150 THE USE OF THE MICROSCOPE 



iiiatic objective, was, as already stated, sometimes given 

 too short, this would result in wrong figures for the magnifi- 

 cation of the eyepiece, if found by dividing the total 

 magnification by the calculated magnification of the 

 objective. (For the lower powers of objectives, however, 

 the optical tube length is below 180 millimeters, because 

 of the lesser distance of the posterior focal plane from 

 the image.) The designation of objectives and eyepieces 

 by their magnifications avoids these discrepancies, es- 

 pecially in the apochromatic series; and this is why its 

 introduction is favored. In the Huyghenian pattern, 

 the fieldlens reduces the image, and the eyelens alone 

 magnifies. Hence, in this form, the magnification of the 

 whole eyepiece is less than the magnification of the eyelens 

 alone. It has already been pointed out that the chromatic 

 and spherical errors are mainly corrected by this con- 

 struction, which shows that achromatic combinations can 

 be attained otherwise than by a convex lens of crown 

 combined with a concave lens of flint. In the higher 

 compensating and orthoscopic eyepieces, however, where a 

 triple lens and a single lens are close together, both act 

 as a compound magnifier and, hence, can be larger than 

 if they had also to make up for the diminution produced 

 by a fieldlens, or if only one lens were used. Hence also 

 the eyepoint is well above such a larger eyelens. 



Eyepieces should be parfocal; for, if not, the focus must 

 be changed with every change of eyepiece, thus altering 

 the magnification of the objective and the optical tube 

 length. A collar of appropriate depth ensures this con- 

 dition. Hence, a series of oculars not parfocal should 

 usually be rejected for scientific work. 



The eyepiece circle (Ramsden circle, or exit pupil) 

 should be in or near the plane of the pupil of the eye. 

 Since there are the cornea, eyelashes, and perhaps spectacle 

 glasses between these two, the need of eyepieces with 

 fairly high eyepoints is apparent. If the pupil of the eye 

 is too much above the eyepiece circle, the field of view is 

 bounded by this circle (instead of by the eyepiece dia- 



