Special Objectives. While objectives are 

 divided into two classes, and although there are objec- 

 tives, distinguished by specific names, of more com- 

 plicated construction and greater capacity than .those 

 generally employed, there are other objectives, designed 

 for special purposes, which are not suitable for ordinary 

 investigations. An exception may be made in the 

 so-called variable objective, a low power objective in 

 which a variation in power is possible by varying the 

 distance of the lenses by mechanical means. 



. Illuminating Objectives. These are absolutely 

 necessary in metallurgical investigations and for examin- 

 ing rulings on metallic surfaces. The powers used range 

 between 11-2 and 1-2 inch. A rectangular prism is fixed 

 back of the front lens with the diagonal surface over 

 and covering one-half of its opening. The light passes 

 into the prism from the side of the microscope and is 

 deflected by the diagonal surface of the prism through 

 the front lens, which, acting as a condenser, concen- 

 trates the light on the surface of the object. The 

 remaining one-half of the objective serves in the usual 

 way to form the image. Satisfactory as these objectives 

 are, they are almost useless on ordinary opaque objects 

 on account of the reflections due to the prism. 



Photographic Microscope Objectives. The 

 regular objectives, especially low and medium powers, 

 are not suited for photographic purposes on account of 



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