64 THEORY OF THE MICKOSCOPE. 



a lens is placed so that it forms virtual images instead of real 

 ones. This will be apparent if, in Fig. 21, the lines drawn towards 

 o and o are produced backwards ; then, in consequence of the 

 spherical aberration, the peripheral image-points will move beyond 

 the position they would occupy if the lens were aplanatic. The 

 margins of the image will therefore appear more magnified than 

 the central parts, and the network will consequently present 

 the appearance of Fig. 24. This is the distortion which is 

 observed in a greater or less degree with every non-achromatic 

 magnifying-lens, and which was formerly erroneously called 

 the curvature of the field of view. Since it is obvious that the 

 eye-lens causes a similar distortion, acting, of course, in an 

 opposite direction to the field-lens, a ratio may always be found 

 by which the final virtual image of the Microscope will exhibit 

 a nearly uniform enlargement, that is, will appear tolerably 

 fiat. The so-called flatness of the field of view is dependent on 

 the practical attainment of this ratio. We shall return to this 

 point in a special chapter. 



Spherical aberration of the field-lens and eye-lens always 

 implies, therefore, under the conditions of microscopic vision, a 

 distortion of the real image (in the sense that the margins are 

 slightly less enlarged than the centre), and a distortion of the final 

 virtual image in an opposite direction. It is quite immaterial in 

 this case whether the image-points lie in a plane or in a curved 

 surface ; the (actual) curvature of the image-surface is a phe- 

 nomenon of quite another kind, which was erroneously regarded 

 by the early microscopists as the cause of the distortion. We 

 shall discuss this point more fully in the proper place ; the only 

 question here is, whether, and in what way, spherical aberration 

 influences the image-surface itself? or, in other words, what 

 changes will occur in the image formed by an aplanatic lens, 

 if a non-aplanatic one of equal focal length (for central rays) 

 is substituted for it ? The question is easily answered. A non- 

 aplanatic lens acts on the peripheral pencils of light, since they 

 meet only a small part of the lens, exactly in the same way as 

 an aplanatic one of shorter focal length, and therefore, cocteris 

 paribm, brings the real image-points somewhat nearer, and forms 

 the virtual ones at a greater distance. Spherical aberration of 

 the field-lens acts, moreover, so to say, attractively upon the 

 margins of the real image, while spherical aberration of the eye- 



