126 THE USE OF THE MICROSCOPE 



used alternately for some years, in each case with yellow- 

 green light. The slight superiority of the apochromatics 

 was obvious. Much good work can be done, however, 

 with achromatic objectives and green screens. Coles 

 preferred fluorite oil-immersion objectives with a green 

 screen to apochromatics for research on blood parasites, 

 and the writer finds a fluorite 100 objective often useful 

 with a yellow-green screen. 



Centering Objectives. — We may distinguish two kinds 

 of centering: making the optic axis of the objective parallel 

 to or coincident with that of the eyepiece, and making the 

 field of the low or medium objectives concentric with the 

 field of the high-power objective. Centering of the first 

 kind is especially important with objectives of high aperture. 

 Centering of the second kind is useful and necessary in 

 practice, but must not interfere with the first kind of 

 centering, at least with the high-power objectives. The 

 Zeiss objective changers, and objective-centering rings 

 permit the first kind of centering to be done perfectly, 

 if the shoulders of the objectives are accurately at right 

 angles to their optic axes. In this case the centering of 

 the high powers can be done with a test object, such as 

 Surirella gemma in hyrax; and the low and medium powers 

 can be made fairly concentric with these tested high powers. 



If, as is mostly the case in practice, a revolving nosepiece 

 is used, then we may put two or three centering collars 

 on a double or triple nosepiece (shortening the tube length 

 by 15 millimeters to compensate). Otherwise we should 

 test a satisfactory high objective with and without the 

 nosepiece. It should center precisely the same. If not, 

 the nosepiece requires alteration or adjustment. In Zeiss 

 nosepieces, adjustment from side to side is made by the 

 snap, and adjustment from front to back by lateral play 

 of the screw when screwing up. When the nosepiece is 

 correct, the lower powers can be made more concentric 

 with the tested high power by putting a film of gum of 

 the right thickness on the right spot of the shoulder. 



