,,! ,,microscope4        ,,be+ ,,!      
,,article ,,contribut$ ,,by             
,,&rew ,,ross          ,,to ,,! 8,,p5ny 
,,cyclop,,dia10 ,,publi%$ ,,by ,,!      
,,society      ,,= ,,! ,,diffusion ,,(  
,,useful ,,k4        ,,fully            
,,illu/rat$4          ,,new ,,york3     
,,! ,,indu/rial ,,publication ,,company4
      #ahgg4          ,,! ,,microscope4 
,microscope1 ! "n ( an in/rum5t = enabl+
 ! eye to see  di/9ctly objects :i* are 
plac$ at a v %ort di/ance f x1  or to   
see magnifi$ images ( small objects1 &  
!re=e to see  small] objects ?an w\ld   
o!rwise be visible4 ,! "n is d]iv$  f ! 
two ,greek words1 express+ ?is prop]ty1 
,,mikros1 small1  & ,,skopeo1 to see4   
,s little is kn[n ( ! e>ly hi/ory ( !   
microscope1 & s  c]ta9 is x t ! magnify+
 p[] ( l5ses mu/ h be5  discov]$ z soon 
z l5ses 7 made1 t !re is no reason =    
haz>d+ any d\btful speculations on ! "q 
( discov]y4 ,we  %all proce$ !re=e at   
once to describe ! simple/ =ms (        
microscopes1 to expla9 _! lat] & m      

important improvem5ts1  & f9ally to     
exhibit ! in/rum5t 9 its pres5t p]fect  
/ate4    ,9 do+ ?is we %all assume t !  
read] is famili> ) !  in=mation conta9$ 
9 ! articles 8,light10 8,l5s10          
8,a*romatic10  8,ab]ration10 & ! o!r    
sub-divisions ( ! sci5ce ( ,optics1  :i*
 are treat$ ( 9 ?is "w4    ,! use ( !   
t]m magnify+ has l$ _m into a           
misconception (  ! nature ( ! effect    
produc$ by convex l5ses4 ,x is n always 
und]/ood t ! s-call$ magnify+ p[] ( a   
l5s appli$ to  ! eye1 z 9 a microscope1 
is d]iv$ f its enabl+ ! eye to  approa* 
m ne>ly to its object ?an w\ld o!rwise  
be compatible  ) di/9ct vision4 ,!      
common occurr5ce ( walk+ across !  /reet
 to read a bill is 9 fact magnify+ !    
bill by approa*2 &  ! obs]v]1 at e /ep  
he takes1 makes a *ange 9 ! optical     
arrangem5t ( 8 eye1 to adapt x to !     
less5+ di/ance betwe5  himself & !      
object ( 8 inquiry4 ,?is p[] (          
spontane\s  adju/m5t is s unconsci\sly  
ex]t$1 t unless ! att5tion be  call$ to 

x by circum/ances1 we are totally unaw>e
 ( its ex]cise4    ,9 ! case j m5tion$ !
 bill w\ld be read ) eyes 9 a v  diff]5t
 /ate ( adju/m5t f t 9 :i* x 0 discov]$ 
on  ! opposite side ( ! /reet1 b no     
conviction ( ?is fact w\ld  be impress$ 
^u ! m9d4 ,if1 h[ev]1 ! suppos$         
individual  %\ld p]ceive on "s "p ( !   
pap] a small speck1 :i* he  suspects to 
be a m9ute insect1 & if he %\ld attempt 
a v close  approa* ( 8 eye = ! purpose (
 v]ify+ 8 suspicion1 he  w\ld pres5tly  
f9d t ! p[] ( natural adju/m5t has a    
limit2  = :5 8 eye has arriv$ )9 ab\t t5
 in*es1 he w discov]  t a fur!r approa* 
produces only confusion4 ,b if1 z he    
cont9ues to approa*1 he 7 to place be=e 
8 eye a s]ies (  prop]ly arrang$ convex 
l5ses1 he w\ld see ! object gradually & 
di/9ctly increase 9 app>5t size by ! m]e
 cont9uance ( !  op]ation ( approa*+4   
,yet ! glasses appli$ to ! eye dur+  !  
approa* f t5 in*es to "o in*1 w\ld h    
done no?+ m  ?an _h be5 previ\sly done  
by ! eye itself dur+ ! approa*  f fifty 

feet to "o foot4 ,9 bo? cases ! magnify+
 is effect$  really by ! approa*1 !     
l5ses m]ely r5d]+ ! latt] p]iods  ( !   
approa* compatible ) di/9ct vision4     
,a v /rik+ pro( ( ?is /atem5t may be    
obta9$ by !  foll[+ simple & in/ructive 
exp]im5t4 ,take any m9ute object1 a  v  
small insect = in/ance1 held on a p9 or 
gumm$ to a slip (  glass2 !n pres5t x to
 a /rong light1 & look at x ?r\gh !     
f9e/ ne$le-hole 9 a black5$ c>d plac$   
ab\t an in* be=e x4  ,! insect w appe> q
 di/9ct1 & ab\t t5 times l>g] ?an  its  
usual size4 ,!n sudd5ly )draw ! c>d )\t 
di/urb+ !  object1 :i* w in/antly become
 indi/9ct & ne>ly invisible4  ,! reason 
is1 t ! nak$ eye _c see at s small a    
di/ance z  "o in*4 ,b ! c>d ) ! hole    
hav+ enabl$ ! eye to  approa* )9 an in*1
 & to see di/9ctly at t di/ance1 is  ?us
 prov$ to be z decid$ly a magnify+      
in/rum5t z any l5s or  comb9ation (     
l5ses4    ,?is description ( magnify+   
p[] does n apply to su*  in/rum5ts z !  
sol> or gas microscope1 by :i* we look n

 at  ! object itself1 b at its %ad[ or  
picture on ! wall2 & !  description w   
require "s modification 9 treat+ ( !    
comp\nd  microscope1 :]e1 z 9 !         
telescope1 an image or picture is =m$   
by "o l5s1 t image or picture be+ view$ 
z an orig9al object  by ano!r l5s4    ,x
 is nev]!less s important to obta9 a    
cle> notion ( ! real  nature ( ! effect 
produc$ by a l5s appli$ to ! eye1 t we  
w adduce ! in/ance ( spectacles to r5d] 
! po9t m  famili>4 ,if ! p]son :o has   
be5 suppos$ to cross ! /reet =  !       
purpose ( read+ a bill _h be5 ag$1 !    
limit to ! p[] (  adju/m5t w\ld h be5   
discov]$ at a great] di/ance1 &  )\t s  
sev]e a te/ z ! suppos$ insect4 ,! eyes 
( ! v  ag$ g5]ally lose ! p[] ( adju/m5t
 at a di/ance ( ?irty or  =ty in*es     
in/ead ( t51 & ! spectacles worn 9      
consequ5ce  are z mu* magnify+ glasses  
to !m z ! l5ses employ$ by  y\ng] eyes  
to exam9e ! mo/ m9ute objects4          
,spectacles are  magnify+ glasses to !  
ag$ because !y enable su* p]sons to see 

z closely to _! objects z ! "y1 & !re=e 
to see !  objects l>g] ?an !y c\ld      
!mselves o!rwise see !m1 b n  l>g] ?an  
!y are se5 by ! unassi/$ y\ng] eye4     
,9 say+ t an object appe>s l>g] at "o   
"t1 or to "o p]son1  ?an ano!r1 x is    
necess>y to gu>d aga9/ misconception4   
,by !  app>5t size ( an object we mean !
 angle x subt5ds at ! eye1  or ! angle  
=m$ by two l9es drawn f ! c5tre ( ! eye 
to  ! extremities ( ! object4 ,9 ,fig4  
#a1 ! l9es ,a ,e & ,b ,e drawn  f ! arr[
 to ! eye =m ! angle ,a ,e ,b1 :i*1 :5 !
 angle  is small1 is ne>ly twice z great
 z ! angle ,c ,e ,d1 =m$ by l9es  drawn 
f a simil> arr[ at twice ! di/ance4 ,!  
arr[ ,a ,b w  !re=e appe> ne>ly twice z 
long z ,c ,d1 be+ se5 "u twice  ! angle1
 & 9 ! same proportion = any great] or  
less]  diff]5ce 9 di/ance4 ,! angle 9 "q
 is call$ ! angle (  vision1 or ! visual
 angle4    ,illu/ration3 ,fig4 #a4    ,!
 angle ( vision mu/1 h[ev]1 n be        
conf\nd$ ) ! angle (  ! p5cil ( light by
 :i* an object is se51 & :i* is expla9$ 

9 ,fig4 #b4 ,"h we h drawn two arr[s    
plac$ 9 relation to ! eye  z be=e1 & f !
 c5tre ( ea* h drawn l9es exhibit+ !    
quantity ( light :i* ea* po9t w s5d into
 ! eye at !  respective di/ances4       
,illu/ration3 ,fig4 #b4    ,n[ if ,e ,f 
repres5t ! diamet] ( ! pupil1 ! angle ,e
 ,a ,f %[s  ! size ( ! cone or p5cil (  
light :i* ent]s ! eye f !  po9t ,a1 & 9 
l mann] ! angle ,e ,b ,f is t ( ! p5cil 
emanat+ f ,b1 & ent]+ ! eye4 ,!n1 s9ce  
,e ,a ,f is d\ble ,e  ,b ,f1 x is evid5t
 t ,a is se5 by f\r times ! quantity (  
light  :i* c\ld be receiv$ f an equally 
illum9at$ po9t at ,b2 s  t ! ne>] body  
w\ld appe> bright] if x did n appe>     
l>g]2 b z its app>5t area is increas$   
f\r times z well z  its light1 no       
diff]5ce 9 ?is respect is discov]$4 ,b  
if we  c\ld f9d means to s5d into ! eye 
a l>g] p5cil ( light1 z =  in/ance t %[n
 by ! l9es ,g ,a ,h1 )\t increas+ !     
app>5t size 9 ! same proportion1 x is   
evid5t t we %\ld  obta9 a b5efit totally
 di/9ct f t ( increas$ magnitude1  & "o 

:i* is 9 "s cases ( ev5 m importance ?an
 size 9  develop+ ! /ructure ( :at we   
wi% to exam9e4 ,?is1 x w be  h]eaft]    
%[n1 is sometimes done2 = ! pres5t1 we  
wi% m]ely to  expla9 cle>ly ! di/9ction 
betwe5 app>5t magnitude1 or !  angle "u 
:i* ! object is se51 & app>5t           
brightness1 or !  angle ( ! p5cil (     
light by :i* ea* ( its po9ts is se51 &  
) ^! explanations we %all cont9ue to    
employ ! common  expressions magnify+   
glass & magnify+ p[]4    ,illu/ration3  
,fig4 #c4    ,! magnify+ p[] ( a s+le   
l5s dep5ds ^u its focal l5g?1  ! object 
be+ 9 fact plac$ ne>ly 9 its pr9cipal   
focus1 or s  t ! light :i* div]ges f ea*
 po9t may1 aft] refraction by  ! l5s1   
proce$ 9 p>allel l9es to ! eye1 or z    
ne>ly s z is  requisite = di/9ct vision4
 ,9 ,fig4 #c1 ,a ,b is a d\ble convex   
l5s1  ne> :i* is a small arr[ to        
repres5t ! object "u exam9ation1  & !   
cones drawn f its extremities are       
portions ( ! rays (  light div]g+ f ^?  
po9ts & fall+ ^u ! l5s4 ,^!  rays1 if   

suff]$ to fall at once ^u ! pupil1 w\ld 
be too  div]g5t to p]mit _! be+ br\ght  
to a focus ^u ! ret9a by  ! optical     
arrangem5ts ( ! eye4 ,b be+ fir/ pass$  
?r\gh  ! l5s1 !y are b5t into ne>ly     
p>allel l9es1 or into l9es  div]g+ f "s 
po9ts )9 ! limits ( di/9ct vision1 z  f 
,c & ,d4 ,?us alt]$1 ! eye receives !m  
precisely z if !y  emanat$ f a l>g] arr[
 plac$ at ,c ,d1 :i* we may suppose to  
be  t5 in*es f ! eye1 & !n ! diff]5ce   
betwe5 ! real &  ! imag9>y arr[ is call$
 ! magnify+ p[] ( ! l5s 9  "q4    ,f :at
 has be5 said x w be evid5t t two p]sons
 ^: eyes  diff]$ z to ! di/ance at :i*  
!y obta9$ di/9ct vision1  w\ld give     
diff]5t results z to ! magnify+ p[] ( a 
l5s4 ,to  "o :o c see di/9ctly ) ! nak$ 
eye at a di/ance ( five  in*es1 !       
magnify+ p[] w\ld seem & w\ld inde$ be  
only half  :at we h assum$4 ,su*        
in/ances are1 h[ev]1 r>e2 ! focal  l5g? 
( ! eye usually ranges f six to twelve  
or f\rte5  in*es1 s t ! di/ance we fir/ 
assum$ ( t5 in*es is v  ne> ! true      

av]age1 & is a conv5i5t numb]1 inasmu* z
 a  ciph] add$ to ! d5om9ator ( !       
fraction :i* expresses !  focal l5g? ( a
 l5s gives its magnify+ p[]4 ,?us a l5s 
^:  focal l5g? is "o-sixte5? ( an in* is
 said to magnify #afj times4    ,:5 !   
focal l5g? ( a l5s is v small1 x is     
difficult to  measure accurately !      
di/ance betwe5 its c5tre & its object4  
,9  su* cases ! be/ way to obta9 ! focal
 l5g? = p>allel or  ne>ly p>allel rays  
is to view ! image ( "s di/ant object   
=m$ by ! l5s 9 "q ?r\gh ano!r l5s ( "o  
in* sol>  focal l5g?1 keep+ bo? eyes op5
 & comp>+ ! image pres5t$  ?r\gh ! two  
l5ses ) t ( ! nak$ eye4 ,! proportion   
betwe5 ! two images s se5 w be ! focal  
l5g? requir$4 ,?us  if ! image se5 by ! 
nak$ eye is t5 times z l>ge z t %[n  by 
! l5ses1 ! focal l5g? ( ! l5s 9 "q is   
"o-t5?  ( an in*4 ,! panes ( glass 9 a  
w9d[1 or c\rses ( bricks 9 a  wall1 are 
conv5i5t objects = ?is purpose4    ,9   
:i*ev] way ! focal l5g? ( ! l5s is      
asc]ta9$1 !  rules giv5 = d$uc+ its     

magnify+ p[] are n rigor\sly  correct1  
?\gh !y are suffici5tly s = all         
practical purposes1  p>ticul>ly z ! :ole
 re/s on an assumption 9 reg>d to !     
focal l5g? ( ! eye1 & z x does n 9 any  
way affect !  actual measurem5t ( !     
object4 ,to calculate ) great precision 
! magnify+ p[] ( a l5s ) a giv5 focal   
l5g? ( eye1 x is  necess>y t ! ?ickness 
( ! l5s be tak5 into ! acc\nt1  & also !
 focal l5g? ( ! eye itself4    ,we h    
hi!rto consid]$ a magnify+ l5s only 9   
ref]5ce to its  enl>gem5t ( ! object1 or
 ! increase ( ! angle "u :i*  ! object  
is se54 ,a fur!r & equally important    
consid]ation is  t ( ! numb] ( rays or  
quantity ( light by :i* e po9t  ( !     
object is r5d]$ visible4 ,! nak$ eye1 z 
%[n 9 ,fig4 #b1  admits f ea* po9t ( e  
visible object a cone ( light hav+  !   
diamet] ( ! pupil = its base1 & mo/     
p]sons are famili>  ) t beautiful       
provision by :i* 9 cases ( excessive    
brilliancy ! pupil spontane\sly         
contracts to r$uce ! cone (  admitt$    

light )9 be>able limits4 ,?is effect is 
/ill fur!r  produc$ 9 ! exp]im5t already
 describ$1 ( look+ at an object  ?r\gh a
 ne$le-hole 9 a c>d1 :i* is equival5t to
 r$uc+ !  pupil to ! size ( a           
ne$le-hole4 ,se5 9 ?is way ! object     
becomes comp>atively d>k or obscure2    
because ea* po9t is se5 by  means ( a v 
small cone ( light1 & a little          
consid]ation w  suffice to expla9 !     
diff]5t effects produc$ by ! ne$le-hole 
& ! l5s4 ,bo? *ange ! angul> value ( !  
cone ( light  pres5t$ to ! eye1 b ! l5s 
*anges ! angle by b5d+ !  extreme rays  
)9 ! limits suit$ to di/9ct vision1 :ile
 !  ne$le-hole effects ! same purpose by
 cutt+ (f ! rays :i*  exce$ ^? limits4  
,x has be5 %[n t remov+ a brilliant     
object to a great]  di/ance w r$uce !   
quantity ( light :i* ea* po9t s5ds into 
! eye1 z effectually z view+ x ?r\gh a  
ne$le-hole2 &  magnify+ an object by a  
l5s has be5 %[n to be ! same ?+ 9  "s   
respects z remov+ x to a great] di/ance4
 ,we h to see !  magnifi$ picture by !  

light emanat+ f ! small object1 & x     
becomes a matt] ( difficulty to obta9 f 
ea* po9t a suffici5t  quantity ( light  
to be> ! diffusion ( a great magnify+   
p[]4  ,we want to p]=m an op]ation j !  
rev]se ( apply+ ! c>d  ) ! ne$le-hole to
 ! eye--we want 9 "s cases to br+ into  
! eye ! l>ge/ possible p5cil ( light f  
ea* po9t ( !  object4    ,ref]r+ to     
,fig4 #c1 x w be obs]v$ t if ! eye c\ld 
see !  small arr[ at ! di/ance !re %[n  
)\t ! int]v5tion (  ! l5s1 only a v     
small portion ( ! cones ( light drawn f 
its extremities w\ld ent] ! pupil2 :]eas
 we h suppos$ t  aft] be+ b5t by ! l5s !
 :ole ( ?is light ent]s ! eye z  "p ( ! 
cones ( small] angle ^: summits are at  
,c & ,d4 ,^!  cones w fur!r expla9 !    
diff]5ce betwe5 l>ge & small  p5cils (  
light2 ^? f ! small arr[ are l>ge       
p5cils2 !  dott$ cones f ! l>ge arr[ are
 small p5cils4    ,9 assum+ t ! :ole (  
?is light c\ld h be5 suff]$ to  ent] !  
eye ?r\gh ! l5s ,a ,b1 we did s = ! sake
 ( n  p]plex+ ! read] ) too _m          

consid]ations at once4 ,he mu/  n[ le>n 
t s l>ge a p5cil ( light pass+ ?r\gh a  
s+le  l5s w\ld be s di/ort$ by !        
sph]ical figure ( ! l5s1 & by  !        
*romatic disp]sion ( ! glass1 z to      
produce a v confus$  & imp]fect image4  
,?is confusion may be greatly dim9i%$ by
  r$uc+ ! p5cil2 = in/ance1 by apply+ a 
/op1 z x is  call$1 to ! l5s1 :i* is    
nei!r m nor less ?an !  ne$le-hole      
appli$ to ! eye4 ,a small p5cil ( light 
may be ?us  transmitt$ ?r\gh a s+le l5s 
)\t suff]+ f sph]ical  ab]ration or     
*romatic disp]sion any am\nt ( di/ortion
 :i* w  mat]ially affect ! figure ( !   
object2 b ?is quantity ( light  is      
insuffici5t to be> diffusion ov] !      
magnifi$ picture1 :i* is  !re=e too     
obscure to exhibit :at we mo/ desire to 
see--^?  beautiful & delicate m>k+s by  
:i* "o k9d ( organic matt] is  di/+ui%$ 
f ano!r4 ,) a small ap]ture ^! m>k+s are
  n se5 at all3 ) a l>ge ap]ture & a    
s+le l5s !y exhibit  a fa9t nebul\s     
appe>ance envelop$ 9 a *romatic mi/1 a  

/ate  :i* is ( c\rse utt]ly valueless to
 ! naturali/1 & n ev5  amus+ to !       
amateur4    ,x becomes !re=e a mo/      
important problem to reconcile a l>ge   
ap]ture ) di/9ctness1 or1 z x is call$1 
def9ition2 &  ?is has be5 done 9 a      
consid]able degree by effect+ ! requir$ 
am\nt ( refraction ?r\gh two or m l5ses 
in/ead ( "o1 ?us  r$uc+ ! angles (      
incid5ce & refraction1 & produc+ o!r    
effects :i* w be %ortly notic$4 ,?is 0  
fir/ accompli%$ 9  a satisfactory mann] 
by--        ,,dr4 ,,wolla/on',s         
,,d\blet1    inv5t$ by ! celebrat$      
philosoph] ^: "n x be>s2 x  consi/s (   
two plano-convex l5ses 7,fig4 #d7 hav+  
_! focal  l5g?s 9 ! proportion ( #a to  
#c1 or ne>ly s1 & plac$ at a  di/ance   
:i* c be asc]ta9$ be/ by actual         
exp]im5t4 ,_!  plane sides are plac$    
t[>ds ! object1 & ! l5s ( %orte/  focal 
l5g? next ! object4    ,illu/ration3    
,fig4 #d4    ,x appe>s t ,dr4 ,wolla/on 
0 l$ to ?is inv5tion by consid]+  t !   
,a*romatic ,huygh5ean ,eye-piece1 :i* w 

be h]eaft]  describ$1 w\ld1 if rev]s$1  
possess simil> good prop]ties z a       
simple microscope4 ,b x w be evid5t :5 !
 eye-piece is  und]/ood1 t !            
circum/ances :i* r5d] x a*romatic are v 
imp]fectly applicable to ! simple       
microscope1 & t ! d\blet1  )\t a nice   
adju/m5t ( ! /op1 w\ld be valueless4    
,dr4  ,wolla/on makes no allusion to a  
/op1 nor is x c]ta9 t he  contemplat$   
its introduction1 al?\gh 8 illness1 :i* 
t]m9at$  fatally soon aft] ! pres5tation
 ( 8 pap]1 may acc\nt = !  omission4    
,! nature ( ! corrections :i* take place
 9 ! d\blet is  expla9$ 9 ! annex$      
diagram 7,fig4 #e71 :]e ,l ,o ,l is !   
object1  ,p a portion ( ! pupil1 & ,d ,d
 ! /op1 or limit+ ap]ture4    ,n[1 x w  
be obs]v$ t ea* ( ! p5cils ( light f !  
extremities ,l ,l ( ! object is r5d]$   
ecc5trical by ! /op1  & ( consequ5ce ea*
 passes ?r\gh ! two l5ses on opposite   
sides ( _! common axis ,o ,p2 ?us ea*   
becomes affect$ by opposite  errors1 :i*
 to "s ext5t balance & correct ea* o!r4 

,to take  ! p5cil ,l1 = in/ance1 :i*    
ent]s ! eye at ,r ,b1 ,r ,b2 x is  b5t  
to ! "r at ! fir/ l5s1 & to ! left at ! 
second2  & z ea* b5d+ alt]s ! direction 
( ! blue rays m ?an  ! r$1 &1 moreov]1 z
 ! blue rays fall ne>] ! m>g9 ( !       
second l5s1 :]e ! refraction1 be+ m     
p[]ful ?an ne> !  c5tre1 comp5sates 9 "s
 degree = ! great] focal l5g? ( !       
second l5s1 ! blue & r$ rays w em]ge v  
ne>ly p>allel1  & ( consequ5ce colorless
 to ! eye4 ,at ! same "t !  sph]ical    
ab]ration has be5 dim9i%$ by !          
circum/ance t !  side ( ! p5cil :i*     
passes "o l5s ne>e/ ! axis passes !  o!r
 ne>e/ ! m>g94    ,?is explanation      
applies only to ! p5cils ne> !          
extremities (  ! object4 ,! c5tral      
p5cil1 x is obvi\s1 w\ld pass bo? l5ses 
symmetrically2 ! same portions ( light  
occupy+ ne>ly ! same  relative places on
 bo? l5ses4 ,! blue light w\ld ent] !   
second  l5s ne>] to its axis ?an ! r$1 &
 be+ ?us less refract$  ?an ! r$ by !   
second l5s1 a small am\nt ( comp5sation 

w\ld  take place1 q diff]5t 9 pr9ciple &
 inf]ior 9 degree to  t :i* is produc$ 9
 ! ecc5trical p5cils4 ,9 ! int]m$iate   
spaces ! corrections are /ill m imp]fect
 & unc]ta92 &  ?is expla9s ! cause ( !  
ab]rations :i* mu/ ( necessity  exi/ ev5
 9 ! be/-made d\blet4 ,x is1 h[ev]1     
inf9itely  sup]ior to a s+le l5s1 & w   
transmit a p5cil ( an angle (  f #ce to 
#ej )\t any v s5sible errors4 ,x        
exhibits1  !re=e1 _m ( ! usual          
te/-objects 9 a v beautiful mann]4      
,illu/ration3 ,fig4 #e4    ,illu/ration3
 ,fig4 #f4    ,! next /ep 9 ! improvem5t
 ( ! simple microscope be>s m  analogy  
to ! eye-piece4 ,?is improvem5t 0 made  
by ,mr4 ,holl&1  & x consi/s 7z %[n 9   
,fig4 #f7 9 sub/itut+ two l5ses =  !    
fir/ 9 ! d\blet1 & reta9+ ! /op betwe5  
!m & !  ?ird4 ,! fir/ b5d+1 be+ ?us     
effect$ by two l5ses in/ead (  "o1 is   
accompani$ by small] ab]rations1 :i* are
 !re=e m  completely balanc$ or correct$
 at ! second b5d+1 9 !  opposite        
direction1 by ! ?ird l5s4 ,?is          

comb9ation1 ?\gh call$  a triplet is    
ess5tially a d\blet1 9 :i* ! ant]ior l5s
 is  divid$ into two4 ,= x mu/ be       
recollect$ t ! fir/ pair (  l5ses m]ely 
accompli%es :at might h be5 done1 ?\gh )
 less  precision1 by "o2 b ! two l5ses (
 ! d\blet are oppos$ to  ea* o!r2 !     
second dim9i%+ ! magnify+ p[] ( ! fir/4 
,! fir/ pair ( l5ses 9 ! triplet concur 
9 produc+ a c]ta9  am\nt ( magnify+ p[]1
 :i* is dim9i%$ 9 quantity &  correct$ z
 to ab]ration at ! ?ird l5s by ! *ange 9
 relation  to ! position ( ! axis :i*   
takes place 9 ! p5cil betwe5  :at is    
virtually ! fir/ & second l5s4 ,9 ?is   
comb9ation !  errors are /ill fur!r     
r$uc$ by ! close approximation to !     
object :i* causes ! refractions to take 
place ne> ! axis4 ,?us  ! transmission (
 a /ill l>g] angul> p5cil1 namely #fe1  
is  r5d]$ compatible ) di/9ctness1 & a m
 int5se image is  pres5t$ to ! eye4     
,e increase 9 ! numb] ( l5ses is att5d$ 
) "o drawback1  f ! circum/ance t a     
c]ta9 portion ( light is lo/ by         

reflection & absorption ea* "t t ! ray  
ent]s a new m$ium4  ,?is loss be>s no   
s5sible proportion to ! ga9 aris+ f !   
increas$ ap]ture1 :i*1 be+ z ! squ>e ( !
 diamet]1  multiplies rapidly2 or1 if we
 e/imate by ! angle ( ! admitt$  p5cil1 
:i* is m easily asc]ta9$1 ! int5sity w  
be z !  squ>e ( twice ! tang5t ( half ! 
angle4 ,to expla9 ?is1 let ,d  ,b 7,fig4
 #g7 repres5t ! diamet] ( ! l5s1 or ( t 
"p ( x  :i* is really employ$2 ,c ,a !  
p]p5dicul> drawn f its  c5tre1 & ,a ,b1 
,a ,d1 ! extreme rays ( ! incid5t p5cil 
( light  ,,dab4 ,!n ! diamet] be+ #b ,c 
,b1 ! area to :i* ! int5sity (  vision  
is proportional w be 7#b ,c ,b71 & ,c ,b
 is evid5tly !  tang5t ( ! angle ,c ,a  
,b1 :i* is half ! angle ( ! admitt$     
p5cil ,d ,a ,b4 ,or1 if a be us$ to     
d5ote ! angul> ap]ture1 !  expression = 
! int5sity is 7#b tan4 a7 :i* increases 
s  rapidly ) ! increase ( a z to make ! 
loss ( light by  reflection & absorption
 ( little consequ5ce4    ,illu/ration3  
,fig4 #g4    ,! comb9ation ( ?ree l5ses 

approa*es1 z has be5 /at$1 v  close to !
 object2 s close1 inde$1 z to prev5t !  
use ( m  ?an ?ree2 & ?is con/itutes a   
limit to ! improvem5t ( !  simple       
microscope1 = x is call$ a simple       
microscope1 al?\gh  consi/+ ( ?ree      
l5ses1 & al?\gh a comp\nd microscope may
 be  made ( only ?ree or ev5 two l5ses2 
b ! diff]5t arrangem5t  :i* gives rise  
to ! t]m comp\nd w be bett] und]/ood :5 
t in/rum5t is expla9$4    ,be=e we      
proce$ to describe ! simple microscope &
 its  app5dages1 x w be well to expla9  
su* o!r po9ts 9 ref]5ce  to ! =m &      
mat]ials ( l5ses z are mo/ likely to be 
int]e/+4    ,a v useful =m ( l5s 0      
propos$ by ,dr4 ,wolla/on1 & call$  by  
him ! ,p]iscopic l5s4 ,x consi/$ ( two  
hemisph]ical l5ses1  cem5t$ toge!r by _!
 plane faces1 hav+ a /op betwe5 !m to   
limit ! ap]ture4 ,a simil> proposal 0   
made ,mr4 ,codd+ton1 :o1  h[ev]1 execut$
 ! project 9 a bett] mann]1 by cutt+ a  
groove  9 a :ole sph]e1 & fill+ ! groove
 ) opaque matt]4 ,8  l5s1 :i* is !      

well-kn[n ,codd+ton l5s1 is %[n 9 ,fig4 
#h4 ,x  gives a l>ge field ( view1 :i*  
is equally good 9 all directions1  z x  
is evid5t t ! p5cils ,a ,a & ,b ,b pass 
?r\gh "u  precisely ! same circum/ances4
 ,its sph]ical =m has ! fur!r  advantage
 ( r5d]+ ! position 9 :i* x is held (   
comp>atively little consequ5ce4 ,x is   
!re=e v conv5i5t z  a h&-l5s1 b its     
def9ition is ( c\rse n s good z t ( a   
well-made d\blet or a*romatic l5s4      
,illu/ration3 ,fig4 #h4    ,ano!r v     
useful =m ( d\blet 0 propos$ by ,sir    
,john ,h]s*el1  *iefly l ! ,codd+ton    
l5s1 = ! sake ( a wide field1 &  *iefly 
to be us$ 9 ! h&4 ,x is %[n 9 ,fig4 #i2 
x consi/s (  a d\ble convex or cross$   
l5s1 hav+ ! radii ( curvature z #a to   
#f1 & ( a plane concave l5s ^: focal    
l5g? is to t ( !  convex l5s z #ac to   
#e4    ,v>i\s1 inde$ innum]able1 o!r =ms
 & comb9ations ( l5ses  h be5 project$1 
"s display+ mu* +5uity1 b few ( any     
practical use4 ,9 ! ,catadioptric l5ses 
! light em]ges at "r  angles f its ent]+

 direction1 be+ reflect$ f a surface cut
  at an angle ( #de degrees to ! axes ( 
! curv$ surfaces4    ,illu/ration3 ,fig4
 #i4    ,x 0 at "o "t hop$1 z ! preci\s 
/ones are m refractive  ?an glass1 & z !
 increas$ refractive p[] is unaccompani$
 by  a correspond5t increase 9 *romatic 
disp]sion1 t !y w\ld  furni% valuable   
mat]ials = l5ses1 inasmu* z !           
refractions  w\ld be accompli%$ by      
%all[] curves1 & consequ5tly )  dim9i%$ 
sph]ical ab]ration4 ,b ^! hopes 7       
disappo9t$2  ev]y?+ t +5uity &          
p]sev]ance c\ld accompli% 0 tri$  by    
,mr4 ,v>ley & ,mr4 ,prit*>d1 "u !       
patronage ( ,dr4 ,gor+4 ,x  appe>$1     
h[ev]1 t ! great reflective p[]1 !      
d\bly-refract+ prop]ty1 ! color1 & !    
het]og5e\s /ructure  ( ! jewels :i* 7   
tri$1 mu* m ?an c\nt]balanc$ !  b5efits 
aris+ f _! great] refractive p[]1 & left
 no  d\bt ( ! sup]iority ( skillfully   
made glass d\blets &  triplets4 ,! idea 
is n[1 9 fact1 ab&on$2 & ! same rem>k is
  applicable to ! attempts at con/ruct+ 

fluid l5ses1 & to !  projects = giv+ to 
glass o!r ?an sph]ical surfaces--none ( 
:i* h come into ext5sive use4    ,by !  
t]m simple microscope is meant "o 9 :i* 
! object is  view$ directly ?r\gh a l5s 
or comb9ation ( l5ses1 j z we  h suppos$
 an arr[ or an insect to be view$ ?r\gh 
a glass held  9 ! h&4 ,:51 h[ev]1 !     
magnify+ p[] ( ! glass is  consid]able1 
9 o!r words1 :5 its focal l5g? is v     
%ort1 &  its prop] di/ance f its object 
( consequ5ce equally %ort1 x  requires  
to be plac$ at t prop] di/ance ) great  
precision3 x  _c1 !re=e1 be held )      
suffici5t accuracy & /ead9ess by  !     
unassi/$ h&1 b mu/ be m\nt$ 9 a frame   
hav+ a rack or  screw to move x t[>ds or
 f ano!r frame or /age :i* holds  !     
object4 ,x is !n call$ a microscope1 & x
 is furni%$1  accord+ to circum/ances1 )
 l5ses & mirrors to collect &  reflect !
 light ^u ! object1 & ) o!r conv5i5ces  
:i*  w n[ be describ$4    ,"o ( ! be/   
=ms ( a /& = a simple microscope is %[n 
9  ,fig4 #aj1 :]e ,a is a brass pill>   

screw$ to a tripod base2 ,b is a  broad 
/age = ! objects1 secur$ to ! /em by    
screws1 ^:  mill$ heads are at ,c4 ,by  
means ( ! l>ge mill$ head ,d1 a         
triangul> b>1 hav+ a rack1 is elevat$   
out ( ! /em ,a1 c>ry+  ! l5s-hold] ,e1  
:i* has a horizontal movem5t 9 "o       
direction1  by means ( a rack work$ by !
 mill$ head ,f1 & 9 ! o!r  direction by 
turn+ on a circul> p94 ,a concave mirror
 ,g reflects  ! light upw>ds ?r\gh !    
hole 9 ! /age1 & a l5s may be  atta*$ to
 ! /age = ! purpose ( ?r[+ light on an  
opaque  object1 9 ! same way t ! =ceps  
,h = hold+ su* objects is  atta*$4 ,?is 
microscope is peculi>ly adapt$1 by its  
broad /age  & its g5]al /ead9ess1 =     
dissect+2 & x is r5d]$ m  conv5i5t = ?is
 purpose by plac+ x betwe5 two incl9$   
planes  ( mahogany1 :i* support ! arms &
 elevate ! wri/s to !  level ( ! /age4  
,?is app>atus is call$ ! dissect+ re/4  
,:5  dissect+ is n a prim>y object1 a   
jo9t may be made at ! l[]  end ( ! /em  
,a1 to all[ ! :ole to take an incl9$    

position2  & !n ! spr+ clips %[n ^u !   
/age are useful to reta9  ! object 9 its
 place4 ,num]\s conv5i5t app5dages may  
be made to  accompany su* microscopes1  
:i* x w be impossible to m5tion 9       
detail2 ! mo/ useful are ,mr4 ,v>ley's  
capill>y cages =  conta9+ animalcul 9   
wat]1 & p>ts ( aquatic plants2 also 8   
tubes = obta9+ & sep>at+ su* objects1 & 
8 phial &  phial-hold] = pres]v+ &      
exhibit+ small liv+ specim5s (  ! ,*>a1 
,nitella1 & o!r simil> plants1 & obs]v+ 
_!  circulation4 ,! phial-microscope    
af=ds facilities = obs]v+ !  op]ations (
 m9ute vegetable & animal life1 :i* w   
probably  lead to ! mo/ int]e/+         
discov]ies4 ,! rec5t volumes ( !        
,transactions ( ! ,society ( ,arts      
conta9 an imm5se mass (  in=mation ( ?is
 sort1 & to ^! we ref] ! read]4         
,illu/ration3 ,fig4 #aj4    ,! mode (   
illum9at+ objects is "o on :i* we mu/   
give "s  fur!r in=mation1 = ! mann] 9   
:i* an object is light$ is  second 9    
importance only to ! excell5ce ( ! glass

 ?r\gh :i*  x is se54 ,9 inve/igat+ any 
new or unkn[n specim51 x %\ld be  view$ 
9 turns by e description ( light1 direct
 & oblique1 z  a transp>5t object & z an
 opaque object1 ) /rong & )  fa9t light1
 ) l>ge angul> p5cils & ) small angul>  
p5cils  ?r[n 9 all possible directions4 
,e *ange w probably develop  "s new fact
 9 ref]5ce to ! /ructure ( ! object1 :i*
  %\ld itself be v>i$ 9 ! mode ( m\nt+ 9
 e possible way4  ,x %\ld be se5 bo? wet
 & dry1 & imm]s$ 9 fluids ( v>i\s       
qualities & d5sities1 su* z wat]1       
alcohol1 oil1 & ,canada  balsam1 =      
in/ance1 :i* la/ has a refractive p[]   
ne>ly equal  to t ( glass4 ,if ! object 
be delicate vegetable tissue1 x w  be 9 
"s respects r5d]$ m visible by g5tle    
heat+ or  scor*+ by a cle> fire plac$   
betwe5 two plates ( glass4 ,9 ?is  way !
 spiral vessels ( asp>agus & o!r simil> 
vegetables may  be beautifully display$4
 ,dye+ ! objects 9 t9cture ( iod9e  w 9 
"s cases answ] ?is purpose bett]4    ,b 
! pr9cipal "q 9 reg>d to illum9ation is 

! magnitude  ( ! illum9at+ p5cil1       
p>ticul>ly 9 ref]5ce to transp>5t       
objects4 ,g5]ally speak+ ! illum9at+    
p5cil %\ld be z l>ge  z c be receiv$ by 
! l5s1 & no l>g]4 ,any light beyond ?is 
produces indi/9ctness & gl>e4 ,!        
sup]flu\s light f !  mirror may be cut  
(f by a scre5 hav+ v>i\s-siz$ ap]tures  
plac$ bel[ ! /age2 b ! be/ mode (       
illum9ation is t  propos$ by ,dr4       
,wolla/on1 & call$ ! ,wolla/on cond5s]4 
,a tube  is plac$ bel[ ! /age ( !       
in/rum5t conta9+ a l5s ,a ,b  7,fig4    
#aa71 :i* c be elevat$ or depress$ )9   
c]ta9 limits at  pleasure2 & at ! l[]   
end is a /op ) a limit$ ap]ture ,c ,d4  
,a plane mirror ,e ,f receives ! rays ( 
light ,l ,l f ! sky or a  :ite cl\d1 :i*
 la/ is ! be/ s\rce ( light1 & reflects 
!m  upw>ds ?r\gh ! ap]ture 9 ,c ,d1 s t 
!y are refract$1 &  =m an image ( !     
ap]ture at ,g1 :i* is suppos$ to be     
ne>ly  ! place ( ! object4 ,! object is 
sometimes be/ se5 :5 !  image ( !       
ap]ture is also be/ se52 & sometimes x  

is be/ to  elevate ! summit ,g ( ! cone 
,a ,b ,g above ! object1 & at o!rs  to  
depress x bel[3 all :i* is done at      
pleasure by ! p[] (  mov+ ! l5s ,a ,b4  
,if artifical light 7z a lamp or c&le7  
be  employ$1 ! flame mu/ be plac$ 9 !   
pr9cipal focus ( a l>ge  deta*$ l5s on a
 /&1 s t ! rays ,l ,l may fall 9 p>allel
  l9es on ! mirror1 or z !y w\ld fall f 
! cl\d4 ,?is w  be f\nd an advantage1 n 
only :5 ! ,wolla/on cond5s] is  employ$1
 b also :5 ! mirror & diaphragm are us$4
 ,a good mode  ( imitat+ artificially ! 
light ( a :ite cl\d opposite ! sun  has 
be5 propos$ by ,mr4 ,v>ley2 he cov]s !  
surface ( ! mirror  "u ! /age ) c>bonate
 ( soda or any simil> mat]ial1 &  !n    
conc5trates ! sun's light ^u its surface
 by a l>ge  cond5s+ l5s4 ,! int5se :ite 
light diffus$ f ! surface (  ! soda =ms 
an excell5t sub/itute = ! :ite cl\d1    
:i*1  :5 opposite ! sun1 & ( consid]able
 size1 is ! be/ daylight1  z ! pure sky 
opposite to ! sun is ! wor/4            
,illu/ration3 ,fig4 #aa4    ,! ,comp\nd 

,microscope may1 z be=e /at$1 consi/ (  
only two  l5ses1 :ile a simple          
microscope has be5 %[n to conta9        
sometimes  ?ree4 ,9 ! triplet = ! simple
 microscope1 h[ev]1 x 0  expla9$ t !    
effect ( ! two fir/ l5ses 0 to d :at    
might  h be5 accompli%$1 ?\gh n s well1 
by "o2 & ! ?ird  m]ely effect$ c]ta9    
modifications 9 ! light be=e x ent]$  ! 
eye4 ,b 9 ! comp\nd microscope ! two    
l5ses h totally  diff]5t functions2 !   
fir/ receives ! rays f ! object1 &1     
br++ !m to new foci1 =ms an image1 :i* !
 second l5s  treats z an orig9al object1
 & magnifies x j z ! s+le  microscope   
magnifi$ ! object itself4               
,illu/ration3 ,fig4 #ab4    ,! annex$   
figure 7#ab7 %[s ! c\rse ( ! rays ?r\gh 
a  comp\nd microscope ( two l5ses4 ,!   
rays proce$+ f ! object  ,a ,b are s    
act$ ^u by ! l5s ,c ,d1 ne> x1 & !nce   
call$ !  object glass1 t !y are conv]g$ 
to foci 9 ,a ,b1 :]e !y  =m an enl>g$   
image ( ! object1 z w\ld be evid5t if a 
piece  ( oil$ pap] or gr\nd glass 7     

plac$ !re to receive !m4 ,!y  are n s   
int]cept$1 & !re=e ! image is n r5d]$   
visible at t place2 b _! fur!r progress 
is simil> to :at  x w\ld h be5 _h !y    
really proce$$ f an object at ,a ,b4    
,!y are at l5g? receiv$ by ! eye-l5s ,l 
,m1 :i* acts ^u !m  z ! simple          
microscope has be5 describ$ to act on ! 
light  proce$+ f its objects4 ,!y are   
b5t s t !y may ent] !  eye at ,e 9      
p>allel l9es1 or z ne>ly s z is         
requisite =  di/9ct vision4 ,:5 we say t
 ! rays ent] ! eye 9 ne>ly  p>allel     
l9es1 we mean only ^? :i* proce$ f "o   
po9t ( !  orig9al object4 ,?us ! two    
p>allel rays ,m ,e h proce$$ f  & are "p
 ( ! cone ( rays ,c ,a ,d1 emanat+ f !  
po9t ,a (  ! arr[2 b !y d n =m two      
pictures 9 ! eye1 because any  numb] (  
p>allel rays :i* ! pupil c receive w be 
conv]g$  to a po9t by ! eye1 & w convey 
! impression ( "o po9t to  ! m9d4 ,9 l  
mann] ! rays ,l ,e are "p ( ! cone (    
rays  emanat+ f ,b1 & ! angle ,l ,e ,m  
is t "u :i* ! eye w  see ! magnifi$     

image ( ! arr[1 :i* is evid5tly _m times
  great] ?an ! arr[ c\ld be made to     
occupy 9 ! nak$ eye at any  di/ance )9 !
 limits ( di/9ct vision4 ,! magnify+ p[]
  dep5ds on two circum/ances3 fir/1 on !
 ratio betwe5 ! ant]ior  di/ance ,a ,c  
or ,b ,d & ! po/]ior focal l5g? ,c ,b or
 ,d ,a2 &  secondly1 on ! p[] ( !       
eye-l5s ,l ,m4 ,! fir/ ratio is !  same 
z t betwe5 ! object ,a ,b & ! image ,a  
,b2 ?is & !  focal l5g? or p[] ( ! eye  
l5s are bo? easily obta9$1 &  _! product
 is ! p[] ( ! comp\nd in/rum5t4    ,s9ce
 ! p[] dep5ds on ! ratio betwe5 !       
ant]ior &  po/]ior foci ( !             
object-glass1 x is evid5t t by increas+ 
t ratio any p[] may be obta9$1 ! same   
eye-glass be+ us$2  or hav+ det]m9$ !   
fir/1 any fur!r p[] may be obta9$ by    
increas+ t ( ! eye-glass2 & ?us1 by a   
pre-arrangem5t (  ! relative proportions
 9 :i* ! magnify+ p[] %all be  divid$   
betwe5 ! object-glass & ! eye-glass1    
almo/ any giv5  di/ance 7)9 c]ta9       
limits7 betwe5 ! fir/ & its object may  

be secur$4 ,?is is "o valuable          
peculi>ity ( ! comp\nd  in/rum5t2 &     
ano!r is ! l>ge field1 or l>ge angle (  
view1  :i* may be obta9$1 e "p ( :i* w  
be ne>ly equally  good2 :]eas ) ! be/   
simple microscopes ! field is small1 &  
is good only 9 ! c5tre4 ,! field ( !    
comp\nd in/rum5t is  fur!r increas$ by  
us+ two glasses at ! eye-end2 ! fir/ be+
  call$1 f its purpose1 ! field-glass1 &
 ! two con/itut+  :at is call$ !        
eye-piece4 ,?is w be m p>ticul>ly       
expla9$  9 ! figure ( ! a*romatic       
comp\nd microscope pres5tly giv54    ,= 
upw>ds ( a c5tury ! comp\nd microscope1 
not)/&+ !  advantages above m5tion$1 0 a
 comp>atively feeble & ineffici5t       
in/rum5t1 ow+ to ! di/ance :i* ! light  
_h to trav]se1 &  ! consequ5t increase (
 ! *romatic & sph]ical ab]rations4 ,to  
expla9 ?is we h drawn 9 ,fig4 #ab a     
second image ne> ,a ,b1 !  fact be+ t ! 
object-glass w\ld n =m "o image1 z has  
be5  suppos$1 b an inf9ite numb] (      
v>i\sly-color$ &  v>i\s-siz$ images1    

occupy+ ! space betwe5 ! two dott$      
arr[s4 ,^? ne>e/ ! object-glass w\ld be 
r$1 & ^? ne>e/  ! eye-glass w\ld be     
blue4 ,! effect ( ?is is to produce s   
mu*  confusion1 t ! in/rum5t 0 r$uc$ to 
a m]e toy1 al?\gh  ^! errors 7 dim9i%$  
to ! utmo/ possible ext5t by limit+  !  
ap]ture ( ! object-glass1 & ?us re/rict+
 ! angle (  ! p5cil ( light f ea* po9t (
 ! object4 ,b ?is involv$  ! defects1   
already expla9$1 ( mak+ ! picture       
obscure1 s t  on ! :ole ! be/ comp\nd   
in/rum5ts 7 inf]ior to ! simple         
microscopes ( a s+le l5s1 ) :i*1 inde$1 
all ! important  obs]vations ( ! la/    
c5tury 7 made4    ,ev5 aft] ! improvem5t
 ( ! simple microscope by ! use (       
d\blets & triplets1 ! long c\rse ( !    
rays1 & ! l>ge  angul> p5cil requir$ 9 !
 comp\nd in/rum5t1 det]r$ ! mo/  sangu9e
 f anticipat+ ! p]iod :5 !y %\ld be     
conduct$  ?r\gh su* a pa? free bo? f    
sph]ical & *romatic errors4  ,)9 tw5ty  
ye>s ( ! pres5t p]iod1 philosoph]s ( no 
less  em95ce ?an ,m4 ,blot & ,dr4       

,wolla/on pr$ict$ t ! comp\nd  w\ld nev]
 rival ! simple microscope1 & t ! idea (
  a*romatiz+ its object-glass 0         
hopeless4 ,nor c ^! op9ions be  wond]$  
at :5 we consid] h[ _m ye>s ! a*romatic 
telescope  _h exi/$ )\t an attempt to   
apply its pr9ciples to ! comp\nd        
microscope4 ,:5 we consid] ! smallness (
 ! p5cil requir$ by  ! telescope1 & !   
enorm\s increase ( difficulty att5d+ e  
enl>gem5t ( ! p5cil--:5 we consid] fur!r
 t ^!  difficulties _h to be cont5d$ ) &
 remov$ by op]ations on  portions (     
glass s small t !y are !mselves almo/   
microscopic  objects1 we %all n be      
surpris$ t ev5 a cauti\s philosoph]  &  
mo/ able manipulator l ,dr4 ,wolla/on   
%\ld prescribe limits  to improvem5t4   
,=tunately = sci5ce1 & especially = !   
dep>tm5ts ( animal  & vegetable         
physiology1 ^! pr$ictions h be5 %[n to  
be  unf\nd$4 ,! la/ fifte5 ye>s h       
suffic$ to elevate !  comp\nd microscope
 f ! condition we h describ$ to t (  be+
 ! mo/ important in/rum5t ev] be/[$ by  

art ^u !  inve/igator ( nature4 ,x n[   
holds a v high rank among  philosophical
 implem5ts1 :ile ! transc5dant beauties 
( =m1  color & organization1 :i* x      
reveals to u 9 ! m9ute works (  nature1 
r5d] x subs]vi5t to ! mo/ delightful &  
in/ructive  pursuits4 ,to ^! claims on  
our att5tion1 x appe>s likely to add a  
?ird ( /ill high] importance4 ,!        
microscopic exam9ation ( !  blood & o!r 
human organic matt] w 9 all probability 
af=d  m satisfactory & conclusive       
evid5ce reg>d+ ! nature &  seat (       
disease ?an any hi!rto appeal$ to1 & w (
 consequ5ce  lead to simil> c]ta9ty 9 ! 
*oice & application ( rem$ies4    ,we h 
?\ght x necess>y to /ate ?us at l>ge !  
claims ( !  mod]n a*romatic microscope  
^u ! att5tion ( ! read]1 z a            
ju/ification ( ! l5g? at :i* we %all    
give its rec5t hi/ory  & expla9 its     
con/ruction2 & we are fur!r induc$ to   
?is  c\rse by ! consid]ation t ! subject
 is entirely new gr\nd1  & t !re are at 
?is "t n m ?an two or ?ree mak]s (      

a*romatic microscopes 9 ,engl&4    ,soon
 aft] ! ye> #ahbj a s]ies ( exp]im5ts 0 
begun 9 ,france  by ,m4 ,selligues1 :i* 
7 foll[$ up by ,frau5h(] at ,muni*1 by  
,amici at ,mod5a1 by ,m4 ,*evali] at    
,p>is1 & by ! late ,mr4 ,tulley  9      
,london4 ,9 #ahbd ! la/-nam$ excell5t   
arti/1 )\t kn[+  :at _h be5 done on !   
,cont95t1 made ! attempt to con/ruct an 
a*romatic object-glass = a comp\nd      
microscope1 & produc$ "o (  n9e-t5?s (  
an in* focal l5g?1 compos$ ( ?ree l5ses1
 &  transmitt+ a p5cil ( eighte5        
degrees4 ,?is 0 ! fir/ t _h  be5 made 9 
,engl&2 & x is due to ,mr4 ,tulley to   
say1 t z  reg>ds accurate correction    
?r\gh\t ! field1 t glass has n  be5     
excell$ by any subsequ5t comb9ation (   
?ree l5ses4 ,su* an  angul> p5cil1 & su*
 a focal l5g?1 w\ld be> an eye-piece    
adapt$ to produce a gross magnify+ p[] (
 "o hundr$ & tw5ty4  ,mr4 ,tulley       
aft]w>ds made a comb9ation to be plac$ 9
 front ( !  fir/ m5tion$1 :i* increas$ !
 angle ( ! transmitt$ p5cil  to         

?irty-eight degrees1 & bore a p[] ( ?ree
 hundr$4    ,:ile ^! practical          
inve/igations 7 9 progress1 ! subject ( 
a*romatism engag$ ! att5tion ( "s ( !   
mo/ pr(\nd  ma!maticians 9 ,engl&4 ,sir 
,john ,h]s*el1 ,pr(essor ,airy1         
,pr(essor ,b>l[1 ,mr4 ,codd+ton1 & o!rs1
 contribut$ l>gely to  ! !oretical      
exam9ation ( ! subject2 & ?\gh ! results
 (  _! labors 7 n imm$iately applicable 
to ! microscope1 !y  ess5tially promot$ 
its improvem5t4    ,= "s "t prior to    
#ahbi ! subject _h occupi$ ! m9d ( a    
g5tleman1 :o1 n entirely practical1 l ! 
fir/1 nor purely  ma!matical1 l !       
la/-m5tion$ class ( inquir]s1 0 l$ to  !
 discov]y ( c]ta9 prop]ties 9 a*romatic 
comb9ations :i*  _h be5 be=e unobs]v$4  
,^! 7 aft]w>ds exp]im5tally  v]ifi$2 & 9
 ! ye> #ahbi a pap] on ! subject1 by !  
discov]]1 ,mr4 ,joseph ,jackson ,li/]1 0
 read & publi%$ by !  ,royal ,society4  
,! pr9ciples & results ?us obta9$ enabl$
 ,mr4  ,li/] to =m a comb9ation ( l5ses 
:i* transmitt$ a p5cil (  fifty degrees1

 ) a l>ge field correct 9 e "p2 z ?is   
pap]  0 ! f\ndation ( ! rec5t           
improvem5ts 9 a*romatic  microscopes1 & 
z its results are indisp5sable to all :o
 w\ld  make or und]/& ! in/rum5t1 we    
%all give ! m important  p>ts ( x 9     
detail1 & 9 ,mr4 ,li/]'s own words4     
8,i w\ld premise t ! plano-concave =m = 
! correct+ fl9t  l5s has 9 t quality a  
/rong recomm5dation1 p>ticul>ly z x     
obviates ! dang] ( error :i* o!rwise    
exi/s 9 c5t]+ !  two curves1 & !reby    
admits ( correct workman%ip = a %ort]   
focus4 ,to cem5t toge!r also ! two      
surfaces ( ! glass  dim9i%es by v ne>ly 
half ! loss ( light f reflection1  :i*  
is consid]able at ! num]\s surfaces ( a 
comb9ation4 ,i  h ?\ght ! cle>ness ( !  
field & brightness ( ! picture  evid5tly
 increas$ by do+ ?is2 x prev5ts any     
dew9ess or  vegetation f =m+ on ! inn]  
surfaces2 & ,i see no  disadvantage to  
be anticipat$ f x if !y are ( id5tical  
curves1 & press$ closely toge!r1 & !    
cem5t+ m$ium  p]man5tly homog5e\s4      

8,^! two conditions !n1 t ! fl9t l5s    
%all be  plano-concave1 & t x %all be   
jo9$ by "s cem5t to !  convex1 seem     
desirable to be tak5 z a basis = !      
microscopic  object-glass1 provid$ !y c 
be reconcil$ ) ! de/ruction (  !        
sph]ical & *romatic ab]rations ( a l>ge 
p5cil4    8,n[ 9 e su* glass t has be5  
tri$ by me :i* has _h its  correct+ l5s 
( ei!r ,swiss or ,engli% glass1 ) a     
d\ble convex  ( plate1 & has be5 made   
a*romatic by ! =m giv5 to ! out]  curve 
( ! convex1 ! proportion has be5 su*    
betwe5 !  refractive & disp]sive p[]s ( 
its l5ses1 t its figure has  be5 correct
 = rays issu+ f "s po9t 9 its axis n f> 
f  its pr9cipal focus on its plane side1
 & ei!r t5d+ to a  conjugate focus )9 ! 
tube ( a microscope1 or em]g+ ne>ly     
p>allel4    8,let ,a ,b 7,fig4 #ac7 be  
suppos$ su* an object-glass1 & let x be 
r\ghly consid]$ z a plano-convex l5s1 ) 
a curve ,a ,c ,b runn+  ?r\gh x1 at :i* 
! sph]ical & *romatic errors are        
correct$  :i* are g5]at$ at ! two out]  

surfaces2 & let ! glass be  ?us free f  
ab]ration = rays ,f ,d ,e ,g issu+ f !  
radiant  po9t ,f1 ,h ,e be+ a p]p5dicul>
 to ! convex surface1 & ,i ,d to  !     
plane "o4 ,"u ^! circum/ances1 ! angle (
 em]g5ce ,g ,e ,h  mu* exce$s t (       
incid5ce ,f ,d ,i1 be+ probably ne>ly   
?ree  times z great4    8,if ! radiant  
is n[ made to approa* ! glass1 s t !    
c\rse  ( ! ray ,f ,d ,e ,g %all be m    
div]g5t f ! axis1 z !  angles ( incid5ce
 & em]g5ce become m ne>ly equal to ea*  
o!r1 ! sph]ical ab]ration produc$ by !  
two w be f\nd to  be> a less proportion 
to ! oppos+ error ( ! s+le correct+     
curve ,a ,c ,b2 = su* a focus !re=e !   
rays w be  ov]-correct$4                
,illu/ration3 ,fig4 #ac4    8,b if ,f   
/ill approa*es ! glass1 ! angle (       
incid5ce  cont9ues to increase ) !      
increas+ div]g5ce ( ! ray1 till  x w    
exce$ t ( em]g5ce1 :i* has 9 ! mean:ile 
be5  dim9i%+1 & at l5g? ! sph]ical error
 produc$ by !m w  recov] its orig9al    
proportion to ! opposite error ( ! curve

 (  correction4 ,:5 ,f has rea*$ ?is    
po9t ,f 7at :i* ! angle (  incid5ce does
 n exce$ t ( em]g5ce s mu* z x _h at    
fir/  come %ort ( x71 ! rays aga9 pass !
 glass free f sph]ical  ab]ration4      
8,if ,f be c>ri$ f h5ce t[>ds ! glass1  
or outw>ds f its  orig9al place1 ! angle
 ( incid5ce 9 ! =m] case1 or (  em]g5ce 
9 ! latt]1 becomes disproportionately   
effective1 &  ei!r way ! ab]ration      
exce$s ! correction4    8,^! facts h be5
 e/abli%$ by c>eful exp]im5t3 !y accord 
) e appe>ance 9 su* comb9ations ( !     
plano-convex glasses  z h come "u my    
notice1 & may1 ,i believe1 be ext5d$ to 
?is  rule1 t 9 g5]al an a*romatic       
object-glass1 ( :i* ! inn]  surfaces are
 9 contact1 or ne>ly s1 w h on "o side (
 x two  foci 9 its axis1 = ! rays       
proce$+ f :i* x w be truly  correct$ at 
a mod]ate ap]ture2 t = ! space betwe5 ^!
 two  po9ts its sph]ical ab]ration w be 
ov]-correct$1 & beyond  !m ei!r way     
"u-correct$4    8,! long] aplanatic     
focus may be f\nd1 :5 "o ( !            

plano-convex  object-glasses is plac$ 9 
a microscope1 by %ort5+ ! tube1 if  !   
glass %[s ov]-correction2 if            
"u-correction1 by l5g!n+  x1 or by br++ 
! rays toge!r1 %\ld !y be p>allel or    
div]g5t1 by a v small good telescope4 ,!
 %ort] focus is got at  by slid+ ! glass
 be=e ano!r ( suffici5t l5g? & l>ge     
ap]ture t is f9ely correct$1 & br++ x   
=w>ds till x  gives ! reflection ( a    
bright po9t f a globule ( quicksilv]1   
%>p & free f mi/1 :5 ! di/ance c be tak5
 betwe5 !  glass & ! object4    8,!     
long] focus is ! place at :i* to asc]ta9
 ! utmo/  ap]ture t may be giv5 to !    
glass1 & :]e1 9 ! abs5ce (  sph]ical    
error1 its exact /ate ( correction z to 
color is se5  mo/ di/9ctly4    8,!      
correction ( ! *romatic ab]ration1 l t (
 !  sph]ical1 t5ds to excess 9 ! m>g9al 
rays2 s t if a glass  :i* is a*romatic1 
) a mod]ate ap]ture1 has its cell op5$  
wid]1 ! circle ( rays ?us add$ to !     
p5cil w be r  ov]-correct$ z to color4  
8,! same t5d5cy to ov]-correction is    

produc$1 if1 )\t v>y+  ! ap]ture1 !     
div]g5ce ( ! incid5t rays is mu*        
augm5t$1  z 9 an object-glass plac$ 9   
front ( ano!r2 b g5]ally 9  ?is position
 a "p only ( its ap]ture comes into use2
 s t !  two prop]ties m5tion$ neutralize
 ea* o!r1 & its *romatic  /ate rema9s   
unalt]$4 ,if1 = example1 ! out/&+ colors
 7  obs]v$ at ! long] focus to be gre5 &
 cl>et1 :i* %[ t  ! ne>e/ practicable   
approa* is made to ! union ( ! spectrum1
  !y usually cont9ue ne>ly ! same = !   
:ole space betwe5 !  foci1 & = "s       
di/ance beyond !m ei!r way4    8,!      
places ( ^! two foci & _! proportions to
 ea* o!r  dep5d on a v>iety (           
circum/ances4 ,9 sev]al object-glasses t
 ,i  h _h made = trial1 plano-convex1 ) 
_! inn] surfaces  cem5t$1 _! diamet]s ! 
radius ( ! fl9t l5s1 & _!  color pretty 
well correct$1 ^? compos$ ( d5se fl9t & 
light  plate h _h ! rays f ! long] focus
 em]g+ ne>ly  p>allel2 & ?is focus has  
be5 n q ?ree times ! di/ance  ( ! %ort] 
f ! glass3 ) ,engli% fl9t ! rays h _h  m

 conv]g5ce1 & ! %ort] focus has borne a 
r less  proportion to ! long]4    8,if !
 surfaces are n cem5t$1 a /rik+ effect  
is produc$ by  m9ute diff]5ces 9 _!     
curves4 ,x may give "s idea ( ?is1  t 9 
a glass ( :i* ne>ly ! :ole disk 0 cov]$ 
) color  f contact ( ! l5ses1 ! addition
 ( a film ( v>ni%1 s ?9  t ?is color 0 n
 de/roy$ by x1 caus$ a s5sible *ange 9  
! sph]ical correction4    8,i h f\nd t  
:atev] ext5d$ ! long] aplanatic focus1 &
  increas$ ! conv]g5ce ( its rays1      
dim9i%$ ! relative l5g?  ( ! %ort]4 ,?us
 by turn+ to ! concave l5s ! flatt]     
in/ead ( ! deep] side ( a convex l5s1 ^:
 radii 7 to ea*  o!r z #ca to #ce1 !    
p5cil ( ! long] aplanatic focus1 f  be+ 
greatly div]g5t1 0 br\ght to conv]ge at 
a v small  di/ance beh9d ! glass2 & !   
l5g? ( ! %ort] focus1 :i*  _h be5       
"o-half t ( ! long]1 became b "o-six? ( 
x4    8,! direction ( ! aplanatic p5cils
 appe>s to be sc>cely  affect$ by !     
diff]5ces 9 ! ?ickness ( glasses1 if _! 
/ate z to color is ! same4    8,"o o!r  

prop]ty ( ! d\ble object-glass rema9s to
 be  m5tion$1 :i* is1 t :5 ! long]      
aplanatic focus is us$1 !  m>g9al rays (
 a p5cil n co9cid5t ) ! axis ( ! glass  
are di/ort$1 s t a coma is ?r[n outw>ds2
 :ile ! contr>y  effect ( a coma direct$
 t[>ds ! c5tre ( ! field is produc$  by 
! rays f ! %ort] focus4 ,^! peculi>ities
 ( ! coma  seem insep>able att5dants on 
! two foci1 & are z conspicu\s 9  !     
a*romatic m5iscus z 9 ! plano-convex    
object-glass4    ,illu/ration3 ,fig4    
#ad4    8,( sev]al purposes to :i* !    
p>ticul>s j giv5 seem  applicable1 ,i   
mu/ at pres5t conf9e myself to ! mo/    
obvi\s "o4  ,!y furni% ! means ( de/roy+
 ) ! utmo/ ease bo?  ab]rations 9 a l>ge
 focal p5cil1 & ( ?us surm\nt+ :at has  
hi!rto be5 ! *ief ob/acle to ! p]fection
 ( ! microscope4  ,& :5 x is consid]$ t 
! curves ( its dim9utive  object-glasses
 h requir$ to be at lea/ z exactly      
proportion$ z  ^? ( a l>ge telescope to 
give ! image ( a bright po9t equally    
%>p & colorless1 & t any *ange made to  

correct "o  ab]ration 0 liable to di/urb
 ! o!r1 "s idea may be =m$ (  :at !     
am\nt ( t ob/acle mu/ h be54 ,x w1      
h[ev]1 be  evid5t t if any object-glass 
is b made a*romatic1 ) its  l5ses truly 
work$ & cem5t$1 s t _! axes co9cide1 x  
may  ) c]ta9ty be connect$ ) ano!r      
possess+ ! same  requisites & ( suitable
 focus1 s t ! comb9ation %all be  free f
 sph]ical error also 9 ! c5tre ( its    
field4 ,= ?is  ! rays h only to be      
receiv$ by ! front glass ,b 7,fig4 #ad7 
f  its %ort] aplanatic focus ,f1 &      
transmitt$ 9 ! direction (  ! long]     
correct p5cil ,f ,a ( ! o!r glass ,a4 ,x
 is desirable  t ! latt] p5cil %\ld     
nei!r conv]ge to a v %ort focus  nor be 
m ?an v slightly if at all div]g5t2 & a 
little  att5tion at fir/ to ! k9d (     
glass us$ w keep x )9 ?is  range1 ! d5s]
 fl9t be+ suit$ to ! glasses ( %ort]    
focus  & l>g] angle ( ap]ture4    8,!   
adju/m5t ( ! microscope is !n p]fect$1  
if necess>y1 by  slightly v>y+ ! di/ance
 betwe5 ! object-glasses2 & aft]  t is  

done1 ! l5g? ( ! tube :i* c>ries !      
eye-pieces may  be alt]$ greatly )\t    
di/urb+ ! correction1 opposite errors   
:i* balance ea* o!r be+ produc$ by !    
*ange4    8,if ! two glasses :i* 9 !    
diagram are drawn at "s di/ance  ap>t   
are br\ght ne>] toge!r 7if ! place ( ,a1
 = in/ance1 is  c>ri$ to ! dott$        
figure71 ! rays transmitt$ by ,b 9 !    
direction ( ! long] aplanatic p5cil ( ,a
 w pla9ly be d]iv$  f "s po9t ,z m      
di/ant ?an ,f1 & ly+ betwe5 !  aplanatic
 foci ( ,b2 !re=e 7accord+ to :at has   
be5 /at$7  ?is glass1 & consequ5tly !   
comb9ation1 w !n be sph]ically          
ov]-correct$4 ,if1 on ! o!r h&1 !       
di/ance betwe5 ,a & ,b is  increas$1 !  
opposite effects are ( c\rse produc$4   
8,9 comb9+ sev]al glasses toge!r x is   
(t5 conv5i5t to  transmit an "u-correct$
 p5cil f ! front glass1 & to  c\nt]act  
its error by ov]-correction 9 ! middle  
"o4    8,slight errors 9 color may 9 !  
same mann] be de/roy$ by  opposite ones2
 & on ! pr9ciples describ$ we n only    

acquire  f9e correction = ! c5tral ray1 
b by ! opposite effects at  ! two foci  
on ! transv]se p5cil1 all coma c be     
de/roy$1 &  ! :ole field r5d]$          
beautifully flat & di/9ct40    ,mr4     
,li/]'s pap] ent]s into fur!r p>ticul>s1
 :i* are n  ess5tial to ! compreh5sion (
 ! subject4 ,x is suffici5t to say  t 8 
inve/igations & results prov$ to be ( ! 
highe/ value  to ! practical optician1 &
 ! progress ( improvem5t 0 9  consequ5ce
 extremely rapid4 ,! new pr9ciples 7    
appli$ &  exhibit$ by ,mr4 ,hugh ,p[ell 
& ,mr4 ,&rew ,ross ) a degree (  success
 :i* _h nev] be5 anticipat$2 s p]fect   
inde$ 7 !  corrections giv5 to !        
a*romatic object-glass--s completely 7  
! errors ( sph]icity & disp]sion balanc$
 or de/roy$--t  ! circum/ance ( cov]+ ! 
object ) a plate ( ! ?9ne/  glass or    
talc di/urb$ ! corrections1 if !y _h be5
 adapt$ to  an uncov]$ object1 & r5d]$  
an object-glass :i* 0 p]fect  "u "o     
condition s5sibly defective "u ! o!r4   
,?is defect1 if t %\ld be call$ a defect

 :i* arose out (  improvem5t1 0 fir/    
discov]$ by ,mr4 ,ross1 :o imm$iately   
sugge/$ ! means ( correct+ x1 & pres5t$ 
to ! ,society (  ,arts1 9 #ahcg1 a pap] 
on ! subject1 :i* 0 publi%$ 9 ! #ea/    
volume ( _! ,transactions1 & :i*1 z x   
is1 l ,mr4 ,li/]'s  ess5tial to a full  
und]/&+ ( ! ultimate ref9em5ts ( !      
in/rum5t1 we %all extract ne>ly 9 full3 
8,9 ! c\rse ( a practical inve/igation  
7says ,mr4 ,ross7 ) !  view ( con/ruct+ 
a comb9ation ( l5ses = ! object-glass ( 
a  comp\nd microscope1 :i* %\ld be free 
f ! effects (  ab]ration1 bo? = c5tral &
 oblique p5cils ( great angle1 ,i       
comb9$ ! condition ( ! greate/ possible 
di/ance betwe5 !  object & object-glass2
 = 9 object-glasses ( %ort focal l5g?   
_! clos5ess to ! object has be5 an      
ob/acle 9 _m cases to  ! use ( high     
magnify+ p[]s1 & is a con/ant s\rce (   
inconv5i5ce4    8,9 ! improv$           
comb9ation1 ! diamet] is only suffici5t 
to admit  ! prop] p5cil2 ! convex l5ses 
are wr\ght to an edge1 & !  concave h   

only suffici5t ?ickness to support _!   
figure2  consequ5tly ! comb9ation is !  
?9ne/ possible1 & x foll[s  t !re w be !
 greate/ di/ance betwe5 ! object & !    
object-glass4 ,! focal l5g? is "o-eigh? 
( an in*1 hav+ an  angul> ap]ture ( #fj1
 ) a di/ance ( #a-be? ( an in*1 & a     
magnify+ p[] ( #igj times l9e>1 ) p]fect
 def9ition on !  mo/ difficult ,podura  
scales4 ,i h made object-glasses #a-af? 
( an  in* focal l5g?2 b z ! angul>      
ap]ture _c be  advantage\sly increas$1  
if ! greate/ di/ance betwe5 ! object  & 
object-glass is pres]v$1 _! use w be v  
limit$4    8,! quality ( ! def9ition    
produc$ by an a*romatic comp\nd         
microscope w dep5d ^u ! accuracy ) :i* !
 ab]rations1  bo? *romatic & sph]ical1  
are balanc$1 toge!r ) ! g5]al  p]fection
 ( ! workman%ip4 ,n[1 9 ,wolla/on's     
d\blets1 &  ,holl&'s triplets1 !re are  
no means ( produc+ a balance ( !        
ab]rations1 z !y are compos$ ( convex   
l5ses only2 !re=e !  be/ t c be done is 
to make ! ab]rations a m9imum2 !  rema9+

 positive ab]ration 9 ^! =ms produces   
its peculi>  effect ^u objects          
7p>ticul>ly ! detail ( ! ?9 transp>5t   
class71 :i* may lead to misappreh5sion (
 _! true /ructure2 b  ) ! a*romatic     
object-glass1 :]e ! ab]rations are      
correctly  balanc$1 ! mo/ m9ute p>ts (  
an object are accurately display$1  s t 
a satisfactory judgm5t ( _! "* may be   
=m$4    ,illu/ration3 ,fig4 #ae4        
,illu/ration3 ,fig4 #af4    8,x w be se5
 by ,fig4 #ae1 t :5 a c]ta9 angul> p5cil
 ,a ,o ,a  proce$s f ! object ,o1 & is  
incid5t on ! plane side ( !  fir/ l5s1  
if ! comb9ation is remov$ f ! object1 z 
9 ,fig4  #af1 ! extreme rays ( ! p5cil  
imp+e on ! m m>g9al p>ts  ( ! glass1 & z
 ! refractions are great] "h1 !         
ab]rations  w be great] also4 ,n[1 if   
two comp\nd object-glasses h _!         
ab]rations balanc$1 "o be+ situat$ z 9  
,fig4 #ae1 & ! o!r  z 9 ,fig4 #af1 & !  
same di/urb+ p[] appli$ to bo?1 t 9  :i*
 ! angles ( incid5ce & ! ab]rations are 
small w n  be s mu* di/urb$ z :]e !     

angles are great1 & :]e  consequ5tly !  
ab]rations increase rapidly4    8,:5 an 
object-glass has its ab]rations balanc$ 
= view+ an  opaque object1 & x is       
requir$ to exam9e t object by           
transmitt$ light1 ! correction w rema92 
b if x is necess>y  to imm]se ! object 9
 a fluid1 or to cov] x ) glass or talc1 
an ab]ration w arise f ^! circum/ances1 
:i* w di/urb  ! previ\s correction1 &   
consequ5tly det]iorate ! def9ition2  &  
?is effect w be m obvi\s ) ! increase ( 
! di/ance  betwe5 ! object & !          
object-glass4    ,illu/ration3 ,fig4    
#ag4    8,! ab]ration produc$ ) div]g+  
rays by a piece ( flat &  p>allel glass1
 su* z w\ld be us$ = cov]+ an object1 is
  repres5t$ at ,fig4 #ag1 :]e ,g ,g ,g  
,g is ! refract+ m$ium1 or  piece (     
glass cov]+ ! object ,o2 ,o ,p1 ! axis (
 ! p5cil1  p]p5dicul> to ! flat         
surfaces2 ,o ,t1 a ray ne> ! axis2 & ,o 
,t1 ! extreme ray ( ! p5cil incid5t on !
 "u surface ( !  glass2 !n ,t ,r1 ,t ,r1
 w be ! directions ( ! rays 9 !  m$ium1 

& ,r ,e1 ,r ,e1 ^? ( ! em]g5t rays4 ,n[ 
if ! c\rse  ( ^! rays is cont9u$1 z by !
 dott$ l9es1 !y w be f\nd  to int]sect !
 axis at diff]5t di/ances1 ,x & ,y1 f ! 
surface ( ! glass2 & ! di/ance ,x ,y is 
! ab]ration produc$  by ! m$ium :i*1 z  
be=e /at$1 int]f]es ) ! previ\sly       
balanc$ ab]rations ( ! sev]al l5ses     
compos+ !  object-glass4 ,!re are _m    
cases ( ?is1 b ! "o "h select$  s]ves   
be/ to illu/rate ! pr9ciple4 ,i ne$ n   
encumb] !  description ) ! !oretical    
det]m9ation ( ?is quantity1 z x  v>ies )
 exce$+ly m9ute circum/ances :i* we _c  
accurately control2 su* z ! di/ance ( ! 
object f ! "u  side ( ! glass1 & !      
slighte/ diff]5ce 9 ! ?ickness (  !     
glass itself2 & if ^! data c\ld be      
readily obta9$1 !  k w\ld be ( no       
utility 9 mak+ ! correction1 t be+      
:olly ( a practical nature4    8,if an  
object-glass is con/ruct$ z repres5t$ 9 
,fig4 #af1 :]e  ! po/]ior comb9ation ,p 
& ! middle ,m h toge!r an excess  (     
negative ab]ration1 & if ?is be correct$

 by ! ant]ior  comb9ation ,a1 hav+ an   
excess ( positive ab]ration1 !n ?is     
latt] comb9ation c be made to act m or  
less p[]fully ^u ,p  & ,m1 by mak+ x    
approa* to or rec$e f !m2 = :5 !  ?ree  
are 9 close contact1 ! di/ance ( !      
object f !  object-glass is greate/2 &  
consequ5tly ! rays f ! object  are      
div]g+ f a po9t at a great] di/ance ?an 
:5 !  comb9ations are sep>at$2 & z a l5s
 b5ds ! rays m1 or acts  ) great]       
effect1 ! m di/ant ! object is f :i* !  
rays div]ge1 ! effect ( ! ant]ior       
comb9ation ,a ^u ! o!r  two1 ,p & ,m1 w 
v>y ) its di/ance f !nce4 ,:5 !re=e  !  
correction ( ! :ole is effect$ = an     
opaque object ) a  c]ta9 di/ance betwe5 
! ant]ior & middle comb9ation1 if !y    
are !n put 9 contact1 ! di/ance betwe5 !
 object &  object-glass w be increas$2  
consequ5tly ! ant]ior comb9ation  w act 
m p[]fully1 & ! :ole w h an excess (    
positive ab]ration4 ,n[ ! effect ( !    
ab]ration produc$ by a  piece ( flat &  
p>allel glass be+ ( ! negative "*1 x  is

 obvi\s t ! above consid]ations sugge/ !
 means (  correction by mov+ ! l5ses    
ne>] toge!r1 till ! positive  ab]ration 
!reby produc$ balances ! negative       
ab]ration caus$ by  ! m$ium4    8,!     
prec$+ ref]s only to ! sph]ical         
ab]ration1 b ! effect  ( ! *romatic is  
also se5 :5 an object is cov]$ ) a piece
  ( glass2 =1 9 ! c\rse ( my exp]im5ts1 
,i obs]v$ t x  produc$ a *romatic ?ick5+
 ( ! outl9e ( ! ,podura &  o!r delicate 
scales2 & if div]g+ rays ne> ! axis & at
 !  m>g9 are project$ ?r\gh a piece (   
flat p>allel glass1 ) !  v>i\s indices (
 refraction = ! diff]5t colors1 x w be  
se5 t ea* ray w em]ge sep>at$ into a    
beam consi/+ ( !  compon5t colors ( !   
ray1 & t ea* beam is widely diff]5t 9   
=m4 ,?is diff]5ce1 be+ magnifi$ by ! p[]
 ( ! microscope1  readily acc\nts = !   
*romatic ?ick5+ ( ! outl9e j  m5tion$4  
,!re=e to obta9 ! f9e/ def9ition (      
extremely  delicate & m9ute objects1 !y 
%\ld be view$ )\t a cov]+2  if x be     
desirable to imm]se !m 9 a fluid1 !y    

%\ld be cov]$  ) ! ?9ne/ possible film (
 talc1 z1 f ! "* ( !  *romatic          
ab]ration1 x w be se5 t v>y+ ! di/ances 
(  ! comb9ations w n s5sibly affect !   
correction2 ?\gh  object-l5ses may be   
made to include a giv5 fluid or solid   
m$ium 9  _! correction = color4         
,illu/ration3 ,fig4 #ah4    8,! me*anism
 = apply+ ^! pr9ciples to ! correction (
 an  object-glass "u ! v>i\s            
circum/ances1 is repres5t$ 9 ,fig4  #ah1
 :]e ! ant]ior l5s is set 9 ! end ( a   
tube ,a ,a1 :i*  slides on ! cyl9d] ,b  
conta9+ ! rema9d] ( ! comb9ation2  !    
tube ,a ,a1 hold+ ! l5s ne>e/ ! object1 
may !n be mov$  ^u ! cyl9d] ,b1 = !     
purpose ( v>y+ ! di/ance accord+  to !  
?ickness ( ! glass cov]+ ! object1 by   
turn+ !  screw$ r+ ,c ,c1 or m simply by
 slid+ ! "o on ! o!r1 &  clamp+ !m      
toge!r :5 adju/$4 ,an ap]ture is made 9 
! tube  ,a1 )9 :i* is se5 a m>k engrav$ 
on ! cyl9d]1 & on !  edge ( :i* are two 
m>ks1 a long] & a %ort]1 engrav$ ^u !   
tube4 ,:5 ! m>k on ! cyl9d] co9cides ) !

 long] m>k on  ! tube1 ! adju/m5t is    
p]fect = an uncov]$ object2 & :5  !     
co9cid5ce is ) ! %ort m>k1 ! prop]      
di/ance is  obta9$ to balance !         
ab]rations produc$ by glass "o-hundr$? (
  an in* ?ick1 & su* glass c be readily 
suppli$4    8,x is h>dly necess>y to    
obs]ve1 t ! necessity = ?is  correction 
is :olly indep5d5t ( any p>ticul>       
con/ruction ( !  object-glass2 z 9 all  
cases :]e ! object-glass is correct$ =  
an object uncov]$1 any cov]+ ( glass w  
create a diff]5t  value ( ab]ration to !
 fir/ l5s1 :i* previ\sly balanc$ !      
ab]ration result+ f ! re/ ( ! l5ses2 & z
 ?is  di/urbance is effect$ at ! fir/   
refraction1 x is indep5d5t (  ! o!r "p (
 ! comb9ation4 ,! visibility ( ! effect 
dep5ds on ! di/ance ( ! object f !      
object-glass1 ! angle  ( ! p5cil        
transmitt$1 ! focal l5g? ( ! comb9ation1
 !  ?ickness ( ! glass cov]+ ! object1 &
 ! g5]al p]fection  ( ! corrections =   
*romatism & ! oblique p5cils4    8,) ?is
 adju/+ object-glass1 !re=e1 we c h !   

requisites ( ! greate/ possible di/ance 
betwe5 ! object &  object-glass1 an     
int5se & %>ply def9$ image ?r\gh\t !    
field f ! l>ge p5cil transmitt$1 & !    
accurate correction  ( ! ab]rations2    
also1 by ! adju/m5t1 ! means ( pres]v+  
t correction "u all ! v>i$ circum/ances 
9 :i* x may be  necess>y to place an    
object = ! purpose ( obs]vation40    ,9 
! annex$ engrav+1 ,fig4 #ai1 we h %[n ! 
triple a*romatic  object-glass 9        
connection ) ! eye-piece consi/+ ( !    
field-glass ,f ,f1 & ! eye-glass ,e ,e1 
=m+ toge!r ! mod]n  a*romatic           
microscope4 ,! c\rse ( ! light is %[n by
 draw+  ?ree rays f ! c5tre & ?ree f ea*
 end ( ! object ,o4  ,^! rays w\ld1 if  
left to !mselves1 =m an image ( ! object
  at ,a ,a1 b be+ b5t & conv]g$ by !    
field-glass ,f ,f1 !y =m  ! image at ,b 
,b1 :]e a /op is plac$ to int]cept all  
light except  :at is requir$ = ! =mation
 ( ! image4 ,f ,b ,b !re=e  ! rays      
proce$ to ! eye-glass exactly z has be5 
describ$ 9  ref]5ce to ! simple         

microscope & to ! comp\nd ( two glasses4
    ,illu/ration3 ,fig4 #ai4    ,if we  
/opp$ "h we %\ld convey a v imp]fect    
idea ( !  beautiful s]ies ( corrections 
effect$ by ! eye-piece1 & :i*  7 fir/   
po9t$ out 9 detail 9 a pap] on ! subject
 publi%$  by ,mr4 ,v>ley 9 ! #ea/ volume
 ( ! ,transactions ( ! ,society (       
,arts4 ,! eye-piece 9 "q 0 inv5t$ by    
,huygh5s =  telescopes1 ) no o!r view   
?an t ( dim9i%+ ! sph]ical  ab]ration by
 produc+ ! refractions at two glasses   
in/ead ( "o1  & ( increas+ ! field (    
view4 ,x 0 res]v$ = ,boscovi* to  po9t  
out t ,huygh5s _h by ?is arrangem5t     
accid5tally correct$  a great "p ( !    
*romatic ab]ration1 & ?is subject is    
fur!r  inve/igat$ ) mu* skill 9 two     
pap]s by ,pr(essor ,airy 9 !  ,cambridge
 ,philosophical ,transactions1 to :i* we
 ref] !  ma!matical read]4 ,^!          
inve/igations apply *iefly to !         
telescope1 :]e ! small p5cils ( light & 
great di/ance ( !  object exclude       
consid]ations :i* become important 9 !  

microscope1 & :i* are well po9t$ out 9  
,mr4 ,v>ley's pap]  be=e m5tion$4       
,illu/ration3 ,fig4 #bj4    ,let ,fig4  
#bj repres5t ! ,huygh5ean eye-piece ( a 
microscope2 ,f ,f  & ,e ,e be+ !        
field-glass & eye-glass1 & ,l ,m ,n !   
two extreme  rays ( ea* ( ! ?ree p5cils1
 emanat+ f ! c5tre & ends  ( ! object1 (
 :i*1 b = ! field-glass1 a s]ies (      
color$  images w\ld be =m$ f ,r ,r to ,b
 ,b2 ^? ne> ,r ,r be+ r$1  ^? ne> ,b ,b 
blue1 & ! int]m$iate ones gre51 yell[1 &
 s  on1 correspond+ ) ! colors ( !      
prismatic spectrum4 ,?is  ord] ( colors1
 x w be obs]v$1 is ! rev]se ( t         
describ$ 9 treat+ ( ! common comp\nd    
microscope 7,fig4 #ab71 9  :i* ! s+le   
object-glass project$ ! r$ image beyond 
! blue4  ,! effect j describ$1 (        
project+ ! blue image beyond !  r$1 is  
purposely produc$ = reasons pres5tly to 
be giv51 & is  call$ ov]-correct+ !     
object-glass z to color4 ,x is to be    
obs]v$ also t ! images ,b ,b & ,r ,r are
 curv$ 9 ! wrong  direction to be       

di/9ctly se5 by a convex eye-l5s1 & ?is 
is a  fur!r defect ( ! comp\nd          
microscope ( two l5ses4 ,b !            
field-glass1 at ! same "t t x b5ds !    
rays & conv]ges  !m to foci at ,b ,b &  
,r ,r1 also rev]ses ! curvature ( !     
images z !re %[n1 & gives !m ! =m be/   
adapt$ =  di/9ct vision by ! eye-glass  
,e ,e4 ,! field-glass has at ! same  "t 
br\ght ! blue & r$ images clos] toge!r1 
s t !y are  adapt$ to pass uncolor$     
?r\gh ! eye-glass4 ,to r5d] ?is         
important po9t m intelligible1 let x be 
suppos$ t !  object-glass _h n be5      
ov]-correct$1 t x _h be5 p]fectly       
a*romatic2 ! rays w\ld !n h become      
color$ z soon z !y  _h pass$ !          
field-glass2 ! blue rays1 to take !     
c5tral p5cil1  = example1 w\ld conv]ge  
at b & ! r$ rays at r1 :i* is  j !      
rev]se ( :at ! eye-l5s requires2 = z its
 blue focus  is also %ort] ?an its r$1 x
 w\ld dem& r t ! blue  image %\ld be at 
r & ! r$ at b4 ,?is effect we h %[n  to 
be produc$ by ! ov]-correction ( !      

object-glass1 :i*  protrudes ! blue foci
 ,b ,b z mu* beyond ! r$ foci ,r ,r z ! 
sum  ( ! di/ances betwe5 ! r$ & blue    
foci ( ! field-l5s &  eye-l5s2 s t !    
sep>ation ,b ,r is exactly tak5 up 9    
pass+  ?r\gh ^? two l5ses1 & ! :ole ( ! 
colors co9cide z to  focal di/ance z    
soon z ! rays h pass$ ! eye-l5s4 ,b :ile
  !y co9cide z to di/ance1 !y diff] 9   
ano!r respect2 ! blue  images are r5d]$ 
small] ?an ! r$ by ! sup]ior refractive 
p[] ( ! field-glass ^u ! blue rays4 ,9  
trac+ ! p5cil ,l1  = in/ance1 x w be    
notic$ t aft] pass+ ! field-glass1  two 
sets ( l9es are drawn1 "o :ole1 & "o    
dott$1 ! =m]  repres5t+ ! r$1 & ! latt] 
! blue rays4 ,?is is !  accid5tal effect
 9 ! ,huygh5ean eye-piece po9t$ out by  
,boscovi*4 ,?is sep>ation into colors at
 ! field-glass is l !  ov]-correction ( 
! object-glass2 x leads to a subsequ5t  
complete  correction4 ,= if ! diff]5tly 
color$ rays 7 kept toge!r  till !y rea*$
 ! eye-glass1 !y w\ld !n become color$1 
&  pres5t color$ images to ! eye2 b     

=tunately1 & mo/  beautifully1 !        
sep>ation effect$ by ! field-glass      
causes !  blue rays to fall s mu* ne>] !
 c5tre ( ! eye-glass1 :]e1  ow+ to !    
sph]ical figure1 ! refractive p[] is    
less ?an at  ! m>g91 t ! sph]ical error 
( ! eye-l5s con/itutes a  ne>ly p]fect  
balance to ! *romatic disp]sion ( !     
field-l5s1  & ! r$ & blue rays ,l & ,l  
em]ge s5sibly p>allel1  pres5t+1 9      
consequ5ce1 ! p]fect def9ition ( a s+le 
po9t  to ! eye4 ,! same reason+ is true 
( ! int]m$iate colors &  ( ! o!r p5cils4
    ,f :at has be5 /at$ x is obvi\s t we
 mean by an a*romatic  object-glass "o 9
 :i* ! usual ord] ( disp]sion is s f>   
rev]s$ t ! light1 aft] und]go+ ! s+ul>ly
 beautiful  s]ies ( *anges effect$ by ! 
eye-piece1 %all come uncolor$ to  ! eye4
 ,we c give no specific rules = produc+ 
^! results4  ,close /udy ( ! =mul =     
a*romatism giv5 by ! celebrat$          
ma!maticians we h quot$ w d mu*1 b !    
pr9ciples mu/ be  br\ght to ! te/ (     
repeat$ exp]im5t4 ,nor w ! exp]im5ts  be

 wor? any?+1 unless ! curves be mo/     
accurately measur$ &  work$1 & ! l5ses  
c5t]$ & adju/$ ) a degree (  precision  
:i*1 to ^? :o are famili> only )        
telescopes1 w  be q unprec$5t$4    ,!   
,huygh5ean eye-piece :i* we h describ$  
is ! be/ =  m]ely optical purposes1 b :5
 x is requir$ to measure !  magnifi$    
image1 we use ! eye-piece inv5t$ by ,mr4
 ,ramsd51 &  call$1 f its purpose1 !    
micromet] eye-piece4 ,:5 x is /at$  t we
 sometimes require to measure portions (
 animal or vegetable  matt] a hundr$    
times small] ?an any divisions t c be   
artificially made on any measur+        
in/rum5t1 ! advantage (  apply+ ! scale 
to ! magnifi$ image w be obvi\s1 z      
comp>$  ) ! application ( engrav$ or    
me*anical micromet]s to !  /age ( !     
in/rum5t4    ,! arrangem5t is %[n 9     
,fig4 #ba1 :]e ,e ,e & ,f ,f are ! eye &
  field glass1 ! latt] hav+ n[ its plane
 face t[>ds ! object4  ,! rays f !      
object are "h made to conv]ge at ,a ,a1 
imm$iately  9 front ( ! field-glass1 &  

"h also is plac$ a plane glass on  :i*  
are engrav$ divisions ( a hundr$? ( an  
in* or less4 ,!  m>k+s ( ^! divisions   
come into focus !re=e at ! same "t  z ! 
image ( ! object1 & bo? are di/9ctly se5
 toge!r4  ,?us ! measure ( ! magnifi$   
image is giv5 by m]e inspection1  & !   
value ( su* measures 9 ref]5ce to ! real
 object may be  obta9$ ?us1 :i*1 :5 once
 obta9$1 is con/ant = ! same            
object-glass4 ,place on ! /age ( !      
in/rum5t a divid$ scale !  value ( :i*  
is kn[n1 & view+ ?is scale z !          
microscopic  object1 obs]ve h[ _m ( !   
divisions on ! scale atta*$ to !        
eye-piece correspond ) "o ( ^? 9 !      
magnifi$ image4 ,if1 =  in/ance1 t5 ( ^?
 9 ! eye-piece correspond ) "o ( ^? 9  !
 image1 & if ! divisions are kn[n to be 
equal1 !n ! image  is t5 times l>g] ?an 
! object1 & ! dim5sions ( ! object  are 
t5 times less ?an indicat$ by !         
micromet]4 ,if ! divisions  on !        
micromet] & on ! magnifi$ scale 7 n     
equal1 x  becomes a m]e rule-(-?ree sum1

 b 9 g5]al ?is tr\ble is tak5  by ! mak]
 ( ! in/rum5t1 :o furni%es a table %[+ !
  value ( ea* division ( ! micromet] = e
 object-glass )  :i* x may be us$4      
,illu/ration3 ,fig4 #ba4    ,:ile on !  
subject ( measur+ x may be well to      
expla9 ! mode  ( asc]ta9+ ! magnify+ p[]
 ( ! comp\nd microscope1 :i*  is g5]ally
 tak5 on ! assumption be=e m5tion$1 t ! 
nak$  eye sees mo/ di/9ctly at ! di/ance
 ( t5 in*es4    ,place on ! /age ( !    
in/rum5t1 z be=e1 a kn[n divid$  scale1 
& :5 x is di/9ctly se51 hold a rule at  
t5 in*es  di/ance f ! dis5gag$ eye1 s t 
x may be se5 by t eye1  ov]lapp+ or ly+ 
by side ( ! magnifi$ picture ( ! o!r    
scale4 ,!n move ! rule till "o or m (   
its kn[n divisions  correspond ) a numb]
 ( ^? 9 ! magnifi$ scale1 & a  comp>ison
 ( ! two gives ! magnify+ p[]4    ,hav+ 
n[ expla9$ ! optical pr9ciples ( !      
a*romatic comp\nd  microscope1 x rema9s 
only to describe ! me*anical arrangem5ts
  = giv+ ^? pr9ciples _! full effect4 ,!
 me*anism ( a  microscope is ( mu* m    

importance ?an might be imag9$ by ^?  :o
 h n /udi$ ! subject4 ,9 ! fir/ place1  
/ead9ess1 or  fre$om f vibration1 & mo/ 
p>ticul>ly fre$om f any  vibrations :i* 
are n equally communicat$ to ! object "u
  exam9ation1 & to ! l5ses by :i* x is  
view$1 is a po9t (  ! utmo/ consequ5ce4 
,:51 = in/ance1 ! body conta9+ !  l5ses 
is screw$ by its l[] extremity to a     
horizontal arm1 we h  "o ( ! mo/        
vibratory =ms conceivable2 x is         
precisely ! =m  ( ! inv]t$ p5dulum1 :i* 
is expressly contriv$ to indicate       
o!rwise ins5sible vibrations4 ,! tremor 
necess>ily att5dant on  su* an          
arrangem5t is magnifi$ by ! :ole p[] ( !
 in/rum5t2  & z ! object on ! /age      
p>takes ( ?is tremor 9 a  comp>atively  
ins5sible degree1 ! image is se5 to     
oscillate s  rapidly1 z 9 "s cases to be
 :olly undi/+ui%able4 ,su*  microscopes 
_c possibly be us$ ) high p[]s 9 ord9>y 
h\ses abutt+ on any pav$ /reets ?r\gh   
:i* c>riages are  pass+1 nor inde$ are  
!y adapt$ to be us$ 9 h\ses 9 :i* !     

ord9>y int]nal s\rces ( %ak+ exi/4      
,"o ( ! be/ modes ( m\nt+ a comp\nd     
microscope is %[n 9  ! annex$ view      
7,fig4 #bb71 :i*1 ?\gh too m9ute to     
exhibit all  ! details1 w s]ve to expla9
 ! *ief features ( !  arrangem5t4    ,a 
massy pill> ,a is screw$ into a solid   
tripod ,b1 & is surm\nt$  by a /rong    
jo9t at ,c1 on :i* ! :ole in/rum5t      
turns1 s z to  enable x to take a       
p]fectly horizontal or v]tical position1
 or any  int]m$iate angle1 su*1 =       
in/ance1 z t %[n 9 !  engrav+4    ,?is  
movable portion ( ! in/rum5t consi/s (  
"o solid ca/+ ,d  ,e ,f ,g2 f ,f to ,g  
be+ a ?ick pi]c$ plate c>ry+ ! /age &   
its app5dages4 ,! comp\nd body ,h is    
atta*$ to ! b> ,d ,e1 &  moves up & d[n 
^u x by a rack & p9ion work$ by ei!r ( !
  mill$ heads ,k4 ,! piece ,d ,e ,f ,g  
is atta*$ to ! pill> by !  jo9t ,c1 :i* 
be+ ! s\rce ( ! requir$ movem5t 9 !     
in/rum5t1 is obvi\sly its weake/ "p1 &  
ab\t :i* no d\bt  consid]able vibration 
takes place4 ,b inasmu* z ! piece ,d ,e 

,f ,g  ( necessity transmits su*        
vibrations equally to ! body ( !        
microscope & to ! objects on ! /age1 !y 
hold always ! same  relative position1 &
 no visible vibration is caus$1 h[ mu*  
soev] may really exi/4 ,to ! "u side ( !
 /age is atta*$ a  circul> /em ,l1 on   
:i* slides ! mirror ,m1 plane on "o side
 &  concave on ! o!r1 to reflect ! light
 ?r\gh ! ap]ture 9 !  /age4 ,b5ea? !    
/age is a circul> revolv+ plate conta9+ 
?ree ap]tures ( v>i\s sizes1 to limit ! 
angle ( ! p5cil (  light :i* %all be    
all[$ to fall on ! object "u exam9ation4
  ,besides ^! conv5i5ces ! /age has a   
d\ble movem5t produc$ by  two racks at  
"r angles to ea* o!r1 & work$ by mill$  
heads  b5ea?4 ,x has also ! usual       
app5dages ( =ceps to hold m9ute         
objects1 & a l5s to cond5se ! light ^u  
!m1 all ( :i* are  well und]/ood1 & if  
n1 w be r5d]$ m intelligible by a  few  
m9utes' exam9ation ( a microscope ?an by
 ! mo/ l5g!n$  description4 ,"o o!r po9t
 rema9s to be notic$4 ,! movem5t        

produc$ by ! mill$ head ,k is n         
suffici5tly delicate to adju/  ! focus (
 v p[]ful l5ses1 nor inde$ is any rack  
movem5t4  ,only ! f9e/ screws are adapt$
 to ?is purpose2 & ev5 ^! are  improv$  
by means = r$uc+ ! rapidity ( ! screw's 
movem5t4  ,= ?is purpose ! l[] end ( !  
comp\nd body ,h1 :i* c>ries  !          
object-glass1 consi/s ( a piece ( small]
 tube slid+ 9  p>allel guides 9 ! ma9   
body1 & kept con/antly press$ upw>ds  by
 a spiral spr+ b x c be drawn d[nw>d by 
a lev] cross+  ! body1 & act$ on by an  
extremely f9e screw ^: mill$ head  is   
se5 at ,n1 & ! f95ess ( :i* is tripl$ by
 means ( !  lev] ?r\gh :i* x acts on !  
object-glass4 ,! in/rum5t is (  c\rse   
r\ghly adju/$ by ! rack movem5t1 & f9i%$
 by !  screw1 or by su* o!r means z are 
*os5 = ! purpose4 ,"o v  +5i\s          
contrivance1 b appli$ to ! /age1 in/ead 
( ! body  ( ! microscope1 inv5t$ by ,mr4
 ,p[ell1 w be f\nd describ$ 9  ! #ej?   
volume ( ! ,transactions ( ! ,society ( 
,arts4    ,illu/ration3 ,fig4 #bb4    ,!

 great] "p ( ! directions = view+ &     
illum9at+  objects giv5 9 ref]5ce to !  
simple microscope are applicable to  !  
comp\nd4 ,an arg& lamp plac$ 9 ! focus (
 a l>ge deta*$  l5s s z to ?r[ p>allel  
rays ^u ! mirror1 is ! be/  artificial  
light2 & = opaque objects ! light s ?r[n
 up may be  reflect$ by metallic specula
 7call$1 f _! inv5tor1  ,lieb]khuns7    
atta*$ to ! object-glasses4    ,x has   
be5 rec5tly propos$ by ,sir ,david      
,brew/] & by ,m4 ,duj>d9  to r5d] !     
,wolla/on cond5s] a*romatic1 & !y h     
accord+ly be5 made ) ?ree pairs (       
a*romatic l5ses in/ead (  ! s+le l5s    
be=e describ$1 ) v excell5t effect4 ,!  
la/-m5tion$ g5tleman has also project$  
an +5i\s app>atus1  call$ !             
,hyptioscope1 atta*$ to ! eye-piece = ! 
purpose (  erect+ ! magnifi$ picture4   
,! erector commonly appli$ to ! comp\nd 
microscope consi/s ( a  pair ( l5ses    
act+ l ! erect+ eye-piece ( ! telescope4
  ,b ?is1 ?\gh x is conv5i5t = ! purpose
 ( dissection1 v  mu* impairs ! optical 

p]=mance ( ! in/rum5t4    ,illu/ration3 
,fig4 #bc4    ,= draw+ ! images pres5t$ 
by ! microscope ! be/ app>atus  consi/s 
( a mirror ,m 7,fig4 #bc71 compos$ ( a  
?9 piece ( r  d>k-color$ glass cem5t$ on
 to a piece ( plate-glass incl9$ at  an 
angle ( #de 9 front ( ! eye-glass ,e4 ,!
 light escap+ f  ! eye-glass is assi/$ 9
 its reflection upw>ds to ! eye by !    
d>k glass1 :i* effects ! fur!r useful   
purpose ( r5d]+ !  pap] less brilliant1 
& ?us enabl+ ! eye bett] to see !       
reflect$ image4 ,! l5s ,l bel[ !        
reflector is to cause ! light  f ! pap] 
& p5cil to div]ge f ! same di/ance z t  
receiv$ f ! eye-glass2 9 o!r words1 to  
cause x to rea* !  eye 9 p>allel l9es4  
,illu/ration3 ,fig4 #bd4    ,dr4        
,wolla/on's cam]a lucida1 z %[n 9 ,fig4 
#bd1 is sometimes  atta*$ to ! eye-piece
 ( ! microscope = ! same purpose4 ,9    
?is in/rum5t ! rays suff] two int]nal   
reflections )9 !  glass prism1 z w be   
se5 expla9$ 9 ! article 8,cam]a         
,lucida40  ,9 ?is m9ute figure we h     

omitt$ to trace ! reflect$ rays1  m]ely 
to avoid confusion4    ,illu/ration3    
,fig4 #be4    ,illu/ration3 ,fig4 #bf4  
,illu/ration3 ,fig4 #bg4    ,annex$ are 
f\r engrav+s ( microscopic objects1 !   
true "*  ( :i* x is1 h[ev]1 impossible  
to give 9 wood1 & is difficult  inde$ to
 accompli% by any description ( engrav+4
    ,illu/ration3 ,fig4 #bh4    ,fig4   
#be %[s a scale ( ! small insect call$  
,podura ,plumbea1 !  common ,skiptail1  
magnifi$ ab\t five hundr$ times4 ,to    
def9e !  m>k+s on ?is scale cle>ly is ! 
highe/ te/ ( a deep  a*romatic          
object-glass2 & ?is draw+ is giv5 r to  
expla9  :at ! obs]v] %\ld look =1 ?an z 
a v correct  repres5tation4 ,fig4 #bf is
 a scale or fea!r ( ! ,m5elaus          
,butt]fly2 ,fig4 #bg is ! hair ( a s+ul>
 insect1 ! ,d]me/es2  & ,fig4 #bh is a  
longitud9al cutt+ ( fir1 %[+ ! circul>  
gl&s on ! vessels :i* di/+ui% conif]\s  
woods4 ,^! latt]  objects may be se5 by 
half-in* or qu>t]-in* a*romatic glasses4
  ,opaque objects are g5]ally bett]     

exhibit$ by in* & two-in*  glasses1 :5 a
 g5]al view ( !m is requir$1 & by high] 
p[]s  :5 we wi% to exam9e _! m9ute      
/ructure4 ,9 ! latt] case !  light mu/  
be obta9$ by cond5s+ l5ses in/ead ( !   
metallic  specula4    ,al?\gh ! reflect+
 microscope is n[ v little us$1 x may be
  expect$ t we %\ld m5tion x4 ,9 ?is    
in/rum5t1 at ,fig4 #bi1  ! object ,o is 
reflect$ by ! incl9$ face ( ! mirror ,m1
 &  ! rays are aga9 reflect$ & conv]g$  
by ! ellipsoidal  reflector ,r ,r1 :i*  
effects ! same purpose z ! object-glass 
(  ! comp\nd microscope4 ,x =ms an image
 :i* is n susceptible (  !              
ov]-correction z to color be=e describ$1
 & :i* !re=e  becomes color$ 9 pass+    
?r\gh ! eye-piece4 ,?is fact1 & !  loss 
( light by reflection1 w probably always
 r5d] !  reflect+ microscope inf]ior to 
! a*romatic refract+4    ,illu/ration3  
,fig4 #bi4    ,! sol> microscope has be5
 s ne>ly sup]s$$ by !  oxy-hydrog51 t a 
brief description ( ! latt] mu/ suffice1
  p>ticul>ly z _! optical pr9ciples are 

simil>4    ,! prim>y object 9 bo? is to 
?r[ an int5se light ^u !  object1 :i* is
 sometimes done by mirrors1 & sometimes 
by l5ses4  ,9 ,fig4 #cj1 ,l repres5ts ! 
cyl9d] ( burn+ lime1 ,r ,r !  reflector1
 :i* conc5trates ! light ^u ! object ,o 
,o2 ! rays  f :i*1 pass+ ?r\gh ! two    
plano-convex l5ses1 are br\ght  to foci 
^u a scre5 plac$ at a great di/ance1 &  
^u :i* is  =m$ ! magnifi$ image4        
,illu/ration3 ,fig4 #cj4    ,fig4 #ca   
%[s a comb9ation ( l5ses to cond5se !   
light ^u !  object4 ,9 ei!r case !      
optical arrangem5ts by :i* ! image is   
=m$ admit ( ! same p]fection z ^? :i* h 
be5 describ$  = ! comp\nd microscopes4  
,a few a*romatic glasses =  oxy-hydrog5 
microscopes h be5 made1 & !y w          
ultimately  become valuable in/rum5ts = 
illu/rat+ lectures on natural  hi/ory & 
physiology4 ,"o made by ,mr4 ,ross 0    
exhibit$ a few  mon?s s9ce at ! ,society
 ( ,arts to illu/rate a lecture on !    
physiology ( woods4 ,x %\ld be obs]v$1  
h[ev]1 t !  oxy-hydrog5 or sol>         

microscope requires ei!r a sph]ical     
scre51  or t ! objects %\ld be m\nt$    
betwe5 sph]ical glasses1 9  ord] to br+ 
! :ole into focus at "o "t4 ,?is latt]  
plan 0  adopt$ on ! occasion j m5tion$ )
 p]fect success4    ,illu/ration3 ,fig4 
#ca4