On the Achromatic Microscope. 267 



> plat 



Inch 



Radius of the 1st surface, 0.16 . , 



0.23 ^^''^^^° 



0.24 ^fl'"^- 



> plate. 



0.24 



0-23 5 ^. „ 



0.12 ) , ^ ^'^'^' 



0.155 \ P'^^^ 



ais ^ fl^"^' 



0.18 , , , 

 Q^j55> plate. 



\ 



|plE 

 > plate, 

 flhit. 



0.09 



0.08 



0.08 



0.12 



0.12 ) T , 



0.09 ^^^^^'^ 



Focus of the compound lens, 0.102 



Diameter of the 1st set 0.14 



do. 2d " 0.12 



do. 3d " 0.08 



Clear aperture, 0.12 to 0.14 



Each of the three kinds shown in the figures has been constructed 

 and found to perform well. The convex lenses of the object glass 

 shown in Fig. 3, should be made as thin as is practicable, that the 

 focus of the microscope may be as far as possible from the last sur- 

 face. The thickness of the flint lenses at their centres may be 

 about j^n of an inch. If they are much thinner they will bend and 

 become disfigured while polishing. 



The lenses of the object glass of an achromatic microscope must 

 be centred with great accuracy ; and they may be considered as well 

 centred, when their optical centres do not vary from a right line more 

 than Tolo o^ ^"^ inch. 



Flint glass is preferable which has the greatest dispersive ratio 5 

 for the secondary aberration of figure is less, when the aperture is 

 the same in relation to tlie focal distance ; although it is greater, 

 when the aperture is the same in relation to the first radius of the 

 flint lens. 



