lack of coincidence of the visual and chemical rays 

 on the photographic plate ; while an image may be 

 sharply defined when viewed by the eye, that made on 

 the plate will be indistinct. For very low powers 

 standard photographic lenses of short focus may be 

 used, but the microscope objective requires its chro- 

 matic corrections changed while retaining the focus, 

 which very much detracts from its usefulness for 

 regular microscopical work. 



While the regular medium powers give fair results, 

 they, as well as the low powers, should be adapted to 

 these changed conditions to give the best results and 

 particularly with a view to obtaining a greater amount 

 of illumination. 



Projection Objectives. These objectives are 

 used to throw an enlarged image of an object on a 

 white screen or wall fifteen feet or more from the 

 apparatus. 



Systems. An objective is said to consist of 

 systems which may vary in number from one to four 

 or five. Two systems are generally used in low 

 power objectives, three and sometimes four in the 

 medium powers, and four or five in the high powers. 

 They are the individual portions consisting of one, two 

 or three lenses. When a system is composed of more 

 than one lens, the lenses are cemented together by 



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