34 ELEMENTARY CHEMICAL MICROSCOPY 



to one side of the center. The disks furnished with the conden- 

 ser, consisting of a central stop with narrow slots, yield very 

 oblique illumination but a black background, and serve an en- 

 tirely different purpose which is discussed elsewhere under the 

 head Dark-ground Illumination. In the highest grades of mi- 

 croscopes the substage mounting is arranged so as to provide a 

 lateral movement of the iris diaphragm by means of rack and 

 pinion. Oblique illumination is then obtained by closing the 

 diaphragm to a small opening and racking it to one side. 



Oblique illumination is often essential to a proper interpre- 

 tation of structure and to a sharp differentiation of refractive 

 indices. 



The ordinary Abbe condenser is corrected for neither chromatic 

 nor for spherical aberration and although it answers all the 

 purposes of illumination in ordinary microscopy with standard 

 objectives, in photomicrography or in combination with objec- 

 tives of the highest grade and in work of the finest kind, its use is 

 injudicious. Recourse should be had in such cases to achromatic 

 or specially constructed condensers. Since investigations of 

 this kind are rare in chemical laboratories, space forbids their 

 consideration. 



In accurate crystallographic studies the microscope condenser 

 must be especially free from both chromatic and spherical aber- 

 ration; and instruments for this class of work are never provided 

 with condensers of the Abbe type, but are always fitted with 

 light-concentrating devices of special construction. 



It is essential that the optic axis of the condenser shall coincide 

 with the optic axis of the microscope, or, in other words, the con- 

 denser must be accurately centered. In the low-priced micro- 

 scopes no provision is made for any adjustment of the mounting, 

 the proper position being fixed by the manufacturer. Not 

 infrequently through carelessness of workmen and inadequate 

 inspection of the finished instrument, microscopes are sold 

 whose substage condensers are so badly out of center as to render 

 them unfit for high grade work. 



To test the adjustment of an Abbe condenser in a fixed mount- 

 ing, close its iris diaphragm to the smallest obtainable opening, 



