THE FLATNESS OF THE FIELD OF VIEW. 



71 



image a 1' at equal distances. A uniform magnification would 

 therefore clearly imply that the deflected pencils of light cut the 

 axis of the Microscope in the same point. This supposition is 

 not, however, correct, since the point of intersection of the peri- 

 pheral rays always lies nearer to the eye-lens than that of the 

 central rays. The final virtual image is thus necessarily more 

 or less distorted. For if c" (Fig. 34) is the image of the object- 

 point c lying in the axis, and p" that of any 

 point p of one of the divisions, a point q at 

 twice the distance would, in the case of uniform 

 magnification, have to be formed in the image 

 also at twice the distance, therefore in q". In 

 reality, however, the image-forming pencil of 

 light is revolved somewhat more round its 

 point of incidence r in the upper surface of 

 the eye-lens, since it cuts the axis at a slightly 

 less distance. The point q" lies therefore 

 somewhat further outwards, as the dotted line 

 indicates ; consequently, in the virtual image, 

 c" 4' < 2 x c" p", and p" <?" < c" p". This 

 means that the magnification increases with 

 the distance from the axis. The increase is 

 limited, however, as is readily seen, to the 

 radial direction; the tangential direction is 

 affected only because the displacement of the 

 image-points in a radial direction causes a pro- 

 portional change in their distances. Straight 

 lines in the object, which (if necessary produced) 

 intersect in the centre of the field of view, must therefore appear 

 in the image as straight lines ; in every other direction, however, 

 an irregular magnification involves their curvature, which is the 

 stronger the greater their distance from the centre. 



We may, consequently, regard it as established, that curvature 

 and distortion are two entirely different phenomena, which are 

 not to be confounded. Distortion depends, we repeat, upon the 

 spherical aberration of the refracting surfaces ; curvature, on the 

 other hand, upon the unequal distances of the respective object- 

 points (in which the image of the first surface is to be regarded 

 as the object for the next succeeding surface). Even if spherical 

 aberration is eliminated, it still does not follow that the micro- 



FIG. 34. 



