Y2 THEORY OF THE MICROSCOPE. 



scopic image is rendered plane, and, conversely, a perfectly plane 

 image may appear more or less distorted. Both defects must be 

 corrected, if the flatness of the field of view, in the traditional 

 sense of the expression, is to be complete. 



Distortion as well as curvature can be eliminated in two different 

 ways : first, by making the eye-piece aplanatic and orthoscopic by the 

 addition of plano-concave flint-lenses ; secondly, by skilful selection 

 and combination of simple plano-convex lenses for the field- 

 lens and the eye-lens, the opposing aberrations being so regulated 

 that they cancel each other. 



As the principles upon which aplanatism is based have been 

 explained in a previous chapter, we need not return to the 

 subject. It is evidently a much easier task to construct aplanatic 

 eye-pieces than aplanatic objectives, because it is possible to 

 arrange the curvatures of the lenses in accordance with the re- 

 quirements of calculation. The points we have to discuss are, 

 how aberration can be corrected in an ordinary eye-piece, and 

 how, in general, the curvature of the refracting surfaces influences 

 the curvature of the image? 



The following will afford the necessary data with regard to the 

 first point : Let I q (Fig. 35) be the path of a peripheral pencil 



FIG. 35. 



after refraction through an aplanatic field-lens, or, otherwise 

 expressed, the direction which a refracted pencil would take in 

 order to give a perfectly correct real image, i.e., corresponding 

 precisely with the object ; further, let b p be the path of the 

 pencil after its passage through a single positive lens of equal 

 power but non-aplanatic. It is then evident that the aberration 

 of this last pencil is only completely cancelled if, after its passage 

 through the eye-piece, it appears to come from a point which 

 corresponds to the direction of the pencil b q after a refraction 



