I4O THE A B C OF PHOTO - MICROGRAPHY 



low and medium powers, from fifty to two 

 hundred diameters, occasionally reaching five 

 hundred, but only in more or less rare cases. 

 In bacterial work one thousand diameters is 

 the generally accepted enlargement at home 

 and abroad. It is sufficiently high to show 

 the more minute forms very satisfactorily, is 

 within the compass of all students' micro- 

 scopes supplied with the usual A oil immer- 

 sion lens, and if universally adopted in pub- 

 lished reproductions, affords a very useful 

 standard for comparative sizes of the various 

 forms of bacteria. 



Ocular or No Ocular? The constantly in- 

 creasing use of apochromatic objectives, due 

 both to their many points of superiority over 

 achromats and reduced cost since their early 

 introduction, has been a large factor, I believe, 

 in the nearly universal employment of an eye- 

 piece for photo-micrography at the present 

 day. Time was, and at no very distant date, 

 when exactly the reverse might have been 

 said. The image of an enlarged microscopic 

 subject was projected by the object glass 

 alone; and increased amplification was ob- 

 tained by adding to the length of bellows 

 in the camera. The late Dr. Col. J. J. Wood- 

 ward, whose wonderful work has rarely been 



