1 #aai ,cr[ns di/+ui%$ f fl9ts1 #fd ,curves1 /ruggle = non-sph]ical1 #ah ,d ,davon micro-telescope1 #adh ,dawes' ,limit1 #bfa 9 physiological factors1 #bfc ,decl9ation circle1 #ajh adju/m5t (1 #bci ,decl9ation circle1 adju/m5t by1 #bcg facilitates sett+ up in/rum5t1 #aaj ,def9ition condition = excell5ce (1 #bed good 9 situations widely diff]5t1 #bed ,de,rheita1 #ab con/ruct$ b9ocul>s1 #ac t]re/rial ocul>1 #ac ,desc>tes' dioptrics1 publication (1 #aa l5s ) elliptical curvature1 #ab ,dew cap1 #bai ,diaphragms1 importance (1 #dc ,diffraction figure = bright l9e1 #bfi patt]n1 #bef solid1 app>5t diamet] (1 #bfb solid ( planet1 #bfi solid = a />1 #bfj spectra1 #aij sy/em1 scale (1 #bfj v>ies inv]sely ) ap]ture1 #bfj ?r\gh objective1 #beh ,digges1 acc\nt sugge/s cam]a obscura1 #g ,dim5sions1 cu/om>y1 telescope (1 #bd ,discs1 inspection ( glass1 #ff r\gh+ to =m1 #fi ,di/ortion1 #hf ,doll&1 ,john1 #bh publi%$ 8 discov]y ( a*romatism1 #bi ,pet]1 e>ly triple objective1 #bi ,dome :olly ( galvaniz$ iron1 #bej ,domes1 #bdf ,driv+ clock1 a simple1 #agd p5dulum controll$1 #agg clocks spr+ op]at$1 #age ,e ,engli% equatorial1 #aaj m\nts1 me*anical /ability (1 #aac ,equatorial1 adju/m5ts (1 #bcj ,equatorial1 c\d1 #abd m\nt1 diff]5t situations 9 us+1 #bbi m\nt1 fir/ by ,%ort1 #ajd m\nt1 pi] ov]hung1 #aae m\nt 9 section1 #ajg two motions necess>y 91 #ajf ,equilibrat+ lev]s1 devis$ by ,t4 ,grubb1 #ci ,ev]%$1 direct vision sol> spectroscope1 #ahi ,eye l5s1 simple1 pref]r$ by ,sir ,w4 ,h]s*el1 #acf ,eyepiece1 comp5sat+1 #adb ,huyg5ian1 #aci ,huyg5ian1 a*romatism (1 #adj ,huyg5ian1 ) cross wires1 #adj ,huyg5ian1 field (1 #ada ,huyg5ian focal l5g? 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