,,radiation ,,by ,p4 ,,phillips ,d4,sc4 7,b',,ham71 ,b4,sc4 7,,london71 ,b4,a4 7,,cantab47 ,,london3 ,t4 ,c4 ,e4 ,c4 ,,jack #fg ,,long ,,acre1 ,w4,c41 ,,& ,,ed9burgh ,,new ,,york3 ,,dodge ,,publi%+ ,,co4 #aiab ,,cont5ts ,,*ap4 ,,introduction ,i4 ,,! ,,nature ,,( ,,radiant ,,heat ,,& ,,light ,,ii4 ,,graphic ,,repres5tation ,,( ,,waves ,,iii4 ,,! ,,mean+ ,,( ,,! ,,spectrum ,,iv4 ,,! ,,laws ,,( ,,radiation ,v4 ,,full ,,radiation ,,vi4 ,,! ,,trans=mation ,,( ,,absorb$ ,,radiation ,,vii4 ,,pressure ,,( ,,radiation ,,viii4 ,,! ,,relation ,,betwe5 ,,radiant ,,heat ,,& ,,electric ,,waves ,,index vii ,,introduction ,we are s famili> ) ! re/lessness ( ! sea1 & ) ! havoc :i* x works on our %ipp+ & our coa/s1 t we ne$ no demon/ration to conv9ce u t waves c c>ry en]gy f "o place to ano!r4 ,few ( u1 h[ev]1 realise t ! en]gy 9 ! sea is z no?+ comp>$ ) t 9 ! space ar\nd u1 yet su* is ! conclusion to :i* we are l$ by an enorm\s am\nt ( exp]im5tal evid5ce4 ,! sea waves are only ne> ! surface & ! effect ( ! wilde/ /orm p5etrates b a few y>ds bel[ ! surface1 :ile ! waves :i* c>ry light & heat to u f ! sun fill ! :ole space ab\t u & br+ to ! e>? a cont9u\s /ream ( en]gy ye> 9 ye> out equal to m ?an #cjj million million horsep[]4 ,! mo/ important "p ( ! /udy ( ,radiation ( en]gy is ! inve/igation ( ! *>act]s ( ! waves :i* con/itute heat & light1 b !re is ano!r me?od ( transf]5ce ( en]gy includ$ 9 ! t]m ,radiation2 ! s\rce ( ! en]gy behaves l a batt]y ( guns po9t+ 9 all directions & p\r+ out a cont9u\s hail ( bullets1 :i* /rike aga9/ ob/acles & s give up ! en]gy due to _! motion4 ,?is me?od is relatively unimportant1 & is usually treat$ ( sep>ately :5 consid]+ ! subject ( ,radioactivity4 ,we %all !re=e n consid] x 9 ?is book4 #i ,,radiation ,,*apt] ,i ,,! ,,nature ,,( ,,radiant ,,heat ,,& ,,light ,simil>ity ( ,heat & ,light4--,t light & heat h ess5tially ! same *>act]s is v soon made evid5t4 ,bo? light & heat travel to u f ! sun across ! n9ety odd millions ( miles ( space unoccupi$ by any mat]ial4 ,illu/ration3 ,figure #a ,bo? are reflect$ 9 ! same way f reflect+ surfaces4 ,?us if two p>abolic mirrors be plac$ fac+ ea* o!r z 9 ! diagram 7,fig4 #a71 ) a s\rce ( light ,l at ! focus ( "o ( !m1 an inv]t$ image ( ! light w be =m$ at ! focus ,i ( ! o!r "o1 & may be receiv$ on a small scre5 plac$ !re4 ,! pa?s ( two ( ! rays are %[n by ! dott$ l9es4 ,if ,l be n[ replac$ by a heat$ ball & a#a black5$ !rmomet] bulb be plac$ at ,i1 ! !rmomet] w indicate a %>p rise ( temp]ature1 %[+ t ! rays ( heat are focuss$ !re z well z ! rays ( light4 #a ,see page #cg4 #aj ,bo? heat & light behave 9 ! same way 9 pass+ f "o transp>5t sub/ance to ano!r1 e4g4 f air into glass4 ,?is c be readily %[n by =m+ images ( s\rces ( heat & ( light by means ( a convex l5s1 z 9 ! diagram 7,fig4 #b74 ,illu/ration3 ,,fig4 #b4 ,! s\rce ( light is repres5t$ z an electric light bulb1 & two ( ! rays go+ to =m ! image ( ! po9t ( ! bulb are repres5t$ by ! dott$ l9es4 ,! image is also dott$ & c be receiv$ on a scre5 plac$ 9 t position4 ,if n[ ! electric light bulb be replac$ by a heat$ ball or "s o!r s\rce ( heat1 we f9d by us+ a black5$ !rmomet] bulb aga9 t ! rays ( heat are br\ght to a focus at almo/ ! same position z ! rays ( light4 ,! po9ts ( simil>ity betwe5 radiant heat & light might be multipli$ indef9itely1 b z a numb] ( !m w appe> 9 ! c\rse ( ! book ^! few fundam5tal ones w suffice at ?is po9t4 ,! ,corpuscul> ,!ory4--,a little ov] a c5tury ago ev]yone believ$ light to consi/ ( almo/ inconceivably small p>ticles or corpuscles %oot+ out at enorm\s spe$ f e lum9\s surface & caus+ ! s5sation ( sight :5 imp++ #aa on ! ret9a4 ,?is 0 ! corpuscul> !ory4 ,x readily expla9s :y light travels 9 /raight l9es 9 a homog5e\s m$ium1 & x c be made to expla9 reflection & refraction4 ,reflection4--,to expla9 reflection1 x is suppos$ t ! reflector repels ! p>ticles z !y approa* x1 & s ! pa? ( "o p>ticle w\ld be l t indicat$ by ! dott$ l9e 9 ! diagram 7,fig4 #c74 ,illu/ration3 ,,fig4 #c4 ,until rea*+ ! po9t ,a we suppose t ! p>ticle does n feel appreciably ! repulsion ( ! surface4 ,aft] ,a ! repulsion b5ds ! pa? ( ! p>ticle r\nd until ,b is rea*$1 & aft] ,b ! repulsion becomes inappreciable aga94 ,! effect is ! same z a p]fectly ela/ic ball b\nc+ on a p]fectly smoo? surface1 & consequ5tly ! angle to ! surface at :i* ! corpuscle comes up is equal to ! angle at :i* x dep>ts4 ,refraction4--,to expla9 refraction1 x is suppos$ t :5 ! corpuscle comes v close to ! surface ( ! transp>5t sub/ance x is attract$ by ! d5s] sub/ance1 e4g4 glass1 m ?an by ! light] sub/ance1 e4g4 air4 ,?us a p>ticle mov+ along ! dott$ l9e 9 air 7,fig4 #d7 w\ld rea* ! #ab po9t ,a be=e ! attraction becomes appreciable1 & !re=e w\ld be mov+ 9 a /raight l9e4 ,betwe5 ,a & ,b ! attraction ( ! glass w be felt & w !re=e pull ! p>ticle r\nd 9 ! pa? indicat$4 ,beyond ,b1 ! attraction aga9 becomes inappreciable1 because ! glass w attract ! p>ticle equally 9 all directions1 & !re=e ! pa? w aga9 become a /raight l9e4 ,we notice t by ?is process ! direction ( ! pa? has become m ne>ly normal to ! surface1 & ?is is z x %\ld be4 ,fur!r1 by treat+ ! angles betwe5 ! two pa?s & ! normal ma!matically we may d$uce ! laws ( refraction :i* h be5 obta9$ exp]im5tally4 ,"o o!r important po9t %\ld be notic$4 ,s9ce ! surface has be5 attract+ ! p>ticle betwe5 ,a & ,b ! spe$ ( ! p>ticle w be great] 9 ! glass ?an 9 ! air4 ,illu/ration3 ,,fig4 #d ,ejection & ,refraction at ! same ,surface4--,a difficulty v soon arises f ! fact t at ne>ly all transp>5t surfaces "s light is reflect$ & "s refract$4 ,h[ c ! same surface sometimes repel & sometimes attract a corpuscle8 ,newton surm\nt$ ?is difficulty by attribut+ a pol>ity to ea* p>ticle1 s t "o end 0 repell$ & ! o!r attract$ by ! reflect+ & refract+ #ac surface4 ,?us1 :e!r a p>ticle 0 reflect$ or refract$ dep5d$ simply ^u :i* end happ5$ to be =emo/ at ! "t4 ,by attribut+ suitable *>act]i/ics to ! corpuscles1 ,newton ) 8 sup]human +5uity 0 able to acc\nt = all ! kn[n facts1 & z ! corpuscles 7 s small t direct obs]vation 0 impossible1 & z ,newton's au?ority 0 s great1 !re 0 no "o to say him nay4 ,wave ,!ory4 ,rectil9e> ,propagation4--,true1 ,huygh5s 9 #afgh _h prop\nd$ ! !ory t light consi/s ( waves ( "s sort />t+ out f ! lum9\s body1 & he _h %[n h[ readily x express$ a numb] ( ! obs]v$ facts2 b light travels 9 /raight l9es1 or appe>s to d s1 & waves b5d r\nd corn]s & no "o at t "t 0 able to expla9 ! discrepancy4 ,?us = ne>ly a c5tury ! !ory :i* 0 to be univ]sally accept$ rema9$ lifeless & discr$it$4 ,! answ] ( ! wave !ory to ! objection n[ is1 t light does b5d r\nd corn]s ?\gh only slightly & t ! smallness ( ! b5d is q simply due to ! extreme %ortness ( ! light waves4 ,! long] waves are1 ! m !y b5d r\nd corn]s4 ,?is c be notic$ 9 any h>b\r ) a tortu\s entrance1 = ! small *oppy waves are practically all cut (f :]eas a consid]able am\nt ( ! long swell manages to get into ! h>b\r4 ,int]f]5ce ( ,light4 ,illu/ration by ,ripples4--,! revival ( ! wave !ory dates f ! discov]y by ,dr4 ,"y ( ! ph5om5on ( int]f]5ce ( light4 ,9 ord] to und]/& ?is we w #ad consid] ! same effect 9 ! ripples on ! surface ( m]cury4 ,a tun+-=k1 ,t 7,fig4 #e71 has two small /yles1 ,s ,s1 plac$ a little di/ance ap>t & dipp+ into ! m]cury conta9$ 9 a l>ge %all[ tr\gh4 ,:5 ! tun+-=k is set into vibration1 ! two /yles w move up & d[n 9 ! m]cury at exactly ! same "t & ea* w />t a sy/em ( ripples exactly simil> to ! o!r4 ,at any in/ant ea* sy/em w be a s]ies ( conc5tric circles ) its c5tre at ! /yle1 & ! cre/s ( ! ripples w be at equal di/ance f ea* o!r ) ! tr\ghs half-way betwe5 ! cre/s4 ,illu/ration3 ,,fig4 #e4 ,! ripples f "o /yle w cross ^? f ! o!r1 & a curi\s patt]n1 some?+ l t 9 ,fig4 #f1 w be =m$ on ! m]cury4 ,s ,s repres5ts ! position ( ! two /yles1 :ile ! pla9 circles d5ote ! positions ( ! cre/s & ! dott$ circles ! positions ( ! tr\ghs at any in/ant4 ,:]e two pla9 circles cross x is evid5t t bo? sy/ems ( ripples are produc+ a cre/1 & s ! two produce an exagg]at$ cre/4 ,simil>ly :]e two dott$ circles cross an exagg]at$ tr\gh is produc$4 ,?us 9 ! %ad$ portions ( ! diagram we get m viol5t ripples ?an ^? due to a s+le /yle4 ,:]e a pla9 circle cuts a dott$ "o1 h[ev]1 "o sy/em ( ripples produces a #ae cre/ & ! o!r a tr\gh1 & betwe5 !m ! m]cury is nei!r depress$ bel[ nor rais$ above its normal level4 ,at ^! po9ts1 !re=e1 ! effect ( "o s]ies ( ripples is j neutralis$ by ! effect ( ! o!r & no ripples are produc$ at all4 ,?is occurs 9 ! un%ad$ regions ( ! diagram4 ,! mutual de/ruction ( ! effects ( ! two sets ( waves is ,int]f]5ce4 ,illu/ration3 ,,fig4 #f4 ,n[ imag9e a r[ ( little floats plac$ along ! l9e ,,edcbabcde4 ,at ! lett]$ po9ts ! floats w be viol5tly agitat$1 b at ! po9ts midway betwe5 ! lett]s !y w be unmov$4 ,?is exactly repres5ts ! effect ( two int]f]+ s\rces ( light ,s1 ,s1 s5d+ light :i* is receiv$ by a scre5 at ! dott$ l9e ,,edcbabcde4 ,! lett]$ po9ts w be brightly illum9at$ :ile ! int]m$iate po9ts w be d>k4 ,9 practice x is f\nd impossible to make two #af s\rces ( light ^: vibrations />t at exactly ! same "t & are exactly simil>1 b ?is difficulty is surm\nt$ by us+ "o s\rce ( light & splitt+ ! waves f x into two portions :i* int]f]e4 ,"y's ,exp]im5t4--,dr4 ,"y's arrangem5t is diagrammatically repres5t$ 9 ,fig4 #g4 ,light ( a c]ta9 wave l5g? is admitt$ at a n>r[ slit ,s1 & is int]cept$ by a scre5 9 :i* !re are two n>r[ slits ,a & ,b p>allel to ! fir/ "o4 ,illu/ration3 ,,fig4 #g4 ,a scre5 receives ! light em]g+ f ! two slits4 ,if ! old corpuscul> !ory 7 true !re w\ld be two bright b&s ( light1 ! "o at ,p & ! o!r at ,q1 b in/ead ,dr4 ,"y obs]v$ a :ole s]ies ( p>allel bright b&s ) d>k spaces 9 betwe5 !m4 ,evid5tly ! two small fractions ( ! orig9al waves :i* pass ?r\gh ,a & ,b spread out f ,a & ,b & int]f]e j z if !y 7 indep5d5t s\rces l ! two /yles 9 ! m]cury ripples exp]im5t4 #ag ,spe$ ( ,light 9 ,r>e & ,d5se ,m$ia4--,! discov]y ( int]f]5ce aga9 br\ght ! wave !ory into prom95ce1 & 9 #ahej ! dea?-bl[ 0 giv5 to ! corpuscul> !ory by ,f\cault1 :o %[$ t light travels m sl[ly 9 a d5se m$ium su* z glass or wat] ?an 9 a light m$ium su* z air4 ,?is is :at ! wave !ory anticipates1 :ile ! rev]se is anticipat$ by ! corpuscul> !ory4 ,b if light & heat consi/ ( waves1 :at k9d ( waves are !y & h[ are !y produc$8 ,ela/ic ,solid ,!ory4--,9 ! e>li] days ( ! wave !ory x 0 suppos$ t ! :ole ( space 0 fill$ ) some?+ :i* act$ l an ela/ic solid mat]ial 9 :i* ! vibrations ( ! atoms ( a lum9\s body />t$ waves 9 all directions1 j z ! vibrations ( a m>ble emb$d$ 9 a jelly w\ld s5d out waves ?r\gh ! jelly4 ,^! waves are q easily imag9$ 9 ! foll[+ way4 ,if "o end ( an ela/ic /r+ be made to oscillate to & fro a s]ies ( waves travels along ! /r+4 ,if a l>ge numb] ( ^! /r+s are atta*$ to an oscillat+ po9t & /ret* out 9 all directions1 ! waves w travel along ea* /r+1 & if ! /r+s are all exactly alike w travel at ! same spe$ along all ( !m4 ,any p>ticul> cre/ ( a wave w ?us at any in/ant lie on ! surface ( a sph]e ^: c5tre is ! oscillat+ po9t4 ,if n[ we imag9e t ! /r+s are s num]\s t !y fill ! :ole ( ! space we h a conception ( ! transmission ( waves by an ela/ic solid4 ,electromagnetic ,waves4--,s9ce ,maxwell publi%$ #ah 8 electromagnetic !ory 9 #ahgc x has be5 univ]sally held t heat & light consi/ ( electro-magnetic waves4 ,^! are by no means s easy to imag9e z ! ela/ic waves1 z !re is no actual movem5t ( ! m$ium2 an alt]nat+ condition ( ! m$ium is c>ri$ onw>d1 n an oscillation ( position4 ,:5 a /ick ( seal+-wax or ebonite is rubb$ ) flannel x becomes possess$ ( c]ta9 prop]ties :i* x did n h be=e4 ,x w attract light pieces ( pap] or pi? t are br\ght ne> to x1 x w repel a simil> rubb$ piece ( seal+-wax or ebonite & w attract a rod ( qu>tz :i* has be5 rubb$ ) silk4 ,! qu>tz rod :i* has be5 rubb$ ) silk has ! same prop]ty ( attract+ light bodies :i* ! ebonite & seal+-wax rod has1 b x repels ano!r rubb$ qu>tz rod & attracts a rubb$ ebonite or seal+-wax rod4 ,positive & ,negative ,electrification4--,! ebonite is said to be negatively electrifi$ & ! qu>tz positively electrifi$4 ,:5 ! two rods1 "o positively & ! o!r negatively electrifi$1 are plac$ ne> to "o ano!r1 we may imag9e ! attraction to be due to _! be+ jo9$ by /ret*$ /r+s fill+ up all ! space ar\nd !m4 ,if a v small positively electrifi$ body be plac$ betwe5 ! two x w t5d to move f ! qu>tz to ! ebonite1 i4e4 9 ! direction ( ! arr[s4 ,illu/ration3 ,,fig4 #h4 ,! ,electric ,field4 ,l9es ( ,=ce4--,! space #ai surr\nd+ ! electrifi$ /icks 9 :i* ! =ces due to !m are appreciable is call$ ! electric field1 & ! direction 9 :i* a small positively electrifi$ p>ticle t5ds to move is call$ ! direction ( ! field4 ,! l9es along :i* ! small positive *>ge w\ld move are call$ l9es ( =ce4 ,! conception ( ! electric field z made up ( /ret*$ ela/ic /r+s is1 ( c\rse1 a v crude "o1 b !re is evid5tly "s *ange 9 ! m$ium 9 ! electric field :i* is some:at analog\s to x4 ,illu/ration3 ,,fig4 #i4 ,electric ,oscillations4--,if ! position ( ! two rods is rev]s$1 !n ( c\rse ! direction ( ! field at a po9t betwe5 !m is rev]s$1 & if ?is rev]sal is repeat$ rapidly1 we %all h ! direction ( ! field alt]nat+ rapidly4 ,if ^! alt]nations become suffici5tly rapid !y are convey$ outw>ds 9 mu* ! same way z ! oscillations ( position are convey$ 9 an ord9>y ripple4 ,?us suppose ! two rods are sudd5ly plac$ 9 ! position 9 ! diagram4 ,! field is n e/abli%$ in/antane\sly1 ! l9es ( =ce tak+ a %ort "t to e/abli% !mselves 9 _! ultimate positions4 ,dur+ ?is "t ! l9es ( =ce w be travell+ outw>ds to ,a 9 ! direction ( ! dott$ arr[4 #bj ,be=e !y rea* ,a let u suppose t ! position ( ! rods is rev]s$4 ,!n ! direction ( ! l9es is rev]s$ & ^! rev]s$ l9es w travel outw>ds t[>ds ,a1 foll[+ 9 ! track ( ! orig9al l9es4 ,?us a cont9u\s procession ( l9es ( =ce1 fir/ 9 "o direction & !n 9 ! opposite direction1 w be mov+ out p]p5dicul> to !mselves 9 ! direction ( ! dott$ arr[4 ,?is con/itutes an electric wave4 ,magnetic ,oscillation1 ,l9es ( ,=ce1 & ,field4--,almo/ exactly ! same k9d ( description applies to a magnetic wave4 ,! space ne> to ! ,nor? & ,s\? poles ( a magnet is modifi$ 9 some:at ! same way z t betwe5 ! electrifi$ rods1 & ! magnetic l9es ( =ce are ! l9es along :i* a small ,nor? magnetic pole w\ld move4 ,we may imag9e a rapid alt]nation ( ! magnetic field by ! rapid rev]sal ( ! positions ( ! ,nor? & ,s\? poles1 & we may imag9e ! transmission ( ! alt]nations by means ( ! procession ( magnetic l9es ( =ce4 ,*anges 9 ,magnetic ,field4--,b exp]im5t %[s t :5ev] ! magnetic field at any place is *ang+ an electric field is produc$ dur+ ! alt]ation1 & vice-v]sa4 ,electric & magnetic waves mu/ !re=e always accompany "o ano!r1 & ! two sets ( waves toge!r con/itute electro-magnetic waves4 ,^! are ! waves :i* a huge am\nt ( exp]im5tal evid5ce leads u to believe con/itute heat1 light1 ! electric waves us$ 9 wireless telegraphy1 #ba & ! invisible ultraviolet waves :i* are s active 9 induc+ *emical action4 ,oscillation ( ,electric ,*>ges )9 ! ,atom4--,we h se5 h[ ^! waves might be produc$ by ! oscillation ( two electrifi$ rods1 & x is suppos$ t ! light com+ f lum9\s bodies is produc$ 9 a simil> way4 ,!re are _m reasons = believ+ t !re exi/ 9 ! atoms ( all sub/ances1 m9ute negatively electrifi$ p>ticles :i* may rotate 9 small orbits or oscillate to & fro )9 ! atom4 ,!re also exi/s an equal positive *>ge )9 ! atom4 ,z ! negative p>ticles rotate or oscillate 9 ! atom1 x is evid5t t ! field betwe5 !m & ! positively electrifi$ "p ( ! atom alt]nates1 & s electro-magnetic waves are s5t out4 #bb ,,*apt] ,,ii ,,graphic ,,repres5tation ,,( ,,waves ,a sy/em ( ripples on ! surface ( wat] appe>s 9 v]tical section at any in/ant some:at z 9 ,fig4 #aj4 ,! dott$ l9e ,,ab repres5ts ! undi/urb$ surface ( ! waf]1 & ! solid l9e ! actual surface4 ,if ! di/urbance :i* is caus+ ! ripples is an oscillation ( p]fectly regul> p]iod ! individual ripples w be all alike1 except !y w get %all[] z !y become m remote f ! di/urbance4 ,illu/ration3 ,,fig4 #aj4 ,wave-l5g?4--,! di/ance betwe5 two successive cre/s w be ! same ev]y:]e1 & ?is di/ance or ! di/ance betwe5 any two correspond+ po9ts on two successive ripples is call$ ! wave-l5g?4 ,evid5tly1 ! wave-l5g? is ! di/ance 9 :i* ! :ole wave repeats itself4 ,phase4--,! position ( a po9t 9 ! wave is call$ ! phase ( ! po9t4 ,?us ! diff]5ce ( phase betwe5 ! two po9ts ,a & ,c is a qu>t] #bc ( a wave-l5g?4 ,z ! waves move on along ! surface x is evid5t t ea* drop ( wat] executes an up & d[n oscillation1 & at ! po9ts ,c1 ,c ! drop has rea*$ its highe/ position & at ! po9ts ,t1 ,t its l[e/4 ,amplitude4--,! l>ge/ displacem5t ( ! drop1 i4e4 ! di/ance f ! dott$ l9e to ,c or to ,t1 is call$ ! amplitude ( ! wave4 ,! "t tak5 = a drop to complete "o :ole oscillation1 i4e4 ! "t tak5 = a wave to travel "o :ole wave-l5g? =w>d1 is call$ ! p]iod ( ! wave4 ,! numb] ( oscillations 9 "o second1 i4e4 ! numb] ( wave-l5g?s travell$ 9 "o second1 is call$ ! frequ5cy4 ,illu/ration3 ,,fig4 #aa4 ,al?\gh !re is no visible displacem5t 9 ! waves ( light & heat1 yet we may repres5t !m 9 mu* ! same way4 ,?us if ,,ab1 ,fig4 #aj1 repres5ts ! l9e along :i* a ray ( light is travell+1 ! l5g? ,,np is drawn to scale to repres5t ! value ( ! electric field at ! po9t ,n1 & is drawn upw>ds f ! l9e ,,ab :5 ! field is 9 "o direction & d[nw>ds :5 x is 9 ! opposite direction4 ,?us ! direction ( ! field at diff]5t po9ts 9 ! wave ,,xy1 ,fig4 #aa1 is %[n by ! dott$ arr[s z if due to electrifi$ rods ( qu>tz & ebonite plac$ above & bel[ ,,xy4 ,9 ! case ( ! electromagnetic wave1 ! #bd amplitude w be ! maximum value to :i* ! electric field atta9s 9 ei!r direction1 & ! o!r t]ms--wave-l5g?1 phase1 p]iod & frequ5cy--w h ! same mean+ z = wat] ripples4 ,wave ,=m4--,waves n only diff] 9 amplitude1 wave-l5g?1 & frequ5cy1 b also 9 wave =m4 ,waves may h any =m1 e4g4 ,fig #ab4 ,or we may h a solit>y irregul> di/urbance su* z is caus$ by ! spla% ( a /one 9 wat]4 ,illu/ration3 ,,fig4 #ab4 ,b !re is "o =m ( motion ( a p>ticle 9 a wave :i* is look$ ^u z ! simple/ & fundam5tal =m4 ,x is t =m :i* is execut$ by ! bob ( a p5dulum1 ! balance :eel ( a wat*1 ! prong ( a tun+-=k1 & mo/ o!r vibrations :]e ! controll+ =ce is provid$ by a spr+ or by "s o!r ela/ic solid4 ,x is call$ ,simple ,h>monic ,motion or ,simple ,p]iodic ,motion1 & ! ess5tial feature ( x is t ! =ce re/or+ ! displac$ p>ticle to its undi/urb$ position is proportional to its displacem5t f ! undi/urb$ position4 ,a wave 9 :i* all ! p>ticles execute simple h>monic motion has ! =m 9 ,fig4 #aj or ,fig4 #aa1 :i* is !re=e look$ ^u z ! fundam5tal wave =m or simple wave =m4 ,simple waves w v>y only 9 amplitude1 wave-l5g?1 & frequ5cy1 & ! en]gy 9 ! wave w dep5d ^u ^! quantities4 #be ,en]gy 9 a ,simple ,wave4--,if ! velocity is ! same = all wave-l5g?s1 !n ! frequ5cy w evid5tly be inv]sely proportional to ! wave-l5g? & ! en]gy w dep5d ^u ! amplitude & ! wave-l5g?4 ,! k9etic en]gy ( any mov+ body1 i4e4 ! en]gy due to its motion1 is proportional to ! squ>e ( its velocity1 & we may apply ?is to ! motion ( ! p>ticles 9 a wave & to %[ h[ ! en]gy dep5ds ^u ! amplitude & wave-l5g?4 ,s9ce ! di/ance travell$ by a p>ticle 9 a s+le p]iod ( ! wave w be equal to f\r times ! amplitude1 ! velocity at any po9t 9 ! wave mu/ be proportional to ! amplitude & !re=e ! k9etic en]gy is proportional to ! squ>e ( ! amplitude4 ,) ! same amplitude b ) diff]5t wave-l5g?s1 we see t ! "t 9 :i* ! oscillation is complet$ is proportional to ! wave-l5g? & t ! velocity is !re=e inv]sely proportional to ! wave-l5g?4 ,! k9etic en]gy is !re=e inv]sely proportional to ! squ>e ( ! wave-l5g?4 ,addition ( ,waves4--,! sup]position ( two waves s z to obta9 ! effect ( bo? waves at ! same place is c>ri$ out v simply4 ,! displacem5ts at any po9t due to ! two waves sep>ately are algebraically add$ toge!r1 & ?is sum is ! actual displacem5t4 ,9 ,fig4 #ac ! dott$ l9es repres5t two simple waves1 "o ( :i* has d\ble ! wave-l5g? ( ! o!r4 ,at any po9t ,p on ! solid l9e1 ! displacem5t ,,pn is equal to #bf ! algebraic sum ( ! displacem5t ,,nq due to "o ( ! waves & ,,nr due to ! o!r4 ,! solid l9e1 !re=e1 repres5ts ! result+ wave4 ,we may repeat ?is process = any numb] ( simple waves1 & by suitably *oos+ ! wave-l5g? & amplitude ( ! simple waves we may build up any desir$ =m ( wave4 ,! ma!matician ,f\ri] has %[n t any =m ( wave1 ev5 ! s+le irregul> di/urbance1 c ?us be express$ z ! sum ( a s]ies ( simple waves & t ! wave-l5g?s ( ^! simple waves are equal to ! orig9al wave-l5g?1 "o-half ( x1 "o-?ird1 "o-qu>t]1 "o-fif?1 & s on 9 an inf9ite s]ies4 ,f\ri] has also %[n t only "o su* s]ies is possible = any p>ticul> =m ( wave4 ,illu/ration3 ,,fig4 #ac4 ,! importance ( ?is ma!matical expression lies 9 ! fact t 9 a numb] ( ways ,f\ri]'s s]ies ( simple waves is manufactur$ f ! orig9al wave & ! diff]5t memb]s ( ! s]ies become sep>at$4 ,?us ! mo/ useful way 9 :i* we c repres5t any wave is1 n to draw ! actual =m ( a wave1 b to repres5t :at simple waves g to =m x & to %[ h[ mu* en]gy !re is 9 ea* p>ticul> simple wave4 #bg ,en]gy--,wave-l5g? ,curve4--,?is c be done q simply z 9 ,fig4 #ad4 ,! di/ance ,,pn f ! l9e ,,oa be+ drawn to scale to repres5t ! en]gy 9 ! simple wave ^: l5g? is repres5t$ by ,,on4 ,illu/ration3 ,,fig4 #ad4 ,?us ! simple wave ( l5g? ,,ox has ! greate/ am\nt ( en]gy 9 x4 ,illu/ration3 ,,fig4 #ae4 ,fig4 #ae wall repres5t a simple wave ( wave-l5g? ,,ox1 ! en]gy 9 all ! o!r waves be+ z]o4 #bh ,! ?ree curves giv5 9 ,fig4 #af give a comp>ison ( ! waves f ! sun1 an arc lamp1 & an ord9>y gas-burn]4 ,illu/ration3 ,,fig4 #af4 #bi ,,*apt] ,,iii ,,! ,,mean+ ,,( ,,! ,,spectrum ,! ,spectrum4 ,disp]sion4--,:5 a n>r[ beam ( :ite light is transmitt$ ?r\gh a prism ( glass or ( any o!r transp>5t sub/ance1 x is deflect$ f its orig9al direction & is at ! same "t spread out into a small fan ( rays in/ead ( rema9+ a s+le ray4 ,if a scre5 is plac$ 9 ! pa? ( ^! rays a col\r$ b& is =m$ on x1 ! lea/ deflect$ "p ( ! b& be+ r$ & ! col\rs rang+ f r$ ?r\gh orange1 yell[1 gre51 blue1 & indigo1 to violet at ! mo/ deflect$ end ( ! b&4 ,?is b& ( col\rs is call$ ! spectrum ( ! :ite light us$1 & ! spread+ out ( ! rays is call$ disp]sion4 ,newton's ,exp]im5t4--,newton fir/ discov]$ ?is fact ) an arrangem5t l t 9 ,fig4 #ag4 ,illu/ration3 ,,fig4 #ag4 ,if by any means ! fan ( col\r$ rays be comb9$ aga9 into a s+le beam1 :ite light is re=m$1 & ,newton !re=e came to ! conclusion t :ite light 0 a mixture ( ! v>i\s col\rs 9 ! spectrum1 & t ! only function ( ! prism 0 to sep>ate ! con/itu5ts4 ,( ! nature ( ! con/itu5ts ,newton _h little k1 s9ce he _h reject$ ! wave !ory1 :i* c\ld alone give ! clue4 #cj ,we n[ believe t :ite light is an irregul> wave1 & t ! prism manufactures f x ! ,f\ri]'s s]ies ( waves to :i* x is equival5t4 ,x is suppos$ t ! manufacture is effect$ by means ( ! pr9ciple ( resonance4 ,z an example ( resonance let a small tap be giv5 to a p5dulum j z x comm5ces ea* sw+4 ,!n because ! taps are s tim$ t ea* ( !m increases ! sw+ ( ! p5dulum by a small am\nt1 !y w v soon cause ! p5dulum to sw+ v viol5tly ev5 ?\gh ! effect ( a s+le tap c sc>cely be detect$ at all4 ,?us :5 any body :i* has a free p]iod ( vibration is subject to p]iodic impulses ( ! same p]iod z its own1 x w vibrate v vigor\sly & absorb ne>ly all ! en]gy ( ! impulses4 ,electrons & _! ,vibrations4--,!re is conclusive evid5ce to %[ t 9 ! atoms ( all sub/ances1 & !re=e ( ! glass ( :i* ! prism is compos$1 !re are a numb] ( m9ute negatively electrifi$ p>ticles :i* are call$ electrons4 ,^! are held 9 position by a positive *>ge on ! re/ ( ! #ca atom1 & if !y are displac$ f _! usual positions by any means !y w vibrate ab\t ^! positions4 ,! "t ( vibration ( ! electron w dep5d ^u its position 9 ! atom & ^u ! position ( neighb\r+ atoms4 ,9 solid or liquid bodies ! neighb\r+ atoms are s ne> t !y h a consid]able influ5ce 9 modify+ ! p]iod ( an electron or a sy/em ( electrons1 & consequ5tly we may f9d almo/ any p]iod ( vibration 9 "o or o!r ( ^! electrons or sy/ems4 ,z ! wave ( light ) its alt]nat+ electric fields comes up to ! prism1 ! field w fir/ displace ! electrons 9 "o direction & !n 9 ! o!r1 & s on4 ,if ! p]iod ( "o p>ticul> type ( electron happ5s to co9cide ) ! p]iod ( ! wave1 t electron w vibrate viol5tly & w 9 its turn s5d out a s]ies ( waves 9 ! glass4 ,if ! wave is an irregul> "o x w />t all ! electrons vibrat+1 b ^? electrons w vibrate mo/ viol5tly ^: p]iods are equal to ! p]iods ( ! ,f\ri]'s con/itu5ts :i* h ! greate/ en]gy4 ,?us we %all actually h ! ,f\ri]'s con/itu5t waves sep>at$ into ! vibrations ( diff]5t electrons4 ,b ! spe$ ) :i* any simple wave travels 9 glass or 9 any transp>5t m$ium1 o!r ?an a vacuum1 is dep5d5t ^u its p]iod4 ,! %ort] ! p]iod1 i4e4 ! %ort] ! wave-l5g?1 ! sl[] is ! spe$ 9 mo/ transp>5t sub/ances4 ,b ! sl[] ! spe$ 9 ! prism ! m is ! ray deviat$1 & !re=e we conclude t ! violet end ( ! spectrum consi/s ( ! %orte/ waves :ile ! r$ end consi/s ( ! #cb longe/ waves1 & t ! diff]5t p>ts ( ! spectrum are simple waves ( diff]5t p]iod4 ,! ,:ole ,spectrum4--,! visible spectrum is by no means ! :ole ( ! s]ies ( ,f\ri]'s waves1 h[ev]4 ,! eye is s5sitive only to a v small range ( p]iod1 :ile !re exi/s 9 sunlight a range _m times z great4 ,^? waves ( %ort] p]iod ?an ! violet end ( ! visible spectrum w be deviat$ ev5 m ?an ! violet1 & w !re=e be beyond ! violet4 ,!y are call$ ! ultra violet rays1 & c easily be detect$ by means ( _! *emical activity4 ,!y cause a numb] ( sub/ances to gl[1 & !re=e by coat+ ! scre5 on :i* ! spectrum is receiv$ ) "o ( ^! sub/ances1 ! violet end ( ! spectrum is ext5d$ by ?is gl[4 ,! waves ( long] p]iod ?an ! r$ rays w be deviat$ less ?an ! r$1 & w !re=e lie beyond ! r$ end ( ! visible spectrum4 ,!y are call$ ! infra-r$ rays1 & are *iefly rem>kable = _! heat+ effect4 ,all ! rays are absorb$ :5 !y fall on to a p]fectly dull1 black surface1 & _! en]gy is conv]t$ into heat4 ,?is heat+ effect provides ! be/ way ( measur+ ! en]gy 9 ! diff]5t p>ts ( ! spectrum1 & ( ?us con/ruct+ curves simil> to ^? giv5 9 ,fig4 #af4 ,! in/rum5t moat commonly us$ is call$ ,langley's bolomet]4 ,x consi/s ( a f9e /rip ( black5$ plat9um1 :i* c be plac$ 9 any "p ( ! spectrum at w & ?us absorb ! waves ov] a v small range ( wave-l5g?4 ,x is heat$ by #cc !m1 & ! rise 9 temp]ature is f\nd by measur+ ! electrical resi/ance ( ! /rip4 ,! electrical resi/ance ( all conductors v>ies ) ! temp]ature1 & s9ce resi/ance c be measur$ ) extreme accuracy ?is =ms a v s5sitive & accurate me?od4 ,spectrum ( an ,inc&esc5t ,solid or ,liquid4--,! spectra giv5 by diff]5t s\rces ( light %[ c]ta9 m>k$ diff]5ces4 ,an inc&esc5t solid or liquid gives a cont9u\s spectrum1 i4e4 all ! diff]5t wave-l5g?s are repres5t$1 b ! "p ( ! spectrum :i* has ! greate/ en]gy is diff]5t = diff]5t sub/ances & = diff]5t temp]atures3 cf4 arc & gas flame 9 ,fig4 #af4 ,?is is q 9 keep+ ) ! idea already sugge/$ t 9 solids & liquids !re are electrons ( almo/ e p]iod ( vibration4 ,:5 !y are agitat$ by be+ heat$1 a mixture ( simple waves ( all p]iods w be s5t out giv+ a v irregul> wave4 ,gases may also become inc&esc5t4 ,?us :5 any comp\nd ( sodium is put into a col\rless flame ! flame becomes col\r$ an int5se yell[4 ,?is is due to ! vap\r ( sodium1 & ! agitation ( ! electrons 9 x is probably due to ! *emical action 9 :i* ! comp\nd is split up into sodium & "s o!r p>ts4 ,we may also make ! gas inc&esc5t by enclos+ x at l[ pressure 9 a vacuum tube & pass+ an electrical dis*>ge ?r\gh x4 ,! gl[ 9 ! tube gives ! spectrum ( ! gas4 ,inc&esc5t gases give a v *>act]i/ic k9d ( spectrum4 #cd ,x consi/s usually ( a limit$ numb] ( n>r[ l9es1 ! re/ ( ! spectrum be+ almo/ p]fectly d>k4 ,! light !re=e consi/s ( a few simple waves ( p]fectly def9ite p]iod4 ,?is w\ld sugge/ t 9 ! atom ( a gas !re are only a few electrons :i* are conc]n$ 9 ! emission ( ! light waves4 ,?us ! spectra ( gases & ( inc&esc5t solids are repres5t$ 9 "* by ! curves 9 ,fig4 #ah4 ,illu/ration3 ,,fig4 #ah4 ,spectrum ,analysis4--,! l9es 9 a gas spectrum are s %>ply def9$ & are s def9itely *>act]i/ic ( ! p>ticul> gas t !y s]ve z a delicate me?od ( detect+ ! pres5ce ( "s elem5ts4 ,^! spectra :i* are emitt$ by inc&esc5t bodies are call$ emission spectra4 ,b n only d diff]5t mat]ials emit diff]5t k9ds ( light :5 rais$ to inc&esc5ce1 b !y also absorb light diff]5tly :5 x passes ?r\gh !m4 ,:5 :ite light is pass$ ?r\gh "s transp>5t solids or liquids & !n ?r\gh a prism1 x is f\nd t :ole regions ( ! spectrum are abs5t4 ,?us a potassium p]manganate solution #ce :i* is n too conc5trat$ absorbs ! :ole ( ! middle "p ( ! spectrum1 all[+ ! r$ & blue rays to pass ?r\gh4 ,s9ce ) solids & liquids ! absorb$ regions are l>ge & some:at ill-def9$1 ! absorption spectra are n ( any great use 9 ! detection ( sub/ances4 ,! absorption spectra ( gases %[ ! same %>ply def9$ *>act]i/ics z ! emission spectra4 ,?us if :ite light f an arc lamp passes ?r\gh a flame col\r$ yell[ ) sodium vap\r1 ! spectrum ( ! issu+ light has two %>ply def9$ n>r[ d>k l9es close toge!r 9 ! yell[ "p ( ! spectrum 9 exactly ! same position z ! two bright yell[ l9es :i* inc&esc5t sodium vap\r itself gives out4 ,! flame has !re=e absorb$ j ^? waves :i* x gives out4 ,?is is p]fectly g5]al1 & applies to solids & liquids z well z to gases4 ,x is p]fectly 9 keep+ ) our view ( ! refraction ( light by ! resonance ( electrons to ! ,f\ri]'s con/itu5ts :i* h ! same p]iod4 ,= if ! electrons h a c]ta9 p]iod ( vibration !y w res\nd to waves ( t p]iod & !re=e absorb _! en]gy4 ,spectrum ( ! ,sun4--,"o ( ! mo/ int]e/+ examples ( ! absorption by inc&esc5t gases ( _! own *>act]i/ic l9es is provid$ by ! sun4 ,! spectrum ( ! sun is cross$ by a l>ge numb] ( f9e d>k l9es :i* 7 mapp$ out by ,fraunhf] & are !re=e call$ ,fraunhf] l9es4 ,^! l9es are f\nd to be 9 ! position ( ! *>act]i/ic l9es ( a numb] ( kn[n elem5ts1 #cf & !re=e we assume t ^! elem5ts are pres5t 9 ! sun4 ,! int]ior ( ! sun is liquid or solid ow+ to ! pressure ( ! mass r\nd x4 ,x !re=e emits a cont9u\s spectrum4 ,b ! light has to pass ?r\gh ! out] lay]s ( inc&esc5t vap\r1 & ^! lay]s absorb f ! light _! *>act]i/ic waves & s produce ! d>k l9es 9 ! spectrum4 ,! spectra ( />s %[ simil> *>act]s to ^? ( ! sun1 & !re=e we assume !m to be 9 ! same condition z ! sun4 ,! spectra ( nebul consi/ only ( bright l9es1 & we !re=e assume t nebul consi/ ( inc&esc5t masses ( gas :i* h n yet cool$ en\gh to h liquid or solid nuclei4 #cg ,,*apt] ,,iv ,,! ,,laws ,,( ,,radiation ,absorb+ ,p[]4--,a p]fectly dull black surface is simply "o :i* absorbs all ! light :i* is fall+ on x & reflects or diffuses none ( x back4 ,if ! surface absorbs ! heat z well z ! light completely1 x is call$ a p]fect or full absorb]4 ,o!r surfaces m]ely absorb a fraction ( ! heat & light fall+ on !m1 & ?is fraction1 express$ usually z a p]c5tage1 is call$ ! absorb+ p[] ( ! surface4 ,! absorb+ p[]s ( diff]5t k9ds ( surfaces c be measur$ 9 a great _m ways1 b ! foll[+ may be tak5 z fairly typical4 ,a p]fectly /eady beam ( heat & light is made to fall on a small metallic disc1 & ! am\nt ( heat :i* is absorb$ p] second is calculat$ f ! mass ( ! metal & ! rate at :i* its temp]ature rises4 ,! disc is fir/ coat$ ) lamp-black1 & ! rate at :i* x !n receives heat is tak5 z ! rate at :i* a full absorb] absorbs heat "u ^! conditions4 ,! disc is !n coat$ ) ! surface ^: absorb+ p[] is to be measur$1 & ! exp]im5t is repeat$4 ,!n ! rate at :i* heat is receiv$ 9 ! second case divid$ by ! rate at :i* x is receiv$ 9 ! fir/ is ! absorb+ p[] ( ! second surface4 #ch ,exp]im5ts ) a l>ge numb] ( surfaces %[ t ! light] 9 col\r & ! m poli%$ is ! surface1 ! small] is its absorb+ p[]4 ,radiat+ ,p[]4--,b ! "* ( ! surface affects n only ! rate at :i* heat & light are absorb$1 b also ! rate at :i* !y are emitt$4 ,= example1 if we heat a fragm5t ( a will[ patt]n *9a plate 9 a bl[pipe flame until x is bright r$ hot1 we %all notice t ! d>k patt]n n[ /&s out bright] ?an ! re/4 ,?us ! d>k patt]n1 :i* absorbs m ( ! light :i* falls on x :5 x is cold1 emits m light ?an ! re/ ( ! plate :5 x is hot4 ,?is is "o example ( a g5]al rule1 = x is f\nd t ! mo/ p]fect absorb]s are ! greate/ radiators1 & vice-v]sa4 ,! p]fectly black surface is !re=e tak5 z a /&>d 9 measur+ ! heat & light emitt$ by surfaces1 9 exactly ! same way z = heat & light absorb$4 ,?us ! emissive or radiat+ p[] ( a surface is def9$ z ! quantity ( heat radiat$ p] second by ! surface divid$ by ! am\nt radiat$ p] second by a p]fectly black surface "u ! same conditions4 ,z x is some:at p>adoxical to call a surface a p]fectly black surface :5 x may ev5 be :ite hot1 ! t]m a full radiator has be5 sugge/$ z an alt]native & w be us$ 9 ?is book4 ,illu/ration3 ,,fig4 #ai ,relation betwe5 ,absorb+ & ,radiat+ ,p[]s4--,! exact relation betwe5 ! absorb+ & radiat+ p[]s ( a surface 0 fir/ det]m9$ by ,rit*ie by means ( an +5i\s exp]im5t4 ,two equal air-tight metal *amb]s ,a & ,b 7 connect$ by a glass tube b5t twice at "r angles z #ci 9 ,fig4 #ai4 ,a drop ( m]cury 9 ! horizontal "p ( ?is tube act$ z an indicator4 ,:5 "o ( ! vessels became hott] ?an ! o!r1 ! air 9 x exp&$ & ! m]cury index mov$ t[>ds ! cold] side4 ,betwe5 ! two metal *amb]s a ?ird equal "o 0 m\nt$ :i* c\ld be heat$ up by p\r+ boil+ wat] into x & c\ld ?us act z a radiator to ! o!r two4 ,"o surface ( ?is radiator 0 coat$ ) lamp-black & ! opposite "o ) ! surface "u inve/igation1 e4g4 c9nab>4 ,! inn] surfaces ( ! o!r two vessels 7 coat$ 9 ! same way1 ! "o ) lamp-black1 ! o!r ) c9nab>4 ,! middle vessel 0 fir/ plac$ s t ! lamp-black$ surface 0 opposite to a c9nab> "o1 & vice-v]sa4 ,9 ?is position1 :5 hot wat] 0 p\r$ into x no movem5t ( ! m]cury drop 0 detect$1 & !re=e ! am\nts ( heat receiv$ by ! two out] vessels mu/ h be5 exactly equal4 ,on ! "o side ! heat giv5 out by ! c9nab> surface ( ! middle vessel is only a fraction1 equal to its radiat+ p[]1 ( ! heat giv5 out by ! black surface4 ,all ! heat giv5 out by ! c9nab> surface to ! black surface opposite to x is absorb$1 h[ev]1 :ile ( ! heat giv5 out by ! black surface to ! c9nab> surface opposite x only a fraction is absorb$ equal to ! absorb+ p[] ( ! c9nab> surface4 ,?us on ! "o side only a fraction is s5t out b all ( x is absorb$1 & on ! o!r side all is s5t out & only a fraction absorb$4 ,s9ce #dj ! quantities absorb$ are exactly equal1 x is obvi\s t ! two fractions mu/ be exactly equal1 or ! absorb+ & radiat+ p[]s ( any surface are exactly equal4 ,?is result is kn[n z ,kir*(f's law1 & x applies solely to radiation :i* is caus$ by temp]ature4 ,lat] exp]im5ts h %[n t x applies to ea* individual wave-l5g?1 i4e4 to any portion ( ! spectrum :i* we isolate1 z well z to ! :ole radiation4 ,?us at any p>ticul> temp]ature let ! dott$ l9e 9 ,fig4 #bj repres5t ! wave-l5g?--en]gy curve = a full radiator1 & let ! solid l9e repres5t x = ! surface "u inve/igation4 ,!n = any wave-l5g?1 ,,on1 ! radiat+ p[] ( ! surface w\ld be equal to ,,qn divid$ by ,,pn4 ,illu/ration3 ,,fig4 #bj4 ,n[ a wave-l5g?--en]gy curve may be z easily con/ruct$ = absorb$ z = emitt$ radiation by means ( a ,langley's bolomet]4 ,! /rip ( ! bolomet] is fir/ coat$ ) lamp-black & ! spectrum ( ! incid5t radiation is explor$ 9 exactly ! same way z is describ$ 9 ,*apt] ,,iii4 #da ,! /rip is !n coat$ ) ! surface "u inve/igation & ! spectrum is aga9 explor$4 ,s9ce ! incid5t radiation is exactly ! same 9 ! two exp]im5ts1 ! diff]5ces 9 ! quantities ( heat absorb$ mu/ be due solely to ! diff]5ce 9 ! absorb+ p[]s ( ! two surfaces4 ,9 ,fig4 #ba ! dott$ l9e repres5ts ! wave-l5g?--en]gy curve = ! radiation absorb$ by ! black5$ bolomet] /rip1 & ! solid l9e ! curve = ! /rip coat$ ) ! surface "u inve/igation4 ,illu/ration3 ,,fig4 #ba4 ,! actual =m ( ! curves may & probably w be q diff]5t f ! =m 9 ,fig4 #bj1 b x w be f\nd = ! same wave-l5g? ,,on t ,,pn_/,,qn is exactly ! same 9 ! two figures4 ,x has already be5 m5tion$ t dull1 d>k-col\r$ surfaces radiate ! mo/ heat1 & t poli%$ surfaces radiate ! lea/4 ,a radiator = heat+ a room %\ld !re=e h a dull1 d>k surface1 :ile a vessel :i* is design$ to keep its cont5ts f los+ heat %\ld h a highly poli%$ ext]ior4 ,a p]fectly transp>5t sub/ance w\ld radiate no en]gy1 :atev] ! temp]ature to :i* x is #db rais$1 = its absorb+ p[] is z]o & !re=e its radiat+ p[] is also z]o4 ,no p]fectly transp>5t sub/ances exi/1 b "s sub/ances are a v ne> approa* to x4 ,a fus$ bead ( microcosmic salt heat$ 9 a small loop ( plat9um wire 9 a bl[pipe flame may be rais$ to su* a temp]ature t x is q pa9ful to look at ! plat9um wire1 yet ! bead itself is sc>cely visible at all4 ,any speck ( metallic du/ on ! surface ( ! bead w at ! same "t %9e out l a bright />4 ,gases z ,radiators4--,mo/ gases are an ev5 ne>] approa* to ! p]fectly transp>5t sub/ance1 & consequ5tly1 ) "o or two exceptions1 ! simple heat+ ( gases causes no appreciable radiation f !m4 ,( c\rse1 gases d radiate heat & light "u "s circum/ances1 b ! radiation seems to be produc$ ei!r by *emical action1 z 9 ! flames col\r$ by metallic vap\rs1 or by electric dis*>ge1 z 9 vacuum tubes1 ! arc or ! electric sp>k4 ,! agitation ( ! electrons is ?us produc$ 9 a diff]5t way 9 gases1 & we mu/ n apply ,kir*(f's law to !m1 al?\gh at fir/ sight !y appe> to con=m to x4 ,we h se5 t ! p>ticul> waves :i* an inc&esc5t gas radiates are also absorb$ by x4 ,?is we %\ld expect1 because ! p>ticul> electron :i* has su* a p]iod ( vibration t x s5ds out a c]ta9 wave-l5g? w naturally be 9 tune to exactly simil> waves :i* fall on x1 & w s res\nd to !m1 & absorb _! en]gy4 ,! quantitative law1 h[ev]1 t ! absorb+ p[] is exactly equal to ! radiat+ p[]1 is n true = gases4 #dc ,emission ( ,pol>is$ ,light4--,"o v int]e/+ result ( ,kir*(f's law is ! emission ( pol>iz$ light by gl[+ t\rmal9e & by "o or two o!r cry/al :5 !y are heat$ to inc&esc5ce4 ,9 ord9>y light ! vibrations are 9 all directions p]p5dicul> to ! l9e along w9* ! light travels1 t is1 ! vibrations at any po9t are 9 a plane p]p5dicul> to ?is l9e4 ,n[ any vibration 9 a plane may be express$ z ! sum ( two compon5t vibrations1 "o compon5t 9 "o direction & ! o!r 9 a p]p5dicul> direction4 ,if we divide up ! vibrations all along ! wave 9 ?is way we %all h two waves1 "o ( :i* has its vibrations all 9 "o direction & ! o!r 9 a p]p5dicul> direction4 ,su* waves1 9 :i* ! vibrations all lie 9 "o plane1 are said to be plane pol>is$4 ,t\rmal9e is possess$ ( ! curi\s prop]ty ( absorb+ vibrations 9 "o direction ( ! cry/al mu* m rapidly ?an x does ^? vibrations p]p5dicul> to ?is direction1 & !re=e light :i* passes ?r\gh x em]ges p>tially1 or 9 "s cases :olly1 plane pol>is$4 ,s9ce ! absorb+ p[] ( t\rmal9e is diff]5t = ! two compon5ts1 ! emissive p[] %\ld also be diff]5t1 & t compon5t :i* 0 mo/ absorb$ %\ld be radiat$ mo/ /rongly4 ,?is 0 f\nd to be true by ,kir*(f himself1 :o detect$ & r\ghly measur$ ! pol>is$ light emitt$4 ,subsequ5tly 9 #aijb1 ,pflg] c>ri$ out exact exp]im5ts :i* gave a beautiful confirmation ( ! law4 #dd ,,*apt] ,v ,,full ,,radiation ,! ,full ,radiator4--,we h assum$ t a lamp-black$ surface is a p]fect absorb]1 & consequ5tly a full radiator1 b al?\gh x is a v ne> approa* to ! ideal x is n absolutely p]fect4 ,no actual surface is a p]fectly full radiator1 b ! exact equival5t ( "o has be5 obta9$ by an +5i\s device4 ,a holl[ vessel :i* is black5$ on ! inside has a small ap]ture ?r\gh :i* ! radiation f ! int]ior ( ! vessel c escape4 ,if ! vessel is heat$ up1 !re=e1 ! small ap]ture may act z a radiator4 ,! radiation :i* em]ges ?r\gh ! ap]ture f any small area on ! int]ior ( ! vessel is made up ( two p>ts1 "o "p :i* x radiates itself1 & ! o!r "p :i* x scatt]s back f ! radiation :i* x receives f ! o!r p>ts ( ! int]ior ( ! vessel4 ,^! two toge!r are equal to ! en]gy s5t out by a full radiator1 & !re=e ! small ap]ture acts z a full radiator3 e4g4 suppose ! inn] surface has an absorb+ p[] ( #ij p] c5t41 !n x radiates #ij p] c5t4 ( ! full radiation & absorbs #ij p] c5t4 ( ! radiation com+ up to x !re=e scatt]+ back #aj p] c5t4 ,we h !re=e com+ f ! inn] surface #ij p] c5t4 #de radiat$ & #aj p] c5t4 scatt]$1 & ! radiat$ & scatt]$ toge!r make #ajj p] c5t4 ,illu/ration3 ,,fig4 #bb4 ,"o =m 9 :i* su* radiators h be5 us$ is %[n 9 section 9 ,fig4 #bb4 ,a d\ble wall$ cyl9drical vessel ( brass has a small hole1 a1 9 "o end4 ,/eam c be pass$ ?r\gh ! space betwe5 ! d\ble walls1 ?us keep+ ! temp]ature ( ! inn] surface at #ajj ,c4 ,a scre5 ) a hole 9 x j opposite to ! hole 9 ! vessel1 or r sev]al su* scre5s1 are plac$ 9 front ( ! vessel 9 ord] to %ield any measur+ in/rum5t f any radiation except t em]g+ ?r\gh ! hole4 ,! ,full ,absorb]4--,9 an exactly simil> way an ap]ture 9 a holl[ vessel w act z a full absorb]1 = ! fraction ( ! incid5t radiation :i* is scatt]$ on ! inn] surface aga9 imp+es on ano!r portion ( ! surface & s all is ultimately absorb$ except a m9ute fraction :i* is scatt]$ out aga9 ?r\gh ! ap]ture4 ,! v>iation 9 ! heat radiat$ by a full radiator at diff]5t temp]atures =ms a v important "p ( ! /udy ( radiation1 & a v l>ge numb] ( exp]im5ts & !oretical inve/igations h be5 devot$ to x4 ,^! inve/igations may be divid$ into two sections3 ^? conc]n$ ) ! total quantity ( heat radiat$ at diff]5t temp]atures & ^? conc]n$ ) ! v>iation 9 ! "* ( ! spectrum ) v>y+ temp]atures4 #df ,! exp]im5ts 9 ! fir/ section h be5 c>ri$ out ma9ly 9 two ways4 ,9 ! fir/1 ! rate ( cool+ ( ! full radiator has be5 det]m9$1 & f ! rate ( cool+ at any temp]ature ! rate at :i* heat 0 lo/ by radiation 0 imm$iately calculat$4 ,newton 0 ! fir/ to inve/igate 9 ?is way by obs]v+ ! rate at :i* a !rmomet] bulb cool$ d[n :5 x 0 surr\nd$ by an enclosure :i* 0 kept at a uni=m temp]ature4 ,he f\nd t ! rate ( cool+1 & !re=e ! rate at :i* heat 0 lo/ by ! !rmomet]1 0 proportional to ! diff]5ce ( temp]ature betwe5 ! !rmomet] & its surr\nd+s4 ,?is rule is kn[n z ,newton's ,law ( ,cool+1 & is /ill us$ :5 x is desir$ to correct = ! heat lo/ dur+ an exp]im5t :]e ! temp]ature diff]5ces are small4 ,x is only true1 h[ev]1 = v small diff]5ces ( temp]ature betwe5 ! !rmomet] & its surr\nd+s1 & z e>ly z #agdj ,m>t9e _h f\nd t x 0 only true = a v limit$ range ( temp]ature4 ,prvo/'s ,!ory ( ,ex*anges4--,9 #agib1 ,prvo/ ( ,g5eva1 :5 endeav\r+ to expla9 ! suppos$ radiation ( cold1 introduc$ ! l9e ( ?\ght1 t any body is n to be reg>d$ z radiat+ heat only :5 its temp]ature is fall+1 or absorb+ heat only :5 its temp]ature is ris+1 b t bo? processes are cont9ually & simultane\sly go+ on4 ,! am\nt ( heat radiat$ w dep5d on ! temp]ature & "* ( ! body itself1 :ile ! am\nt absorb$ w dep5d ^u ! condition ( ! surr\nd+s z well z ^u ! nature #dg ( ! body4 ,if ! am\nt ( heat radiat$ is great] ?an ! am\nt absorb$ ! body w fall 9 temp]ature1 & vice-v]sa4 ,?is view ( ,prvo/'s is call$ ! ,!ory ( ,ex*anges1 & we c see t x is a necess>y consequ5ce ( our ideas z to ! production ( heat & light waves by ! agitation ( electrons 9 ! radiat+ body4 ,if ! rate ( cool+ ( a body at a c]ta9 temp]ature is measur$ :5 x is plac$ 9 an enclosure at a l[] temp]ature1 x mu/ be borne 9 m9d t ! rate ( loss ( heat is equal to ! rate at :i* heat is radiat$ m9us ! rate at :i* x is absorb$ f ! enclosure4 ,a second way 9 :i* ! heat lo/ by a body has be5 measur$ at diff]5t temp]atures is by heat+ a conductor su* z a ?9 plat9um /rip by means ( an electric curr5t1 & measur+ ! temp]ature to :i* ! conductor has atta9$4 ,:5 its temp]ature is /eady1 all ! en]gy giv5 to x by ! curr5t mu/ be lo/ z heat1 & !re=e ! electrical en]gy1 :i* c v easily be calculat$1 mu/ be equal to ! heat radiat$ by ! body m9us ! heat receiv$ f ! enclosure4 ,s _m attempts h be5 made to e/abli%1 by "o or o!r ( ^! two me?ods1 ! relation betwe5 ! quantity ( heat radiat$ & ! temp]ature1 t x is impossible to give ev5 a pass+ ref]5ce to mo/ ( !m4 ,un=tunately1 ! results d n %[ ! agreem5t ) "o ano!r :i* we w\ld l1 b probably ! mo/ correct result is t /at$ by ,/efan 9 #ahgh1 aft] a close inspection ( ! exp]im5tal results ( ,dulong & #dh ,petit4 ,he /at$ t ! quantity ( heat radiat$ p] second by a full radiator is proportional to ! f\r? p[] ( its absolute temp]ature4#a ,?us ! quantity ( heat radiat$ by "o squ>e c5timetre ( ! surface ( a full radiator ^: absolute temp]ature is ,t1 is equal to ,,et7sup7#d1 :]e ,e is "s con/ant multipli] :i* mu/ be det]m9$ by exp]im5t & :i* is call$ ! radiation con/ant4 ,if ! absolute temp]ature ( ! enclosure 9 :i* ! surface is plac$ is ,t1 !n ! rate at :i* ! surface is los+ heat w be ,e7,t7sup7#d-,t7sub7#a7sup7#d71 = x w receive heat at ! rate ,,et7sub7#a7sup7#d & w radiate x at ! rate ,,et7sup7#d4 #a ,see page #ef4 ,/efan's f\r? p[] law has be5 v]ifi$ by a numb] ( good exp]im5ts1 notably ^? ( ,lumm] & ,pr+%eim 7,congrs ,int]national de ,physique1 ,vol4 ,,ii4 p4 #gh71 s t al?\gh "s exp]im5ts d n agree ) x1 we are probably ju/ifi$ 9 tak+ x z correct4 ,9 #ahhd ,boltzmann add$ /ill fur!r evid5ce 9 support ( ?is law by d]iv+ x !oretically4 ,he appli$ to a space conta9+ ! waves ( full radiation ! two kn[n laws :i* gov]n ! trans=mation ( en]gy1 by imag9+ ! space to be tak5 ?r\gh a cycle ( compressions & expansions 9 j ! same way z a gas is compress$ & exp&$ 9 :at is kn[n z ,c>not's cycle4 ,v>iation ( ,spectrum ) ,temp]ature4--,! v>iation ( ! "* ( ! spectrum ( a full radiator has be5 det]m9$ ma9ly by ! use ( ,langley's bolomet]1 b ! g5]al nature ( ! *ange may be readily obs]v$ by ! eye4 #di ,z ! temp]ature ( a full radiator rises x fir/ gives out only invisible heat waves2 z soon z its temp]ature exce$s ab\t #ejj ,c4 x beg9s to emit "s ( ! longe/ visible rays2 & z ! temp]ature rises fur!r1 m & m ( ! visible rays 9 ! spectrum are emitt$ until1 :5 ! radiator is :ite hot1 ! :ole ( ! visible spectrum4 is produc$4 ,?us ! high] ! temp]ature ( ! radiator ! m ( ! %ort] waves are produc$4 ,illu/ration3 ,,fig4 #bc4 ,by means ( ,langley's bolomet] ! di/ribution ( en]gy 9 ! spectrum has be5 measur$ accurately1 ) ! results ( confirm+ & amplify+ ! g5]al results j /at$4 ,! en]gy 9 ! spectrum ( ev5 ! hotte/ ( t]re/rial radiators is mo/ly 9 ! long] waves ( ! infra-r$1 b ! position ( ! maximum ( en]gy moves to %ort] & %ort] wave-l5g?s z ! temp]ature rises1 & s m ( ! %ort] waves make _! appe>ance4 ,! sun is n a full radiator1 b is ne>ly s1 & its temp]ature is s high t ! maximum ( en]gy 9 its spectrum is 9 ! visible "p ne> to ! r$ end4 #ej ,fig4 #bc %[s ! results obta9$ by ,lumm] & ,pr+%eim1 & br+s out cle>ly ! %ift ( ! maximum ) ris+ temp]ature & also ! position ( ! greate/ "p ( ! en]gy 9 ! infr>$ region4 ,wi5's ,laws4--,exam9ation ( ! results also %[s t ! wave-l5g? at :i* ! maximum en]gy occurs is inv]sely proportional to ! absolute temp]ature & t ! actual en]gy at ! maximum po9t is proportional to ! fif? p[] ( ! absolute temp]ature4 ,^! two results h bo? be5 d]iv$ !oretically by ,wi5#b 9 a simil> way to t 9 :i* ,boltzmann d]iv$ ,/efan's f\r? p[] law1 i4e4 by imag9+ a space fill$ ) ! radiation to be tak5 ?r\gh a cycle ( compressions & r>efactions4 #b ,wi$4 ,ann41 #df1 p4 #fcc2 #eb1 p4 #acb4 ,wi5 d]iv$ an amplification ( ! la/ result by %[+ t if a wave-l5g? 9 ! spectrum ( a full radiator at "o temp]ature & ano!r wave-l5g? 9 ! spectrum at ano!r temp]ature are s relat$ z to be inv]sely proportional to ! two absolute temp]atures1 !y may be said to correspond to ea* o!r1 & ! en]gy 9 correspond+ wave-l5g?s at diff]5t temp]atures is proportional to ! fif? p[] ( ! absolute temp]ature4 ,we see !re=e t if ! di/ribution ( en]gy 9 ! spectrum ( ! full radiator be kn[n at any "o temp]ature x may be calculat$ = any o!r temp]ature by apply+ ^! two laws ( correspond+ wave-l5g?s & ! en]gy 9 !m4 ,nei!r ( !m give u any in=mation1 h[ev]1 #ea ab\t ! actual di/ribution ( en]gy at any "o temp]ature f :i* we may calculate t at any o!r temp]ature4 ,= t1 "s relation mu/ be f\nd betwe5 ! en]gy & ! wave-l5g?4 ,planck1 by reason+ f\nd$ on ! electromagnetic "* ( ! waves1 d]iv$ su* a relation1 b bo? 8 reason+ & 8 results are a little too complicat$ to be introduc$ "h4 ,8 results h be5 confirm$ 9 ! mo/ /rik+ mann] by exp]im5ts c>ri$ out by ,rub5s & ,kurlbaum 7,ann4 d] ,physik1 #d1 p4 #fdi1 #aija74 ,!y measur$ ! en]gy 9 a p>ticul> wave-l5g? 74#jjea cms41 i4e4 ne>ly #ajj times ! wave-l5g? ( r$ light7 9 ! radiation ( a full radiator f a temp]ature ( #he up to #aggc absolute1 & _! results are giv5 9 ! foll[+ table3 ,absolute ,temp]ature4 ,obs]v$ ,en]gy4 ,en]gy calculat$ f ,planck's ,=mula4 #he -#bj4f -#ba4i #aic -#aa4h -#ab #bic #j #j #ebc #ca #cj4d #ggc #fd4f #fc4h #ajbc #ih4a #ig4b #abgc #acb #acb #aebc #afd #afj #aggc #aif #bjj ,we h !re=e ! means ( calculat+ bo? ! total quantity & ! k9d ( radiation giv5 out by any full radiator at any temp]ature1 & a numb] ( v int]e/+ problems may be solv$ by means ( ! results4 #eb ,effici5cy 9 ,light+4--,"o v simple problem is conc]n$ ) effici5cy 9 light+4 ,we see by ref]5ce to ,fig4 #af1 t 9 ! radiation f ! electric arc v little ( ! en]gy is 9 ! visible "p ( ! spectrum ev5 ?\gh ! temp]ature 9 ! arc is ! highe/ yet obta9$ on ! e>?1 :]eas ! en]gy 9 ! visible "p ( ! spectrum f a gas flame is almo/ :olly negligible4 ,! problem ( effici5t light+ is to get z big a proportion z possible ( ! en]gy into ! visible "p ( ! spectrum1 & !re=e ! high] ! temp]ature ! great] ! effici5cy4 ,?is is ! reason ( ! great] effici5cy ( ! inc&esc5t gas mantle ov] ! ord9>y gas burn]1 = ! introduction ( ! air into ! gas all[s ! combu/ion to be mu* m complete1 & !re=e ! temp]ature ( ! mantle becomes v mu* high] ?an t ( ! c>bon p>ticles 9 ! ord9>y flame4 ,! mod]n metallic filam5t electric lamps h filam5ts made ( metals ^: melt+ po9t is extremely high1 & !y may !re=e be rais$ to a mu* high] temp]ature ?an ! old] c>bon filam5ts4 ,! arc is ev5 m effici5t ?an ! metallic filam5t lamps1 because its temp]ature is high] /ill2 & we mu/ assume t ! temp]ature ( ! sun is v mu* high] ev5 ?an ! arc1 s9ce its maximum ( en]gy lies 9 ! visible spectrum4 ,temp]ature ( ! ,sun4--,! actual temp]ature ( ! sun may be calculat$ approximately by means ( ,/efan's f\r? p[] law4 ,we w fir/ assume t ! e>? & ! sun are bo? full radiators1 & #ec t ! e>? is a good conductor1 s t its temp]ature is ! same all ov]4 ,! fir/ assumption is v ne>ly true1 & we w make a correction = ! small error x introduces2 & ! second1 al?\gh f> f true1 makes v little diff]5ce to ! f9al result1 = x is f\nd t ! values obta9$ on ! opposite assumption t ! e>? is an absolute non-conductor diff] by less ?an #b p] c5t4 f ^? calculat$ on ! fir/ assumption4 ,we w fur!r assume t ! heat radiat$ out by ! e>? is exactly equal to ! heat :i* x receives f ! sun4 ,?is is sc>cely an assumption1 b r an exp]im5tal fact1 = exp]im5t %[s t heat is conduct$ f ! int]ior ( ! e>? to ! ext]ior1 & s is radiat$1 b at su* a small rate t x is p]fectly negligible comp>$ ) ! rate at :i* ! e>? is receiv+ heat f ! sun4 ,! sun occupies j ab\t "o #id1jjj? "p ( ! hemisph]e ( ! heav5s or "o #ahh1jjj? "p ( ! :ole sph]e4 ,if ! :ole sph]e surr\nd+ ! e>? 7 ( sun brightness1 ! e>? w\ld be 9 an enclosure at ! temp]ature ( ! sun1 & w\ld !re=e be at t temp]ature itself4 ,! sph]e w\ld be s5d+ heat at #ahh1jjj times ! rate at :i* ! sun is s5d+ x1 & ! e>? w\ld be radiat+ x at #ahh1jjj times its pres5t rate4 ,b ! rate at :i* x radiates is proportional to ! f\r? p[] ( its absolute temp]ature1 & !re=e its temp]ature w\ld be ! f\r? root ( #ahh1jjj times its pres5t temp]ature1 i4e4 #bj4h times4 ,if ! radiat+ or absorb+ p[] ( ! e>?'s surface be tak5 z #i_/aj1 :i* is some:]e ne> ! m>k1 #ed ! calculation gives ! numb] #ba4e in/ead ( #bj4h4 ,! av]age temp]ature ( ! e>?'s surface is probably ab\t #ag ,c4 or #bij absolute1 & !re=e ! temp]ature ( ! sun is #bij x #ba4e1 i4e4 ab\t #fbjj absolute4 ,x is easy to see t if we _h kn[n ! temp]ature ( ! sun & n ( ! e>?1 we c\ld h calculat$ t ( ! e>? by rev]s+ ! process4 ,by ?is means we c e/imate ! temp]atures ( ! o!r planets1 at any rate ( ^? = :i* we may make ! same assumptions z = ! e>?4 ,probably ^? planets :i* are v mu* l>g] ?an ! e>? are /ill radiat+ a consid]able am\nt ( heat ( _! own1 & !re=e to !m ! calculation w n apply2 b ! small] planets ,m]cury1 ,v5us & ,m>s h probably already radiat$ ne>ly all _! own heat & are n[ radiat+ only su* heat z !y receive f ! sun4 ,! temp]atures calculat$ 9 ?is way are-- ,av]age ,absolute ,temp]ature ,m]cury 4 4 4 4 4 4 4 4 4 #dfg ,v5us 4 4 4 4 4 4 4 4 4 4 #cdb ,e>? 4 4 4 4 4 4 4 4 4 4 #bij ,m>s 4 4 4 4 4 4 4 4 4 4 #bce ,s9ce ! freez+ po9t ( wat] is #bgc absolute1 we see t ! av]age temp]ature ( ,m>s is #ch ,c4 bel[ freez+1 & x is almo/ c]ta9 t no "p ( ,m>s ev] gets above freez+ po9t4 ,9 a v simil> way we may f9d ! temp]ature to :i* a non-conduct+ surface rea*es :5 x is expos$ to full sunlight by equat+ ! heat absorb$ to ! heat radiat$1 & ! result comes #ee to #dab absolute1 i4e4 #aci ,c41 or consid]ably above boil+ po9t4 ,?is w\ld be ! upp] limit to ! temp]ature ( ! surface ( ! moon at a po9t :]e ! sun is at its z5i?4 ,on ! surface ( ! e>? ! sunlight has _h to pass ?r\gh ! atmosph]e1 & 9 p]fectly bright sun%9e x is e/imat$ t only ?ree-fif?s ( ! heat is transmitt$4 ,any surface is also radiat+ out into surr\nd+s :i* are at ab\t #cjj absolute4 ,tak+ into acc\nt ^! two facts1 we f9d t ! upp] limit to a non-conduct+ surface 9 full sun%9e on ! e>? is ab\t #cfe absolute1 or only a few degrees less ?an ! boil+ po9t ( wat]4 ,effective ,temp]ature ( ,space4--,! la/ problem we w attack by means ( ! f\r? p[] law is ! e/imation ( ! effective temp]ature ( space1 i4e4 ! temp]ature ( a full absorb] %ield$ f ! sun & f> away f any planet4 ,x is e/imat$ by exp]im5t t z5i? sun radiation is five million times ! radiation f ! />s4 ,?is e/imate is only v r\gh1 z ! radiation f ! />s is s m9ute4 ,z ! sun only occupies "o #id1jjj? "p ( ! heav5s1 ! radiation f a sunbright hemisph]e w\ld be five million times #id1jjj times />light1 i4e4 #dgj1jjj1jjj1jjj times4 ,! temp]ature ( ! sun is !re=e ! f\r? root ( ?is quantity times ! effective temp]ature ( space1 i4e4 ab\t #gjj times4 ,s9ce ! temp]ature ( ! sun is ab\t #fbjj1 ! temp]ature ( space is a little "u #aj absolute2 i4e4 l[] ?an -#bfc ,c4 #ef ,note on ,absolute ,temp]ature4--,x is f\nd t1 if a gas su* z air has its temp]ature rais$ or l[]$ :ile its pressure is kept uni=m1 = e "o degree c5tigrade rise or fall its volume is increas$ or decreas$ by "o two hundr$ & sev5ty-?ird ( its volume at freez+ po9t1 i4e4 at #j c5tigrade4 ,if !re=e x cont9u$ 9 ! same way "r d[n to -#bgc c5tigrade1 its volume w\ld be r$uc$ to z]o at ?is temp]ature4 ,?is temp]ature is !re=e call$ ! absolute z]o ( temp]ature1 & temp]atures reckon$ f x are call$ absolute temp]atures4 ,to get absolute temp]atures f c5tigrade temp]atures we evid5tly ne$ to add #bgc4 #eg ,,*apt] ,,vi ,,! ,,trans=mation ,,( ,,absorb$ ,,radiation ,no acc\nt ( radiation w\ld be complete )\t m5tion+ :at becomes ( ! radiation :i* bodies absorb1 b a good deal ( ! subject is 9 s unc]ta9 a /ate t v little space w be devot$ to x4 ,absorb$ ,radiation conv]t$ into ,heat4--,! mo/ common effect ( absorb$ radiation is to raise ! temp]ature ( ! absorb+ body1 & s cause x to re-emit long heat-waves4 ,z ! usual arrangem5t is = ! absorb+ body to be at a l[] temp]ature ?an ! radiat+ "o1 ! waves giv5 out by ! absorb] are long] ?an ^? giv5 out by ! radiator1 & s ! net result is ! trans=mation ( %ort] waves into long] ones4 ,b we h se5 by ,prvo/'s !ory ( ex*anges t radiator & absorb] are int]*angeable1 & !re=e we see t ^? waves :i* are emitt$ by ! absorb] & absorb$ by ! radiator are re-emitt$ by ! latt] z %ort] waves4 ,! me*anism by means ( :i* ! waves are conv]t$ into heat 9 ! body is /ill a my/]y4 ,t ! waves %\ld cause ! electrons to vibrate is p]fectly cle>1 b h[ ! vibrations ( ! electrons are conv]t$ into ^? vibrations ( ! atoms #eh & molecules :i* con/itute heat is /ill unsolv$1 & ! rev]se process is1 ( c\rse1 equally puzzl+4 ,! heat+ ( ! body & ! consequ5t re-emission ( heat-waves is n1 h[ev]1 ! only process :i* goes on4 ,9 a l>ge numb] ( sub/ances1 waves are giv5 out "u ! /imulus ( o!r waves )\t any heat+ ( ! body at all4 ,9 mo/ ( ^! cases ! emission /ops z soon z ! /imulat+ waves are )drawn1 & 9 ^! cases ! ph5om5on has be5 call$ fluoresc5ce4 ,! "n has be5 d]iv$ f fluor sp>1 ! sub/ance :i* 0 fir/ obs]v$ to exhibit ?is peculi> emission ( waves4 ,a famili> example ( fluoresc5ce is provid$ by p>aff9-oil1 :i* gl[s ) a blue light :5 x is illum9at$ ) ord9>y sunlight or daylight4 ,p]haps ! easie/ way to view x is to project a n>r[ beam ( light ?r\gh ! p>aff9-oil conta9$ 9 a glass vessel & view ! oil 9 a direction p]p5dicul> to ! beam4 ,! latt] w !n %[ up a brilliant blue4 ,a wat] solution ( sulphate ( qu99e1 made acid by a few drops ( sulphuric acid1 also exhibits a blue fluoresc5ce1 :ile a wat] solution ( scul9 7made by p\r+ hot wat] ov] "s scraps ( horse-*e/nut b>k7 %9es ) a brilliant blue light4 ,"s lubricat+ oils fluoresce ) a gre5 light1 z does also a solution 9 wat] ( fluoresc5e1 nam$ ?us because ( its m>k$ fluoresc5ce4 ,a solution ( *lorophyll 9 alcohol1 :i* c be readily prep>$ by soak+ gre5 leaves 9 alcohol1 %[s a r$ fluoresc5ce2 uranium glass--! can>y glass ( :i* small vases are v frequ5tly #ei made--exhibits a brilliant gre5 fluoresc5ce1 z does also cry/al uranium nitrate4 ,x is f\nd1 on obs]v+ ! spectrum ( ! fluoresc5t light1 t a fairly small range ( waves is emitt$ %[+ a well-m>k$ maximum ( int5sity at a wave-l5g? :i* is *>act]i/ic ( ! p>ticul> fluoresc+ sub/ance4 ,!re also seems to be a limit$ range ( waves :i* c induce ?is fluoresc5ce1 & ?is range also dep5ds ^u ! fluoresc+ sub/ance4 ,z a rule1 ! induc+ waves are %ort] 9 l5g? ?an ! induc$ fluoresc5ce1 b ?is rule has "s v m>k$ exceptions4 ,! fact t only a limit$ range ( waves produces fluoresc5ce expla9s a noticeable *>act]i/ic ( ! ph5om5on4 ,if ! fluoresc+ solutions are at all /rong ! fluoresc5ce is conf9$ to ! region close to :]e ! light ent]s ! solution1 ?us %[+ t ! rays :i* are responsible = induc+ ! gl[ become rapidly absorb$1 :]eas ! rema9d] ( ! light goes on practically unabsorb$4 ,phosphoresc5ce4--,sometimes ! emission ( ! induc$ light cont9ues = "s "t aft] ! induc+ waves are )drawn1 & !n ! ph5om5on is t]m$ phosphoresc5ce1 s9ce phosphorus emits a cont9u\s gl[ )\t rise ( temp]ature4 ,sometimes ! gl[ w cont9ue = sev]al h\rs aft] ! excit+ rays h be5 cut (f1 a good example ( ?is be+ provid$ by ,balma9's lum9\s pa9t1 :i* is a sulphide ( calcium4 ,) o!r sub/ances ! gl[ w only cont9ue = a v small fraction ( a second1 s t x is impossible to #fj say :]e fluoresc5ce ends & :]e phosphoresc5ce beg9s4 ,9 ord] to det]m9e ! duration ( ! gl[ 9 ! case ( ^! small times1 an arrangem5t consi/+ ( two rotat+ discs1 ea* ( :i* h slits 9 !m1 is set up4 ,?r\gh ! slits 9 "o ( !m ! sub/ance is illum9at$1 & ?r\gh ! slits 9 ! o!r ! sub/ance is obs]v$ :ile ! light is cut (f4 ,by adju/+ ! position ( ! discs ) reg>d to ea* o!r ! slits may be made to foll[ "o ano!r aft] great] or %ort] int]vals1 & s ! "t ( obs]vation c be made great] or small] aft] ! illum9ation is cut (f4 ,all ! bodies :i* h be5 obs]v$ to exhibit phosphoresc5ce are solid4 ,!ory ( ,fluoresc5ce4--,x is fairly simple to imag9e a me*anism by :i* fluoresc5ce might be br\ght ab\t1 z we might assume a relation betwe5 ! p]iods ( oscillation ( c]ta9 types ( electron 9 ! sub/ance & ! p]iod ( ! /imulat+ waves4 ,?us resonance might occur1 & ! consequ5t vibrations ( ! electrons w\ld />t a s]ies ( second>y waves4 ,if1 h[ev]1 we assume resonance1 x is difficult to see :y !re is a range ( wave-l5g?s produc$ & ano!r range ( wave-l5g?s :i* may produce !m4 ,we %\ld h expect$ "o def9ite wave-l5g? or a few def9ite ones produc+ "o or a few def9ite wave-l5g?s 9 ! gl[1 :ile if a :ole range ( waves w produce ! effect x is difficult to see :y all bodies d n exhibit ! ph5om5on4 ,b ! ph5om5on ( phosphoresc5ce f9ally #fa disposes ( any su* description1 = ! two ph5om5a h no %>p di/9ction betwe5 !m4 ,"s sub/ances are kn[n 9 :i* ! phosphoresc5ce la/s = su* an extremely small fraction ( a second aft] ! /imulat+ waves are )drawn t x is difficult to "k :e!r to call ! effect fluoresc5ce or phosphoresc5ce4 ,x is probable1 !re=e1 t bo? are due to ! same action4 ,n[ a wave ( orange light completes ab\t five hundr$ million million vibrations 9 "o second1 & !re=e if an orange-col\r$ phosphores5ce 7 to la/ = only "o five-hundr$? ( a second x w\ld mean t ! electrons responsible = x vibrate "o million million times aft] ! /imulus is remov$4 ,?is is h>dly cr$ible1 & becomes m cr$ible :5 we rememb] t 9 "s phosphoresc5t sub/ances ! effect la/s = _m h\rs4 ,*emical ,!ory ( ,phosphores5ce4--,x is m probable t ! /imulat+ rays produce an actual *emical *ange 9 ! phosphoresc5t sub/ance4 ,= in/ance1 x is possible t ! vibrations ( a c]ta9 type ( electron 9 "o k9d ( atom become s viol5t z to deta* x f ! atom & ! tempor>ily free electron atta*es itself imm$iately to ano!r k9d ( atom4 ,! new arrangem5t may be q /able2 x is s 9 ! action ( light on a photographic plate1 b x may only be /able :5 ! electrons are be+ driv5 out ( _! orig9al atoms1 & 9 ?is case ! electrons w beg9 to return to _! old allegiance z soon z ! /imulus is )drawn4 ,9 ! return #fb process ! electrons w naturally be agitat$1 & w !re=e emit waves hav+ _! *>act]i/ic p]iod4 ,! rate at :i* ! return process takes place w evid5tly dep5d ^u ! /ability ( ! new arrangem5t4 ,if x is extremely un/able1 ! :ole return may only occupy a fraction ( a second1 b if x is ne>ly z /able z ! orig9al arrangem5t ! return may be extremely sl[4 ,on ?is view1 !n1 ^? sub/ances w phosphoresce :i* h an electron :i* is fairly easily deta*$ f its atom & :i* w atta* itself to ano!r atom1 =m+ an arrangem5t :i* is less /able ?an ! orig9al4 ,temp]ature & ,phosphoresc5ce4--,a confirmation ( ?is *emical view is provid$ by ! effect ( temp]ature on phosphoresc5ce4 ,! rate ( a *emical *ange is usually v l>gely increas$ by rise ( temp]ature1 & fur!r1 at v l[ temp]atures a l>ge numb] ( *emical *anges :i* take place q readily at ord9>y temp]atures d n take place at all4 ,simil>ly at v l[ temp]atures ! action ( ! light may be m or less /able4 ,= example1 ,dew> cool$ a fragm5t ( ammonium-plat9o-cyanide by means ( liquid hydrog51 & expos$ x to a /rong light4 ,aft] remov+ ! light no phosphoresc5ce 0 obs]v$1 ?\gh at ord9>y temp]atures a brilliant gre5 phosphoresc5ce is exhibit$1 b on all[+ ! fragm5t to w>m up x pres5tly gl[s v brightly4 ,a p>tial /ability is %[n by ,balma9's lum9\s pa9t1 = if x be kept 9 ! d>k until x becomes q non-lum9\s x w beg9 to gl[ aga9 = a #fc %ort "t if w>m$ up 9 any way4 ,by means ( ?is prop]ty ! infra-r$ region ( ! spectrum may be made visible4 ,= ?is purpose a scre5 is coat$ ) ! pa9t1 expos$ to /rong sunlight1 & !n plac$ s z to receive ! spectrum4 ,! fir/ effect ( ! invisible heat rays is to make ! portions ( ! scre5 on :i* !y fall bright] ?an _! surr\nd+s2 b ?is causes ! phosphoresc5ce to be emitt$ m rapidly1 & soon x is all emitt$1 leav+ a d>k region :]e ! heat has de/roy$ ! phosphoresc5ce4 ,on ! :ole1 !n1 ^? sub/ances :i* phosphoresce at ord9>y temp]atures d s m rapidly z ! temp]ature rises4 ,b ,dew> has f\nd a numb] ( sub/ances :i* phosphoresce only at l[ temp]atures1 e4g4 gelat9e1 celluloid1 p>aff91 ivory & horn4 ,?is is n a fatal objection to ! idea ( *emical *ange1 z "s *emical actions w only take place at l[ temp]atures1 b x is an objection z q a l>ge numb] ( sub/ances only phosphoresce at l[ temp]atures1 :]eas !re are n _m *emical reactions :i* w only take place !re4 ,z a matt] ( fact1 ev5 if ! idea ( a *emical *ange be ! true "o1 x is n a v satisfactory "o1 z *emical *anges are und\bt$ly v complicat$ ones1 & x w\ld be too difficult to trace ! *ange f ! vibration ( an electron to ! *emical *ange1 & vice-v]sa4 ,no satisfactory !ory !re=e exi/s to acc\nt = ! absorption & ! remission ( ! waves1 :e!r accompani$ or unaccompani$ by a rise 9 temp]ature ( ! absorb+ body4 #fd ,,*apt] ,,vii ,,pressure ,,( ,,radiation ,pr$iction ( ,pressure by ,maxwell4--,_h ! fact t light ex]ts a pressure be5 kn[n 9 ,newton's "t !re is no d\bt t x w\ld h be5 hail$ z conclusive pro( ( ! sup]iority ( ! corpuscul> !ory ov] ! wave !ory4 ,yet1 ironically en\gh1 x 0 res]v$ = ,james ,cl]k ,maxwell to pr$ict its exi/5ce & calculate its value on ! assumption ( 8 electromagnetic wave !ory2 & fur!r1 ! measurem5t ( its value has giv5 decisive evid5ce 9 fav\r ( ! wave !ory1 = ! value pr$ict$ by ! latt] is only "o-half t pr$ict$ by ! corpuscul> !ory1 & ! measurem5ts by ,ni*olls & ,hull agree to )9 #a p] c5t4 ) ! wave !ory value4 ,maxwell %[$ t all waves :i* come up to & are absorb$ by a surface ex]t a pressure on e squ>e c5timetre ( ! surface equal to ! am\nt ( en]gy conta9$ 9 "o cubic c5timetre ( ! beam4 ,if ! surface is a p]fect reflector1 ! reflect$ waves produce an equal back pressure1 & !re=e ! pressure is d\bl$4 ,z ! waves are reflect$ back along _! orig9al direction1 ! en]gy 9 ! beam w also be d\bl$1 & s #fe ! pressure w /ill be equal to ! en]gy p] cubic c5timetre ( ! beam4 ,z ! en]gy :i* is receiv$ 9 "o second f ! sun on any area c be measur$ by measur+ ! heat absorb$1 & s9ce ! spe$ ( light is kn[n1 we c calculate ! en]gy conta9$ 9 "o cubic c5timetre ( full sunlight1 & h5ce ! pressure on "o squ>e c5timetre ( surface4 ,= ! en]gy receiv$ on "o squ>e c5timetre ( surface 9 "o second mu/ h be5 spread orig9ally ov] a l5g? ( beam equal to ! di/ance :i* ! light has travell$ 9 "o second1 i4e4 ov] a l5g? equal to ! spe$ ( light4 ,if we divide t en]gy1 !re=e1 by ! spe$ ( light1 we %all get ! en]gy 9 a "o-c5timetre l5g? ( ! beam1 & !re=e 9 "o cubic c5timetre4 ,?is turns out to be an extremely small pressure inde$1 be+ only a little m ?an ! weight ( half a milligram1 on a squ>e metre ( surface4 ,maxwell sugge/$ t a mu* great] en]gy ( radiation might be obta9$ by means ( ! conc5trat$ rays ( an electric lamp4 ,su* rays fall+ on a ?91 metallic disc delicately susp5d$ 9 a vacuum might p]haps produce an obs]vable me*anical effect4 ,ne>ly ?irty ye>s aft] ,maxwell's sugge/ion x 0 successfully c>ri$ out by ,pr(4 ,leb$ew ( ,mosc[1 :o us$ precisely ! arrangem5t :i* ,maxwell _h sugge/$4 ,measurem5t ( ! ,pressure4--,a beam ( light f an arc lamp 0 conc5trat$ on to a disc susp5d$ v delicately 9 an exhau/$ glass #ff globe ab\t #h in*es across4 ,actually f\r discs 7 susp5d$1 z 9 ,fig4 #bd1 & arrangem5ts 7 made to conc5trate ! beam on to ei!r side ( any ( ! f\r discs4 ,illu/ration3 ,,fig4 #bd4 ,! susp5sion 0 a v f9e qu>tz fibre q4 ,! discs d1 d1 d1 d1 7 half a c5timetre 9 diamet] & 7 fix$ on two light arms1 s t _! c5tres 7 "o c5timetre f ! glass rod1 g1 :i* c>ri$ !m4 ,a mirror1 m1 s]v$ to measure ! angle ?r\gh :i* ! :ole sy/em 0 twi/$ ow+ to ! pressure ( ! beam on "o ( ! discs4 ,9 ord] to measure ! angle a telescope view$ ! reflection ( a scale 9 m1 & z m turn$ diff]5t divisions ( ! scale came into view4 ,! two discs on ! left 7 poli%$ & !re=e ! pressure on !m %\ld be ab\t twice t on ! black5$ discs on ! "r4 ,hav+ measur$ ! angle ?r\gh :i* a beam ( light has turn$ ! sy/em1 x is a simple matt] to measure ! =ce :i* w\ld cause ?is twi/ 9 ! fibre q4 ,9 ord] to te/ :e!r ! pressure agrees ) ! calculat$ value1 we mu/ f9d ! en]gy 9 ! beam ( light4 ,?is 0 done by receiv+ ! beam on a black5$ block ( copp] & measur+ ! rate at :i* its temp]ature rose4 ,f ?is rate & ! weight ( copp] x is easy to calculate ! am\nt ( heat receiv$ p] second1 & !re=e ! am\nt ( en]gy receiv$ p] second on "o squ>e #fg c5timetre ( ! area4 ,kn[+ ! spe$ ( ! light we c1 z sugge/$ above1 calculate ! en]gy 9 "o cubic c5timetre ( ! beam4 ,leb$ew's result 0 9 v fair accord ) ! calculat$ value4 ,! *ief difficulty 9 ! exp]im5t is to elim9ate ! effects due to ! small am\nt ( gas :i* rema9s 9 ! globe4 ,ea* disc is heat$ by ! beam ( light1 & ! gas 9 contact ) x becomes heat$ & causes convection curr5ts 9 ! gas4 ,at v l[ pressures a slightly diff]5t action ( ! gas becomes a di/urb+ factor4 ,?is effect is due to ! molecules :i* come up to ! disc becom+ heat$ & reb\nd+ f ! disc ) a great] velocity ?an t ) :i* !y approa*$ x4 ,! reb\nd ( ea* molecule causes a backw>d kick on to ! disc1 & ! cont9ual /ream ( molecules causes a /eady pressure4 ,?is w\ld be ! same on bo? sides ( ! disc if bo? sides 7 at ! same temp]ature1 b s9ce ! beam ( light comes up to "o side1 t side becomes hott] ?an ! o!r & !re w be an excess ( pressure on t side4 ,?is action is call$ radiomet] action1 because x 0 fir/ made use ( by ,crookes 9 detect+ radiation4 ,betwe5 ! ,scylla ( convection curr5ts at high] pressures & ! ,*>ybdis ( radiomet] action at l[] pressures1 !re seems to be a *annel at a pressure ( ab\t two or ?ree c5timetres ( m]cury4 ,= "h ! convection curr5ts are small & ! radiomet] action has sc>cely begun to be appreciable4 ,by work+ at ?is pressure & us+ "o or two #fh o!r devices = elim9at+ & all[+ = ! gas action1 ,pr(essors ,ni*olls & ,hull also measur$ ! pressure ( light 9 an exce$+ly c>eful & ma/]ly way4 ,_! results 7 extremely consi/5t among !mselves1 & agre$ ) ! calculat$ value to )9 "o p] c5t4 ,^? :o "k ! difficulty ( measur+ su* m9ute =ces1 & ! greatness ( ! di/urb+ factors1 mu/ recognise 9 ?is result "o ( ! f9e/ exp]im5tal a*ievem5ts ( our "t4 ,effect ( ,light ,pressure 9 ,a/ronomy4--,=ces due to light pressure are s small t we %\ld n expect to be able to detect _! effects on a/ronomical bodies1 & c]ta9ly we _c hope to obs]ve !m 9 ! l>ge bodies ( our sy/em4 ,! pressure ( ! sunlight on ! :ole surface ( ! e>? is ab\t #ge1jjj tons weight4 ,?is does n s\nd small until we comp>e x ) ! pull ( ! sun = ! e>?1 :i* is two hundr$ million million times z great4 ,:5 we consid] v small bodies1 h[ev]1 we f9d t ! pressure ( ! light may ev5 exce$ ! gravitational pull1 & !re=e ^! small p>ticles w be driv5 "r away f our sy/em4 ,9 ord] to %[ t ! light pressure becomes m & m important1 let u imag9e two sph]es ( ! same mat]ial1 "o ( :i* has f\r times ! radius ( ! o!r4 ,!n ! weight ( ! l>g] "o1 t is its gravitational pull1 w be sixty-f\r times z great z t ( ! small] "o1 :ile ! area1 & !re=e ! light pressure1 w be sixte5 times z great4 #fi ,! light pressure is !re=e f\r times z important 9 ! sph]e ( "o-qu>t] ! radius4 ,= a sph]e ^: radius is "o two hundr$ million million? ( ! radius ( ! e>? & ( ! same d5sity1 ! pressure ( ! light w\ld equal ! pull ( ! sun1 & !re=e su* a sph]e w\ld n be attract$ to ! sun at all4 ,?is is an extremely small p>ticle1 mu* small] ?an ! f9e/ visible du/1 b ev5 = mu* l>g] ?+s ! light pressure has an appreciable effect4 ,?us = a sph]e ( "o c5timetre radius & ( ! same d5sity z ! e>?1 ! pressure due to ! sunlight is "o sev5ty-f\r ?\s&? ( ! pull due to gravitation4 ,x !re=e ne$ n move 9 its orbit ) q su* a high spe$ 9 ord] t x may n fall into ! sun1 & its ye> is !re=e l5g!n$ by ab\t ?ree m9utes4 ,! l5g!n+ out ( comets' tails z !y approa* ! sun1 & ! app>5t repulsion ( ! tail by ! sun1 has sometimes be5 attribut$ to pressure ( sunlight1 b x is pretty c]ta9 t ! =ces call$ into play are v mu* great] ?an c be acc\nt$ = by ! light4 ,doppl] ,effect4--,! ,doppl] effect also has "s influ5ce on ! motion ( a/ronomical bodies4 ,:5 a body :i* is receiv+ waves moves t[>ds ! s\rce ( ! waves1 x receives ! waves m rapidly ?an if x 7 /ill1 & !re=e ! pressure is great]4 ,:5 ! body is mov+ away f ! s\rce x receives ! waves less rapidly1 & h5ce ! pressure ( light on x is less ?an = a /ation>y body4 ,if a body is mov+ 9 an elliptical orbit1 x is mov+ t[>ds ! sun 9 "o "p ( its orbit & #gj away 9 ano!r "p2 x w !re=e be ret>d$ 9 bo? p>ts1 & ! ultimate result w be t ! orbit w be circul>4 ,! ,doppl] effect c act 9 ano!r way4 ,a body :i* is receiv+ waves f ! sun on "o side is !reby heat$ & emits waves 9 all directions4 ,z x is mov+ 9 its orbit x w cr[d up ! waves :i* x s5ds out 9 front ( x & l5g!n out ^? :i* x s5ds out beh9d x4 ,b ! en]gy p] cubic c5timetre w be great] :]e ! waves are cr[d$ up ?an :]e !y are drawn out1 & !re=e ! body w exp]i5ce a ret>d+ =ce 9 its orbit4 ,z ! body t5ds to move m sl[ly x falls 9 a little t[>ds ! sun1 & s approa*es ! sun 9 a spiral pa?4 ,?ree ,effects ( ,light ,pressure4--,we ?us h ?ree effects ( light pressure on bodies describ+ an orbit r\nd ! sun4 ,! fir/ effect is to l5g!n _! p]iod ( revolution1 ! second is to make _! orbits m circul>1 & ! ?ird is to make !m gradually approa* ! sun 9 a spiral pa?4 ,^! effects are q inappreciable = bodies any?+ l ! size ( ! e>?1 b = small bodies ( ! ord] ( "o c5timetre diamet] or less ! effects w\ld be q l>ge4 ,our sy/em is full ( su* bodies1 z is evid5c$ by ! numb] ( !m :i* p5etrate our atmosph]e & =m %oot+ />s4 ,! exi/5ce ( su* bodies is some:at ( a problem1 z :atev] e/imate ( ! sun's age we accept z correct1 he is c]ta9ly ( su* an age t if ^! bodies _h exi/$ at 8 beg9n+ !y w\ld all h be5 drawn 9 to him long ago4 ,we mu/ !re=e #ga suppose t !y are cont9ually r5ew$ 9 "s way1 & s9ce we c see no suffici5t s\rce inside ! ,sol> sy/em1 we mu/ come to ! conclusion t !y are r5ew$ f outside4 ,!re is e reason to believe t "s ( !m orig9ate 9 comets :i* h become dis9tegrat$ & spread out along _! orbits4 ,^! =m ! meteoric %[]s4 ,?us ! v f9e/ du/ is driv5 by ! sun "r out ( our sy/em1 & all ! re/ he is gradually draw+ 9 to himself4 #gb ,,*apt] ,,viii ,,! ,,relation ,,betwe5 ,,radiant ,,heat ,,& ,,electric ,,waves ,9 ?is conclud+ *apt] x is propos$ to %[ h[ ! wave-l5g?s ( radiant heat h be5 det]m9$ & to /ate :at range ( wave-l5g?s has be5 exp]im5tally obs]v$4 ,x is !n propos$ to %[ h[ electromagnetic waves h be5 produc$ by /raight=w>d electrical means & h[ _! wave-l5g?s h be5 measur$4 ,! simil>ity 9 prop]ties ( ! radiant heat & ( ! electric waves w be not$1 lead+ to ! conclusion t ! diff]5ce betwe5 ! two sets ( waves is m]ely "o ( wave-l5g?4 ,diffraction ,grat+4--,! be/ me?od ( measur+ ! wave-l5g?s ( heat & light is by means ( ! ,diffraction ,grat+4 ,?is consi/s ess5tially ( a l>ge numb] ( f9e p>allel equidi/ant slits plac$ v close to "o ano!r4 ,= ! measurem5t ( ! wave-l5g?s ( light & ( ! %ort] heat waves1 x is usually produc$ by rul+ a l>ge numb] ( v f9e close equidi/ant l9es on a piece ( glass or on a poli%$ mirror by means ( a diamond po9t4 ,! rul$ l9es are opaque on ! glass & d n reflect on ! mirror1 & consequ5tly ! spaces 9 betwe5 act z slits4 #gc ,r[l&'s ,grat+s4--,! rul+ ( ^! grat+s is a v difficult & t$i\s bus9ess1 b ! difficulties h be5 surm\nt$ 9 a v rem>kable mann] by ,r[l&1 s t ! grat+s rul$ on 8 ma*9e h become /&>d in/rum5ts ?r\gh\t ! _w4 ,he succe$$ 9 rul+ grat+s #f in*es 9 diamet] ) #ad1jjj l9es to ! in*1 truly a rem>kable p]=mance :5 we rememb] t if ! diamond po9t develops ! slighte/ *ip 9 ! process1 ! :ole grat+ is spoilt4 ,illu/ration3 ,,fig4 #be4 ,! action ( ! grat+ c be made cle> by means ( ,fig4 #be4 ,let ,a1 ,b1 ,c1 ,d repres5t ! #gd equidi/ant slits 9 a grat+1 & let ! /raight l9es to ! left ( ! grat+ repres5t at any in/ant ! cre/s ( "s simple plane waves com+ up to ! grat+4 ,! small fractions ( ! orig9al waves em]g+ f ! slits ,a1 ,b1 ,c1 ,d w spread out f ! slits s t ! cre/s ( ! small wavelets may at any in/ant be repres5t$ by a s]ies ( conc5tric circles1 />t+ f ea* slit z c5tre4 ,! s]ies ( cre/s f ea* slit are repres5t$ 9 ! figure4 ,n[ notice t a l9e ,,pq p>allel to ! orig9al waves lies on "o ( ! cre/s f ea* slit1 & !re=e ! wavelets w make up a plane wave p>allel to ! orig9al wave4 ,?is may !re=e be br\ght to a focus by means ( a convex l5s j z if ! grat+ 7 remov$1 except t ! int5sity ( ! wave is less4 ,b a l9e1 ,,lm1 also lies on a s]ies ( cre/s1 ! cre/ f ,a be+ "o wave-l5g? beh9d t f ,b1 ! "o f ,b a wave-l5g? beh9d t f ,c1 & s on4 ,! wavelets w !re=e =m a plane wave ,,lm1 :i* w move 9 ! direction p]p5dicul> to itself 7i4e4 ! direction ,,dk7 & may be br\ght to a focus 9 t direction by means ( a l5s4 ,draw ,,* & ,,dk p]p5dicul> to ,,lm1 & draw ,,ce p]p5dicul> to ,,dk1 i4e4 p>allel to ,,lm4 ,! diff]5ce betwe5 ,,* & ,,dk is evid5tly "o wave-l5g?1 i4e4 ,,de is "o wave-l5g?4 ,if ,greek3 alpha is ! angle betwe5 ! direction ( ,,pq & ,,lm1 ,,de is evid5tly equal to ,,cd s9 ,greek3 alpha & !re=e "o wave-l5g?,,cd s9 ,greek3 alpha4 ,f ! rul+ ( ! grat+ we "k ! value #ge ( ,,cd1 & !re=e by measur+ ,greek3 alpha we c calculate ! wave-l5g?4 ,we f9d t a ?ird l9e ,,rs also lies on a s]ies ( cre/s1 & !re=e a plane wave sets out 9 ! direction p]p5dicul> to ,,rs4 ,we notice "h t ! cre/ f ,a is two wave-l5g?s beh9d t f ,b1 & s on1 & !re=e if ,greek3 beta is ! angle betwe5 ,,rs & ,,pq1 ,,cd s9 ,greek3 beta is equal to two wave-l5g?s4 ,simil>ly we get ano!r plane wave = a ?ree wave-l5g?s diff]5ce1 & s on4 ,! int5sity ( ! wavelets falls (f fairly rapidly z !y become m oblique to _! orig9al direction1 & !re=e ! int5sity ( ^! plane waves also falls (f r rapidly z !y become m oblique to ! direction 9 :i* ,,pq goes4 ,we see t ! ess5tial condition = ! plane wave to set out 9 any direction1 is t ! diff]5ce 9 ! di/ances ( ! plane wave f two successive slits %all be exactly a :ole numb] ( wave-l5g?s4 ,%\ld x dep>t ev] s little f ?is condition we %\ld see1 on draw+ ! l9e1 t !re lie on ! l9e an equal numb] ( cre/s & tr\ghs1 & !re=e1 if a l5s focus waves 9 ?is direction1 ! result+ effect is z]o4 ,! directions ( ! waves ,,pq1 ,,lm1 ,,rs1 c41 w !re=e be v %>ply def9$ & w admit ( v accurate det]m9ation4 ,disp]sion by ,grat+4--,evid5tly ! deviations ,greek3 alpha1 ,greek3 beta w be great] ! great] is ,,de1 i4e4 ! great] ! wave-l5g?1 & !re=e ! light or heat w be disp]s$ into its diff]5t wave-l5g?s z 9 ! prism2 b 9 ?is case ! disp]sion #gf is opposite to t 9 ! normal prism1 ! long waves be+ disp]s$ mo/ & ! %ort waves lea/4 ,evid5tly1 too1 ! small] ! di/ance ,,cd ! great] ! angle1 & !re=e = ! extremely %ort wave-l5g?s ( light & ( ultraviolet rays we require ! di/ance betwe5 successive slits to be extremely small4 ,illu/ration3 ,,fig4 #bf4 ,! ,spectromet]4--,! grat+ is usually us$ ) a spectromet]1 z %[n 9 plan diagrammatically 9 ,fig4 #bf4 ,! slit ,s f :i* ! waves radiate is plac$ at ! pr9cipal focus ( ! l5s ,l1 & !re=e ! waves em]ge f ,l z plane waves :i* come up to ! grat+ ,g4 ,! telescope ,t is fir/ turn$ until x views ! slit directly1 i4e4 until ! plane waves l ,,pq 9 ,fig4 #be are br\ght to a focus at ! pr9cipal focus ,f ( ! objective ( ! telescope4 ,! eyepiece ,e views ! image ( ! slit ,s :i* is =m$ at ,f4 ,! telescope is !n turn$ ?r\gh an angle1 ,greek3 alpha1 until x views ! second image ( ! slit :i* w be =m$ by ! plane waves simil> to ,,lm 9 ,fig4 #be4 ,! angle ,greek3 alpha is c>efully measur$ by ! graduat$ circle on ! spectromet]1 #gg & h5ce ! wave-l5g? ( a p>ticul> k9d ( light1 or ( a p>ticul> "p ( ! spectrum1 is measur$4 ,?is spectromet] me?od is exactly ! me?od us$ = measur+ ! wave-l5g?s 9 ! visible "p ( ! spectrum4 ,= ! ultraviolet rays1 in/ead ( view+ ! image ( ! slit by means ( ! eyepiece ( ! telescope1 a photographic plate is plac$ at ! pr9cipal focus ,f ( ! objective ( ! telescope1 & s]ves to detect ! exi/5ce & position ( ^! %ort] waves4 ,= ! heat rays a ,langley's bolomet] /rip is plac$ at ,f1 9 fact ! bolomet] /rip might be us$ ?r\gh\t1 b x is n q s s5sitive = ! visible & ultraviolet rays z ! eye & ! photographic plate4 ,absorption by ,glass & ,qu>tz4--,two ma9 difficulties arise 9 ^! exp]im5ts4 ,! fir/ "o is t al?\gh glass1 or bett] /ill qu>tz1 is extremely transp>5t to ultraviolet1 visible1 & ! %ort] infra-r$ waves1 yet x absorbs "s ( ! long] heat waves almo/ completely4 ,= ^! waves1 !re=e1 "s arrangem5t mu/ be devis$ 9 :i* !y are n transmitt$ ?r\gh a glass diffraction grat+ or ?r\gh glass or qu>tz l5ses4 ,to effect ?is1 ! convex l5ses are replac$ by concave mirrors & ! rul$ grat+ is replac$ by "o :i* is made ( v f9e wires1 :i* are /ret*$ on a frame p>allel to & equidi/ant f ea* o!r4 ,! wire grat+ _c be con/ruct$ ) su* f9e or close slits z ! rul$ grat+1 b = ! long] waves ?is is unnecess>y4 #gh ,reflect+ ,spectromet]4--,an arrangem5t us$ by ,rub5s is repres5t$ r\ghly 9 plan 9 ,fig4 #bg4 ,l repres5ts ! s\rce ( heat1 ! rays f :i* are reflect$ at ! concave mirror ,m1 & br\ght to a focus on ! slit ,s4 ,em]g+ f ,s ! rays are reflect$ at ,m7sub7#b & are !reby r5d]$ p>allel be=e pass+ ?r\gh ! wire grat+ ,g4 ,aft] pass+ ?r\gh ! grat+1 ! rays are reflect$ at ,m7sub7#c & are !reby focuss$ on to a bolomet] /rip plac$ at ,b4 ,turn+ ! mirror ,m7sub7#c 9 ?is arrangem5t is evid5tly equival5t to turn+ ! telescope 9 ! ord9>y spectromet] arrangem5t4 ,illu/ration3 ,,fig4 #bg4 ,absorption ( ,waves by ,air4--,by us+ a spectromet] 9 an exhau/$ vessel ,s*umann discov]$ t waves exi/$ 9 ! ultraviolet region ( mu* small] wave-l5g? ?an any previ\sly f\nd1 & t ^! waves 7 almo/ completely absorb$ on pass+ ?r\gh a few c5timetres ( air4 ,to all long] waves1 h[ev]1 air seems to be extremely transp>5t4 ,! second difficulty arises f ! fact1 already expla9$1 t a diffraction grat+ produces n "o1 b a numb] ( spectra4 ,if only a small range ( waves exi/s1 ?is w lead to no confusion1 b if a l>ge range is be+ inve/igat$1 we may get two or m ( ^! spectra ov]lapp+4 ,suppose1 = example1 we h "s waves ( wave-l5g? ,,de 79 ,fig4 #be71 "s ( wave-l5g? "o-half #gi ,,de & "s ( "o-?ird ,,de4 ,!n 9 ! direction ,,dk we %all get plane waves ( ea* ( ^! wave-l5g?s sett+ out & be+ br\ght to a focus 9 ! same place4 ,?is difficulty c be fairly simply surm\nt$ :]e ! measurem5t ( wave-l5g? alone is requir$1 by plac+ 9 ! pa? ( ! rays f ! s\rce ( light1 suitable absorb+ scre5s1 :i* w only all[ a v small range ( wave-l5g?s to pass ?r\gh !m4 ,!re w !n be no ov]lapp+ & no confusion4 ,:]e ! actual di/ribution ( en]gy 9 ! spectrum ( any s\rce ( heat is to be det]m9$ ! difficulty becomes m s]i\s1 & probably !re is "s error 9 ! det]m9ations1 especially 9 ! longe/ waves1 :i* are mask$ almo/ completely by ! ov]lapp+ %ort] waves4 ,re/-,/rahl5 or ,residual ,rays4--,a v beautiful me?od ( isolat+ v long heat waves1 & s free+ !m f ! mask+ effect ( ! %ort] waves1 0 devis$ by ,rub5s & ,ni*ols4 ,x is f\nd t :5 a sub/ance v /rongly absorbs any waves t pass ?r\gh x1 x also /rongly reflects at its surface ! same waves4 ,= example1 a %eet ( glass us$ z a fire-scre5 w cut (f mo/ ( ! heat com+ f ! fire1 al?\gh x is p]fectly transp>5t to ! light4 ,if1 n[1 x is plac$ s z to reflect ! light & heat f ! fire1 x is f\nd to reflect v little light b a v l>ge proportion ( ! heat4 ,"s sub/ances h a well-def9$ absorption b&1 i4e4 !y absorb a p>ticul> wave-l5g? v /rongly1 & ^! sub/ances w !re=e reflect #hj ?is same wave-l5g? /rongly4 ,if in/ead ( a s+le reflection a numb] ( successive reflections be arrang$1 at ea* reflection ! proportion ( ! /rongly reflect$ wave-l5g? is increas$ until ultimately !re is practically only ?is "o wave-l5g? pres5t4 ,x c !re=e be v easily measur$4 ,^! waves result+ f a numb] ( successive reflections1 re/-/rahl5 or residual rays z !y h be5 nam$1 h be5 v l>gely us$ = inve/igat+ long waves4 ,qu>tz gives re/-/rahl5 ( l5g? 4#jjjhe c5timetres & v feeble ones ( 4#jjbj c5timetres long4 ,sylvite gives ! longe/ rays yet isolat$1 ! wave-l5g? be+ 4#jjf c5timetres4 ,range ( ! ,waves4--,! l5g?s ( ! waves ?us f> measur$ are3-- ,s*umann waves 4 4 4 4 4 4 4 4 4#jjjja to 4#jjjjb cms4 ,ultraviolet 4 4 4 4 4 4 4 4 4 4#jjjjb to 4#jjjjd ,violet 4 4 4 4 4 4 4 4 4 4 4 4 4#jjjjd ,gre5 4 4 4 4 4 4 4 4 4 4 4 4 4#jjjje ,r$ 4 4 4 4 4 4 4 4 4 4 4 4 4 4#jjjjf to 4#jjjjge ,infra-r$ 4 4 4 4 4 4 4 4 4 4 4#jjjjge to ab\t 4#jjja ,re/-/rahl5 f qu>tz 4 4 4#jjjhe & 4#jjbj ,re/-/rahl5 f ,sylvite 4 4 4#jjfj ,?us ! longe/ waves are six hundr$ times ! l5g? ( ! %orte/4 ,! correspond+ range ( wave-l5g?s ( s\nd w\ld be a little m ?an eight octaves1 ( :i* ! visible "p ( ! spectrum is less ?an "o4 ,electromagnetic ,induction4--,9 ! attempt to expla9 ! nature ( an electromagnetic wave 7pp4 #ag-ba7 x 0 /at$ t an electric wave mu/ always be accompani$ by a magnetic wave4 ,9 ord] to #ha und]/& ! production ( ^! waves1 ! relation betwe5 electric & magnetic l9es ( =ce mu/ be /at$ 9 m detail4 ,a l>ge numb] ( q simple exp]im5ts %[ t :5ev] ! electric field at any po9t is *ang+1 i4e4 :5ev] ! l9es ( =ce are mov+ p]p5dicul> to !mselves1 a magnetic field is produc$ at ! po9t1 & ?is magnetic field la/s :ile ! *ange is tak+ place4 ,an exactly simil> result is obs]v$ :5 ! magnetic field at a po9t is *ang+--an electric field is produc$ :i* la/s :ile ! magnetic field is *ang+4 ,:5 ! electric field *anges1 !re=e1 !re is bo? an action & a reaction--a magnetic field is produc$ & ?is *ange 9 magnetic field produces a correspond+ electric field4 ,?is induc$ electric field is always ( su* a k9d z to delay ! *ange 9 ! orig9al electric field2 if ! orig9al field is becom+ weak] ! induc$ field is 9 ! same direction1 ?us delay+ ! weak5+1 & if ! orig9al field is becom+ /rong] ! induc$ field is 9 ! opposite direction1 ?us delay+ ! increase4 ,mom5tum ( ,mov+ ,electric ,field4--,imag9e n[ a small portion ( an electric field mov+ at a /eady spe$2 x w produce1 ow+ to its motion1 a /eady magnetic field4 ,if n[ ! motion be /opp$1 ! magnetic field w be de/roy$1 & ! *ange 9 ! magnetic field w produce an electric field s z to delay ! *ange1 i4e4 s z to cont9ue ! orig9al motion4 ,! mov+ electric field ?us has mom5tum 9 exactly ! same way z a mov+ mass has4 ,! p>allel betwe5 ! two #hb is /rictly accurate4 ,! mass has en]gy due to its motion1 & 9 ord] to /op ! mass ?is en]gy mu/ be conv]t$ into "s o!r =m ( en]gy & "w mu/ !re=e be done4 ,! electric field has en]gy due to its motion--! en]gy ( ! magnetic field--& !re=e to /op ! motion ( ! electric field1 ! en]gy ( ! magnetic field mu/ be conv]t$ into "s o!r =m1 & "w mu/ !re=e be done4 ,"o consequ5ce ( ! mom5tum ( a mov+ mass is well illu/rat$ by ! p5dulum4 ,! bob ( ! p5dulum is 9 equilibrium :5 x is at its l[e/ po9t1 b :5 x is displac$ f t po9t & all[$ to sw+1 x does n sw+ to its l[e/ po9t & /ay !re1 b is c>ri$ beyond t po9t by its mom5tum4 ,! "w done 9 displac+ ! bob soon br+s x to re/ on ! o!r side1 & x sw+s back aga9 only to ov]%oot ! m>k aga94 ,! friction 9 ! support ( ! p5dulum & ! resi/ance ( #hc ! air to ! motion makes ea* sw+ a little small] ?an ! "o be=e x1 s t ultimately ! sw+ w die d[n to z]o & ! p5dulum w come to re/ at its l[e/ po9t4 ,! graph ( ! displacem5t ( ! bob at diff]5t times w !re=e be some?+ l ,fig4 #bh4 ,%\ld ! p5dulum be put to sw+1 n 9 air1 b 9 "s visc\s m$ium l oil1 its vibrations w\ld be damp$ d[n v mu* m rapidly1 & if ! m$ium be visc\s en\gh ! vibrations may be suppress$1 altoge!r1 ! p5dulum m]ely s9k+ to its l[e/ position4 ,illu/ration3 ,,fig4 #bh4 ,electric ,oscillation4--,^! conditions h _! exact c\nt]p>t 9 ! electric field4 ,to und]/& !m1 ?ree prop]ties ( l9es ( =ce mu/ be borne 9 m9d3 7i47 l9es ( =ce act z if 9 t5sion & !re=e always t5d to %ort5 z mu* z possible2 7ii47 ! ends ( l9es ( =ce c move freely on a conductor2 7iii47 l9es ( =ce 9 motion possess mom5tum4 ,n[ imag9e two conduct+ plates ,a & ,b1 ,fig4 #bi1 *>g$ positively & negatively1 & !re=e connect$ by l9es ( =ce z indicat$4 ,let ! two plates be sudd5ly connect$ by ! wire w1 s t ! ends ( ! l9es ( =ce may freely slide f ,a to ,b or vice-v]sa1 & !re=e all ! l9es w slide upw>ds along ,a & ,b1 & !n t[>ds ea* o!r along w1 until !y %r9k to z]o #hd some:]e 9 w4 ,! condition ( equilibrium w evid5tly be rea*$ :5 all ! l9es h ?us %runk to z]o1 b ! l9es :i* are travell+ f ,a t[>ds ,b w h mom5tum & w !re=e ov]%oot ! equilibrium condition & pass "r on to ,b4 ,t is1 ! positive ends ( ! l9es w travel on to ,b1 & simil>ly ! negative ends w pass on to ,a4 ,! l9es ( =ce betwe5 ,a & ,b w !re=e be rev]s$4 ,! t5sion 9 ! l9es w soon br+ !m to re/1 & !y w slide back aga91 ov]%oot ! m>k aga91 rea* a limit 9 ! orig9al direction & /ill aga9 slide back4 ,! field betwe5 ,a & ,b w !re=e be cont9ually rev]s$1 b ea* "t its value w be a little less1 until ultimately ! vibrations w die d[n to z]o4 ,?us if we 7 to replace ! displacem5t 9 ,fig4 #bi by ! value ( ! field betwe5 ,a & ,b we %\ld h an exactly simil> graph4 ,illu/ration3 ,,fig4 #bi4 ,! am\nt by :i* ! oscillations are damp$ d[n w dep5d ^u ! "* ( ! wire w4 ,if x is a v poor conductor x w (f] a l>ge resi/ance to ! slid+ ( ! l9es along x1 & ! vibrations w be quickly damp$ d[n or1 if ! resi/ance is great en\gh1 be suppress$ altoge!r4 ,?is rapid alt]nation ( ! electric field w s5d out electromagnetic waves :i* die d[n z ! oscillations decrease4 ,! ,sp>k ,dis*>ge4--,9 practice ! wire w is n actually us$1 b ! air itself sudd5ly becomes a conductor & makes ! connection4 ,:5 ! electric field at a po9t 9 ! air exce$s a c]ta9 limit+ /r5g?1 ! air seems to break d[n & #he sudd5ly become a conductor & rema9s "o = a %ort "t4 ,?is break+ d[n is accompani$ by light & heat1 & is kn[n z ! sp>k dis*>ge or electric sp>k4 ,exp]im5ts ( ,h]tz4--,9 ! brilliant exp]im5ts c>ri$ out by ,h]tz at ,k>lsruhe betwe5 #ahhf & #ahia1 he n only demon/rat$ ! exi/5ce ( ! waves produc$ 9 ?is way1 b he %[$ t !y are reflect$ & refract$ l ord9>y light1 he measur$ _! wave-l5g? & r\ghly measur$ _! spe$1 ?is latt] be+ equal to ! spe$ ( light )9 ! errors ( exp]im5t4 ,illu/ration3 ,,fig4 #cj4 ,"o arrangem5t us$ by ,h]tz is %[n 9 plan 9 ,fig4 #cj4 ,a ,ruhmkorff coil ,r s]ves to *>ge ! two conductors ,a & ,b until ! air breaks d[n at ! gap ,g1 & a sp>k passes4 ,be=e ! sp>k is #hf produc$1 ! l9es ( =ce on ! l[] side ( ,,ab w 9 =m be some?+ l ! dott$ l9es 9 ! figure1 b z soon z ! air becomes a conductor1 ! positive ends ( ! l9es w surge f ,a t[>ds ,b & on to ,b1 & ! negative ends w surge on to ,a4 ,^! to & fro surg+s w cont9ue = a little :ile1 b w gradually die out4 ,z ! surg+s are all up & d[n ,,ab1 ! electric vibrations 9 ! electromagnetic waves s5t out #hg w all be p>allel to ,,ab1 & !re=e !y w be pol>is$4 ,illu/ration3 ,,fig4 #ca4 ,?is is *>act]i/ic ( all electric waves1 z no s+le sp>k+ app>atus w produce any?+ b waves p>allel to ! sp>k gap4 ,! electric vibrations com+ up to a conductor plac$ 9 ! position ( ! wire rectangle1 ,m1 w cause surg+ ( ! l9es along x1 &1 if ^! surg+s are p[]ful en\gh1 w cause a sp>k to pass across ! small gap ,s4 ,su* a rectangle 0 !re=e us$ by ,h]tz z a detector ( ! waves1 b s9ce t "t _m detectors ( v mu* great] s5sitiv5ess h be5 devis$4 ,reflection4--,9 ord] to %[ t ^! waves are reflect$ 9 ! same way z light waves1 ,h]tz plac$ ! sp>k+ knobs1 ,g1 at ! focus ( a l>ge p>abolic metallic reflector1 & 8 detector1 ,d1 at ! focus ( a simil> reflector plac$ z 9 ,fig4 #ca1 b mu* f>!r away 7cf4 ,fig4 #a74 ,9 ?is position sp>k+ at ,g produc$ /rong sp>k+ 9 ! detector1 al?\gh ! di/ance 0 su* t no sp>k+ 0 produc$ )\t ! reflectors4 ,refraction4--,! refraction ( ! waves 0 #hh %[n by means ( a l>ge prism made ( pit*4 ,?is _h an angle ( #cj & 0 ab\t #a4e metres high & #a4b metres broad4 ,illu/ration3 ,,fig4 #cb4 ,sett+ x up z %[n 9 plan 9 ,fig4 #cb1 /rong sp>k+ 0 produc$ 9 ! detector1 ?us %[+ t ! rays ( electric waves 7 deflect$ by #bb on pass+ ?r\gh ! prism4 ,mov+ ! mirror & detector 9 ei!r direction f ! l9e ,,lm1 made ! sp>ks decrease rapidly 9 int5sity1 s t ! exact position ( ,,lm c be det]m9$ ) consid]able def9it5ess4 ,wave-l5g?1 by ,/ation>y ,waves4--,! wave-l5g?s ( ! oscillations 7 f\nd by means ( :at are kn[n z /ation>y waves4 ,:5 two exactly simil> sets ( waves are travell+ 9 opposite directions ov] ! same space1 !y produce no effects at c]ta9 po9ts call$ nodes4 ,^! nodes are j half a wave-l5g? ap>t4 ,_! production c be und]/ood by ref]5ce to ,fig4 #cc4 ,! dott$ l9es repres5t ! two waves :i* are travell+ 9 ! direction indicat$ by ! arr[s4 ,9 ,a ! "t is *os5 :5 ! waves are exactly sup]pos$1 & ! resultant displacem5t w be repres5t$ by ! solid l9e4 ,! po9ts m>k$ ) a cross w be po9ts at :i* ! displacem5t is z]o4 ,illu/ration3 ,,fig4 #cc4 ,9 ,b ea* wave has travell$ a di/ance equal to a qu>t] ( a wave-l5g?1 & x w be se5 t ! two sets ( waves cause equal & opposite displacem5ts4 ,! result+ displacem5t is !re=e z]o1 z indicat$ by ! solid l9e4 ,9 ,c ! waves h travell$ ano!r qu>t] ( a wave-l5g? & #hi are sup]pos$ aga91 b 9 ?is case ! displacem5ts w be 9 ! opposite directions f ^? 9 ,a4 ,9 ,d1 /ill ano!r qu>t] wave-l5g? has be5 trav]s$ by ea* wave1 & ano!r qu>t] wave-l5g? w\ld br+ back ! position ,a4 ,9 ,e1 we h ! successive positions ( ! wave drawn 9 "o diagram1 & we notice t ! po9ts indicat$ by a cross are always undisplac$ & _! di/ance ap>t is "o-half a wave-l5g?4 ,h]tz produc$ ^! conditions by sett+ up 8 coil & sp>k+ knobs at "s di/ance f a reflect+ wall1 ,fig4 #cd4 ,!n ! waves :i* are com+ up to ! wall & ^? :i* are reflect$ #ij f ! wall w be travell+ 9 opposite directions ov] ! same space4 ,true1 ! reflect$ waves w be r weak] ?an ! orig9al ones1 s t !re w be a little displacem5t ev5 at ! nodes1 b !re w be a well-m>k$ m9imum4 ,?us :5 ! detector is plac$ at ,a1 ,b1 ,c or ,d no sp>k+ or v feeble sp>k+ occurs1 :ile midway betwe5 ^! po9ts ! sp>k+ is v vigor\s1 & ! di/ance betwe5 two successive m9ima is "o-half a wave-l5g?4 ,illu/ration3 ,,fig4 #cd4 ,! wave-l5g? w dep5d ^u ! size1 =m1 c41 ( ! conductors betwe5 :i* ! sp>k+ occurs1 = ! "t :i* ! l9es ( =ce take to surge backw>ds & =w>ds 9 ! conductors w dep5d ^u ^! ?+s4 ,o!r ?+s be+ equal1 ! small] ! conductors ! small] ! "t & !re=e ! %ort] ! wave-l5g?4 ,! %orte/ wave :i* ,h]tz succe$$ 9 produc+ 0 #bd c5timetres long1 b s9ce !n waves z little z #f millimetres long h be5 produc$4 #ia ,! waves :i* are produc$ 9 a mod]n wireless telegraphy app>atus are miles 9 l5g?4 ,we ?us see t !re is r a l>ge gap betwe5 ! longe/ heat waves :i* h be5 isolat$1 4#jjf cms41 & ! %orte/ electric waves1 4#f cms4 ,! surpris+ fact1 h[ev]1 is t ?is gap is s small1 = ! heat waves are produc$ by vibrations )9 a molecule1 or at mo/ )9 a small gr\p ( molecules1 :]eas ! electric surg+s1 ev5 9 ! smalle/ conductors1 take place ov] _m _m millions ( molecules4 ,9 conclusion1 !re=e1 we see t f ! ,s*umann waves up to ! longe/ heat waves a little ov] eight octaves ( electromagnetic waves h be5 detect$1 !n aft] a gap ( betwe5 five & six octaves ! ord9>y electrically produc$ electromagnetic waves beg9 & ext5d on ?r\gh an almo/ indef9ite numb] ( octaves4 #ib ,,books ,,= ,,fur!r ,,read+ ,j4 ,h4 ,poynt+1 ,! ,pressure ( ,light4 ,e4 ,eds]1 ,heat = ,advanc$ ,/ud5ts3 ! *apt]s on ,radiation4 ,e4 ,eds]1 ,light = ,advanc$ ,/ud5ts3 ! *apt]s on ! ,spectrum4 ,b4 ,w4 ,wood1 ,physical ,optics3 ! *apt]s on ,fluoresc5ce & ,phosphoresc5ce1 ,laws ( ,radiation1 ,nature ( ,:ite ,light1 & ,absorption ( ,light4 #ic ,,index ,,absorb+ p[]1 #cg -- & radiat+ p[]1 #ch ,absorption1 spectra1 #cd -- by glass & qu>tz1 #gg -- by air1 #gh ,addition ( waves1 #be ,amplitude1 #bc ,,balma91 lum9\s pa9t1 #ei ,boltzmann1 laws ( radiation1 #dh ,,convection curr5ts1 #fg ,corpuscul> !ory1 #aj -- reflection & refraction by1 #aa ,crookes' radiomet]1 #fg ,,dew>1 temp]ature & phosphoresc5ce1 #fb ,diffraction grat+1 #gb -- disp]sion by1 #ge -- wire grat+1 #gg ,disp]sion1 #bi1 #ge ,doppl] effect1 #fi ,,effici5cy 9 light+1 #eb ,ela/ic solid !ory1 #ag ,electric field1 #ah ,electric *>ges )9 ! atom1 #ba ,electric oscillations1 #ai1 #hc ,electrification1 positive & negative1 #ah ,electromagnetic induction1 #hj ,electromagnetic waves1 #ag1 #hd ,electrons1 #cj ,en]gy 9 simple wave1 #be ,en]gy--wave-l5g? curve1 #bg ,,fluoresc5ce1 #eh -- !ory (1 #fj ,f\cault1 spe$ ( light 9 diff]5t m$ia1 #ag ,f\ri]'s s]ies ( waves1 #bf1 #cj ,fraunhf] l9es1 #ce ,full radiator & absorb]1 #dd1 #de ,,gases z radiators1 #db ,,huygh5s' wave !ory1 #ac ,h]tz1 exp]im5ts on electric waves1 #he -- reflection1 #hf -- refraction1 #hg -- wave-l5g? by /ation>y waves1 #hh ,,infra-,,r$ rays1 #cb ,int]f]5ce1 #ac ,,kir*(f',s law1 #dj ,,langley1 ,bolomet]1 #cb1 #dh1 #di1 #gg ,leb$ew1 pressure ( light1 #fe ,lumm] & ,pr+%eim1 law ( radiation1 #dh1 #ej ,,magnetic oscillations1 #bj ,maxwell1 electromagnetic !ory1 #ag -- pressure ( light1 #fd ,mom5tum ( mov+ electric field1 #ha ,,newton1 disp]sion1 #bi -- corpuscul> !ory1 #ab -- law ( cool+1 #df ,ni*ols1 ,rub5s &1 ,re/-/rahl51 #gi ,ni*olls & ,hull1 pressure ( light1 #fd1 #fh ,,pfl,,g]1 emission f t\rmal9e1 #dc ,phase1 #bb ,phosphoresc5ce1 #ei -- *emical !ory (1 #fa -- temp]ature & phosphoresc5ce1 #fb ,planck1 en]gy & wave-l5g?1 #ea ,pol>is$ light1 emission f t\rmal9e1 #db ,pressure ( light1 pr$iction ( by ,maxwell1 #fd -- measurem5t by ,leb$ew1 #fe -- measurem5t by ,ni*olls & ,hull1 #fd1 #fh -- on ! e>?1 #fh -- on f9e du/1 #fi -- on comets' tails1 #fi -- ?ree effects ( 9 a/ronomy1 #gj ,prvo/1 ,!ory ( ,ex*anges1 #df ,,radiat+ p[]1 #ch ,radiomet] action1 #fg ,reflection1 corpuscul> !ory1 #aa -- ( electric waves1 #hg ,refraction1 corpuscul> !ory1 #aa -- ( electric waves1 #hg ,resonance1 #cj ,re/-/rahl5 or residual rays1 #gi ,ripples on m]cury1 #ac ,rit*ie1 radiat+ & absorb+ p[]s1 #ch ,r[l&1 grat+s1 #gc ,rub5s & ,kurlbaum1 pro( ( ,planck's law1 #ea ,rub5s & ,ni*ols1 ,re/-/rahl51 #gi ,,s*umann waves1 #gh ,simple h>monic motion1 simple p]iodic motion1 #bd ,sp>k dis*>ge1 #hd ,spectromet]1 #gf -- reflect+1 #gh ,spectrum1 #bi -- ! :ole1 #cb -- inc&esc5t solid or liquid1 #cc -- inc&esc5t gas1 #cc -- analysis1 #cd -- emission & absorption1 #cd -- sun1 #ce -- />s & nebul1 #cf -- & temp]ature1 #dh ,/ation>y waves1 #hh ,/efan1 law ( radiation1 #dg ,,temp]ature1 absolute1 #ef -- ( planets1 #ed -- ( space1 #ee -- ( sun1 #ec ,,ultraviolet rays1 #cb1 #gg ,,wave =m1 #bd ,wave-l5g?1 #bb -- range (1 #hj -- ( electric waves1 #ij ,wave !ory1 rectil9e> propagation1 #ac ,wi51 ,law ( ,radiation1 #ej ,,"y1 int]f]5ce1 #af ,pr9t$ by ,,ballantyne1 ,,hanson ,,co4 ,ed9burgh ,london _/tb ,,! ,,p',s ,,books ,a wond]ful ent]prise1 admirably plann$1 & des]v+ ! highe/ success4--,! ,nation4 ,,! ,,fir/ ,,doz5 ,,volumes #e4 ,,botany3 ,,! ,,mod]n ,,/udy ,,( ,,plants4 ,by ,m4 ,c4 ,,/opes1 ,d4,sc41 ,ph4,d41 ,f4,l4,s4 ,a wond]ful 'multum 9 p>vo1' & _c fail1 by its lucidity & pleasant me?od ( exposition1 to give ! read] n only a cle> conception ( ! sci5ce ( botany z a :ole1 b also a desire = full] k ( plant life4--,notes & ,qu]ies4 #aj4 ,,h]$ity4 ,by ,j4 ,a4 ,s4 ,,watson1 ,b4,sc4 ,accurate1 & writt5 9 a simple mann] :i* w /imulate ^? :o are int]e/$ to wid] read+4--,a!num4 #ab4 ,,organic ,,*emi/ry4 ,by ,pr(essor ,j4 ,b4 ,,coh51 ,b4,sc41 ,f4,r4,s4 ,an excell5tly cle> & effici5t treatise on a subject n easily conf9$ )9 a %ort or unte*nical disc\rse4--,! ,man*e/] ,gu>dian4 #ac4 ,,! ,,pr9ciples ,,( ,,electricity4 ,by ,,norman ,r4 ,,campbell1 ,m4,a4 ,z = ,mr4 ,norman ,campbell's treatise '9 petto' ,i _c b ?9k x a model ( its k9d4 ,he takes next to no?+ = grant$4--,sunday ,times4 #ae4 ,,! ,,sci5ce ,,( ,,! ,,/>s4 ,by ,e4 ,w4 ,,maund]1 ,f4,r4,a4,s41 ( ! ,royal ,obs]vatory1 ,gre5wi*4 ,w convey to ! att5tive read] an enorm\s am\nt ( in=mation 9 a small space1 be+ cle> & abrea/ ( curr5t k4--,! ,a!num4 #bf4 ,,h5ri ,,b]gson3 ,,! ,,philosophy ,,( ,,*ange4 ,by ,h4 ,,wildon ,,c>r4 ,! fact t ,m4 ,b]gson has read ! pro(-%eets ( ,mr4 ,c>r's admirable survey w give x a c]ta9 au?oritativ5ess = ! g5]al read]4--,daily ,news4 #cb4 ,,roman ,,ca?olicism4 ,by ,h4 ,b4 ,,coxon4 ,preface1 ,mgr4 ,r4 ,h4 ,,b5son4 ,?is small book is "o :i* _c fail to be ( use to ^? :o desire to "k :at ,ca?olics d1 & d n1 believe4--,! ,ca?olic ,*ronicle4 #ci4 ,,m>y ,,que5 ,,( ,,scots4 ,by ,e4 ,o',,neill1 ,m4,a4 ,mrs4 ,o',neill1 on ',m>y ,que5 ( ,scots1' is spl5did2 x is an attempt to give ! v tru? ab\t a subject on :i* all feel int]e/ & mo/ lie freely4--,daily ,express4 #dg4 ,,wom5',s ,,suffrage4 ,by ,m4 ,g4 ,,fawcett1 ,,ll4,d4 ,mrs4 ,fawcett's admirably concise & fair-m9d$ hi/orical sket* ( ! wom5's suffrage movem5t4 ,we c\ld h>dly ask = a bett] summ>y ( ev5ts & prospects4--,daily ,news4 #ea4 ,,%akespe>e4 ,by ,pr(essor ,c4 ,h4 ,,h]=d1 ,litt4,d4 ,well wor? a place alongside ,pr(essor ,raleigh's book 9 ! ',engli% ,m5 ( ,lett]s4' 444 ,sets a high note & reta9s x )\t ef=t4--,obs]v]4 #ec4 ,,pure ,,gold--,a ,,*oice ,,( ,,lyrics ,,& ,,sonnets4 ,by ,h4 ,c4 ,o',,neill4 ,an an?ology ( good poetry su* z we might expect f a man ( ta/e4--,daily ,news4 #eg4 ,,dante4 ,by ,a4 ,g4 ,,f]r]s ,,h[ell4 ,x is a f9e piece ( s*ol>%ip1 & %\ld be read by any "o :o is beg9n+ ! /udy ( ,dante1 or inde$ any "o :o is int]e/$ g5]ally 9 ! e>ly process ( ,european lit]ature1 = ! process is "h admirably analys$4--,! ,man*e/] ,gu>dian4 ,,! ,,second ,,doz5 ,,volumes 7,ready7 #a4 ,,! ,,f\ndations ,,( ,,sci5ce4 ,by ,w4 ,c4 ,d4 ,,:e?am1 ,m4,a41 ,f4,r4,s4 #aa4 ,,inorganic ,,*emi/ry4 ,by ,pr(essor ,k4 ,c4 ,c4 ,,baly1 ,f4,r4,s4 #ad4 ,,radiation4 ,by ,p4 ,,phillips1 ,d4,sc4 #bb4 ,,"l ,,kelv94 ,by ,a4 ,,russell1 ,m4,a41 ,d4,sc41 ,m4,i4,e4,e4 #bc4 ,,huxley4 ,by ,pr(essor ,g4 ,,leighton1 ,m4,d4 #cf4 ,,! ,,gr[? ,,( ,,fre$om4 ,by ,h4 ,w4 ,,nev9son4 #da4 ,,julius ,,caes>3 ,,soldi]1 ,,/atesman1 ,,emp]or4 ,by ,,hil>y ,,h>d+e4 #dc4 ,,engl& ,,9 ,,! ,,middle ,,ages4 ,by ,mrs4 ,e4 ,o',,neill1 ,m4,a4 #ed4 ,,francis ,,bacon4 ,by ,pr(essor ,a4 ,r4 ,,skemp1 ,m4,a4 #ee4 ,,! ,,bront,s4 ,by ,miss ,,flora ,,masson4 #fj4 ,a ,,diction>y ,,( ,,synonyms4 ,by ,,au/9 ,k4 ,,gray1 ,b4,a4 #fa4 ,,home ,,rule4 ,by ,l4 ,g4 ,,r$mond ,,h[>d4 ,li/ ( o!r ,volumes 9 ,prep>ation may be _h4 ,,london ,,& ,,ed9burgh3 ,t4 ,c4 ,e4 ,c4 ,,jack ,,new ,,york3 ,,dodge ,,publi%+ ,,co4