432 LIMITS OF OPTICAL CAPACITY OF THE MICROSCOPE. 



visible which may be counted at 3800 to 4000 in the millimeter 

 (lUoVoo of English inch.) 



Thus it appears to me beyond doubt that diffraction of the rays 

 is the the principle cause of the limitation of sharpness of the 

 microscope image. In comparison ■with diffraction, chromatic and 

 spherical aberrations appear to exert but an inconsiderable influence, 

 in spite of the very large angles of incidence and divergence of 

 rays. Considering the extreme care expended on calculation and 

 execution of lenses for telescopes and the photograph-camera, it is 

 justly a matter of surprise that vrith the lenses of the microscope, 

 which are so much more difficult to construct according to prescribed 

 dimensions, and which have so large an aperture, spherical aberra- 

 tion makes itself so little felt. 1 have, however, already pointed 

 out that when there is water between the object and covering glass, 

 and also between this and the objective, the divergence angle is not 

 S7^°, as usually stated, but only 48^° With dry mounted objects 

 an angle of 87^'* can indeed be in action, but onli/ through the 

 minute distance between the object and covering glass ^ so that the 

 spherical aberration arising therefrom is of no importance. 



As wide pencils of light are needed to keep diffraction within 

 moderate limits, the illaminating apparatus should also be capable 

 of emitting pencils of the same angle, in order to show clearly 

 the contour lines of dark objects. 



If there happen to be particles in the object which act like lenses, 

 these may of course convert a small illuminating pencil of rays into 

 strongly divergent rays, and so become clearly visible. Otherwise 

 nothing is seen but a confusion of diffractions at and in the object 

 on one part, and in the (optical) aperture of the microscope on the 

 other part. 



Here lies obviously the explanation why microscopes, otherwise 

 good, but whose illuminating apparatus is not specially arranged for 

 the purpose, yield, with artificial illumination, e. g. a flame, such 

 unserviceable images of the outlines of dark objects. For an 

 immersion lens, the best illuminating apparatus is one constructed 

 according to the same principle — that is to say, a lens of the same 



