104 



Modern Microscopy 



denser on the under side, and the Abbe chromatic and 

 achromatic condensers referred to previously are par- 

 ticularly suitable for this arrangement. 



The Bull's-eye Condenser. 



I Many objects, being opaque, cannot be viewed by light 

 from beneath, and consequently have to be illuminated 



from above. In order to do this 

 a bull's-eye condenser is neces- 

 sary. This usually consists of a 

 plano-convex lens mounted on a 

 stand, as shown in the figure. 

 This has a ball and socket joint 

 and a sliding telescopic tube, by 

 means of which the lens can be 

 placed in any desired position. 

 The plane side of the lens should 

 be turned towards the object, and 

 the convex towards the source of 

 the illumination, whether it be 

 daylight or artificial by lamp. 

 Latterly Mr. Nelson has sug- 

 gested improvements in the con- 

 struction of bull's-eye condensers 

 in order to reduce the large 

 amount of spherical aberration 

 which is a necessary accompani- 

 ment of the single lens. His 

 improved form consists of either 

 two or three lenses in combina- 

 tion, and the advantage obtained, 

 especially in photography, is well 

 worth the additional outlay. The 



bull's-eye condenser is often of use in conjunction with the 

 sub-stage condenser, to enable the field to be evenly illumin- 

 ated with the latter, as mentioned on page. 96. It will be 



FIG. 38. STAND CONDENSER. 



