Illumination and Illuminating Apparatus 89 



Sub-stage Condensers. 



A few years since all that was considered necessary in 

 order to illuminate in a proper manner an object under 

 examination was the mirror, perhaps in conjunction with 

 the bull's-eye stand condenser ; and in many cases the 

 mirror was hung on a tailpiece which could be moved in an 

 arc round the centre of the stage, and by this means light 

 at any angle could be reflected on the object. The day ifor 

 this, however, has gone by, and anyone who requires to get 

 even fair results must use a sub-stage condenser in some 

 form or other. Especially does this apply to high-power 

 objectives. Plenty of illumination can be obtained with 

 the mirror only for low power objectives, but beyond these 

 the object becomes ill-defined and the field dark. More 

 especially since the study of bacteriology has taken so 

 prominent a position has the condenser come to the front. 

 Without its aid it would be almost impossible to distinguish 

 between different species of these minute organisms. To 

 the leading members of the Eoyal Microscopical Society, 

 and especially to Dr. Dallinger and Mr. E. M. Nelson, is 

 due the steady improvement that has taken place in the 

 optical qualities of the sub-stage condenser. The two 

 gentlemen named have been indefatigable in their appeals 

 and demonstrations to microscopists, urging the pre-eminent 

 position that it should occupy in manipulation, and the 

 proper methods of using it. 



One of the most largely used of condensers is a chromatic 

 one, named the Abbe illumin- 

 ator, originated by the firm of 

 Carl Zeiss, and now supplied by 

 nearly all opticians. It is made 

 in two forms, one having a 

 numerical aperture of 1'20, and 



the other of 1-40. The former Fig 24 -Abbe Illuminator. 



(1'20 Optical Paet.) 

 is the more commonly em- 

 ployed, and is principally adopted by students. The 



