78 THEORY OF THE MICROSCOPE. 



r = 20 mm., the quantity jV* will be about '02 mm. less than in 

 the plane image. But this is, in reality, further increased by 

 the fact that the refracting surface is not, as we have supposed, 

 aplanatic, but is accompanied by the aberration due to spherical 

 form. It acts, as we have explained, upon the peripheral pencils 

 of rays, just as an aplanatic surface of shorter focal length, and, 

 in consequence, brings the corresponding image-points somewhat 

 nearer. The curvature must therefore be perceptible, even if the 

 spherical aberration is taken at only '1 of the focal length. 



The action of the eye-lens has already been explained for a 

 particular case. We found that the image of a Hat surface formed 

 by it must be curved, because its peripheral points are further 

 distant from the refracting surface-elements than the central points. 

 Such a curvature must, of course, always take place when the 

 spherical aberration acts in the same manner as in the case of 

 single lenses. The final virtual image ^ of an ordinary eye-piece 

 cannot possibly be flat, unless the objective-image itself is curved 

 and its convex side turned downwards. The most favourable 

 position of the eye-lens will always be that in which the pencils of 

 rays proceeding from the field-lens are incident upon the spherical 

 surface at the smallest possible angle. 



With the Eamsden arrangement of eye-piece, the conditions 

 which influence the curvature are more complicated. Since the 

 lower lens is here turned with its plane surface to the real objective- 

 image, the virtual image formed by it would be curved downwards, 

 if spherical aberration (which, as has been shown, acts in an opposite 

 direction) did not preponderate. The combined effect of the eye- 

 piece is therefore, in general, very nearly the same as in that of 

 Campani. 



From a practical point of view, moreover, a slight curvature of 

 the field of view is unimportant. No eye-pieces are known to us 

 (not excluding the aplanatic and orthoscopic ones of Ploessl, &c.) 

 in which it is completely eliminated. The chief and proper aim of 

 the optician is to eliminate as far as possible the distortion of 

 the differently-coloured images, and to regulate the remaining 

 deviations so that the red and violet image-points coincide perfectly, 

 at least in the central portion of the field of view. If this is attained 

 for a definite length of tube, the eye-piece will reproduce, as 

 distinctly as possible, any objective-image with an equal length of 

 tube. 



