to meet the different standards as used by the makers 

 of objectives. Test objects which are prepared to test 

 the resolving power of objectives and consist of diatoms 

 are generally covered with these thicknesses. The 

 No. i are principally used with oil immersion objec- 

 tives as the working distance of these objectives is 

 very short and more working distance is gained by 

 using thin covers with them. 



An excellent means of determining the thickness of 

 cover glass, as well as of studying spherical and 

 chromatic aberration, is the Abbe test-plate. This 

 consists of a series of cover glasses ranging in thickness 

 from 0.09 mm. to 0.24 mm., silvered on the under 

 surface, with lines cut through the film of silver, and 

 cemented to a slide, each cover being marked with its 

 correct thickness. 



With this test-plate spherical aberration is corrected 

 when the bands or lines show distinctly without any 

 nebulous fringe, and thus indicate the proper thickness 

 of cover to use. 



Chromatic correction may be judged by the character 

 of color bands which show with oblique light. But 

 correction is indicated when, on swinging the mirror to 

 the left, violet appears on the left and apple green on 

 the right side of the bands. 



142 



