74 PHOTOMICROGRAPHY 



paration right up to the moment of exposure. The 

 method of using them is given on page 247. 



Achromatic and Holoscopic Eyepieces. There are other 

 types of ocular made, but not so frequently used. For 

 instance, there are achromatic oculars, and the Holoscopic 

 eyepieces made by Watson. The latter are so arranged 

 that by adjusting the separation of their component lenses 

 they may be used with either achromatic or apochromatic 

 objectives at will. 



Combination of Eyepiece and Objective. The best 

 results with low- and medium-power achromatic objectives 

 are usually obtained without an eyepiece, and in any case 

 it is not advisable to use an ocular with greater magnify- 

 ing power than 8 times. Firstly, because any residual 

 error of the objective is magnified by the eyepiece ; 

 and, secondly, because there is considerable loss of 

 light with the higher eyepieces. The most useful 

 eyepieces are those which magnify about 4, 6, and 8 

 times. 



Apochromatic objectives, on the other hand, should 

 always be used with compensating or projection oculars, 

 their corrections are so good and the brilliance of 

 the image so great that very high-power oculars, giving 

 magnifications up to 18 or 27 diameters, can be used 

 without any serious depreciation of the image. The x 6 

 and X 12 are the most useful of the compensating eyepieces. 

 The higher power achromats (J" and over), with large 

 N.A., are also generally under-corrected, and perform 

 to better advantage with a compensating or projection 

 eyepiece than with a Huyghenian. 



Most makers now compute their eyepieces so that 

 the lower focal planes fall at the same position in the 

 microscope tube (Fig. 22), and, in consequence, a change 

 of eyepiece may be made with, at most, a slight alteration 

 in the fine adjustment. 



Designation of Eyepieces. There is no uniformity 



