THE FLATNESS OF THE FIELD OF VIEW. 



69 



the lens A B forms of the plane a b (assumed to be at right angles 

 to the optic axis), lie in a curved surface convex to the object, and 

 that the peripheral parts of this surface, in consequence of their 

 greater distance, are somewhat more magnified. Similarly, it was 



considered that the opposite distortion of 



real images (Fig. 31), in which the magnifica- 

 tion decreases from the centre outwards, was 

 occasioned by a corresponding curvature of 

 the image-surface (Fig. 32) [cf. Harting, 

 " Das Mikroskop," first edition, pp. 134 and 

 278]. Harting and others have since changed 

 their views (cf . loc. cit., second edition, vol. i. 

 pp. 100 and 141) ; but as the traditional idea 

 crops up here and there, and is, moreover, 

 contained in all the older text-books, we will repeat without 

 alteration in this second (German) edition our investigations on 

 this point. 



That the old explanation is entirely erroneous is plain from 

 what has been said above (pp. 6265). It was there shown that such 



FIG. 31. 



a 



FIG. 32. 



distortions are not to be explained by the curvature of the image- 

 surface, but by the stronger refraction of the peripheral pencils, 

 and are therefore due to the spherical aberration of the image- 

 producing lenses ; it was at the same time pointed out that a real 

 curvature of the image might take place quite independently of 

 this, and that it varies with the spherical aberration of the lenses, 

 according as their convex sides are turned upwards or downwards. 

 Nevertheless, it may not be superfluous if we return once more 

 to this subject, in order to explain, in as simple a case as possible, 

 not only the distortion by the eye-lens, but also the curvature of 

 the virtual image. 



