44 THEORY OF THE MICROSCOPE. 



defined throughout, at all points equally magnified (not distorted), 

 and only very slightly curved. 



The aplanatic eye-pieces of Schieck, Schrceder, and others are 

 constructed on similar principles. In Schrocder's catalogue it is 

 expressly stated that the aplanatic eye-pieces consist of two 

 achromatic lenses, whilst the so-called orthoscopic eye-pieces of the 

 same maker have only one achromatic lens. The orthoscopic eye- 

 pieces of Kellner, on the other hand, are similar in optical action 

 to the Campani eye-piece, but they consist of an achromatic 

 bi-convex field-lens and an ordinary eye-lens. They give a large 

 field of view free from distortion, without, however, appreciably 

 increasing the performance otherwise. This is true also of the 

 4t periscopic " eye-pieces of Seibert and Krafft. The advantages 

 obtained by any improved construction of the eye-piece as, for 

 instance, greater flatness of the field of view, and so on are of 

 minor importance. A real increase in optical power is, in spite 

 of all assertions to the contrary, altogether out of the question. 



The expressions " aplanatic " and " orthoscopic " appear to us to 

 mean much the same thing. In order to eliminate the distortion 

 of the image the incident pencils must be so refracted that their 

 lines of direction cut the optic axis in the same point (vide Chap. 

 VII. On the Flatness of the Field of View): the eye-piece must, 

 therefore, be aplanatic. As in orthoscopic eye-pieces the freedom 

 from distortion is especially commended, their principal value 

 seems to consist in their being aplanatic. 1 Supposing they 

 neutralize the curvature of the image-surface in a more perfect 

 degree than the aplanatic eye-pieces of Plcessl and others, which 

 we will assume to be the fact, the expression " orthoscopic " is not 

 very distinctive, since it is used by some opticians as meaning 

 "inverting" or "erecting" that is, in quite a different sense. 

 The so-called achromatic and periscopic eye-pieces are also 

 avowedly aplanatic as a leading feature. By appropriate com- 

 bination of the lenses, however, achromatism and greater extent 

 of the field of view may be obtained at the same time. 



The special advantage of the " holosteric," or " solid glass eye- 

 pieces," is unknown to us. Most of them magnify very highly ; 



1 In English works on Optics, " Flatness of Field" is distinguished from 

 " Aplanatism" : by the former is meant that the system forms a plane image 

 of objects in a plane perpendicular to the axis ; by the latter, that the system 

 is free from spherical aberration. [ED.] 



