TESTING THE FLATNESS OF THE FIELD OF VIEW. 169 



A distortion as in Fig. 23, where the marginal parts appear less 

 enlarged than those in the centre, may be distinguished thus: 

 when the two lines are in contact in the centre, the ends of the 

 straight line m n, which is in the eye-piece setting, are bent 

 outwards, while the opposite distortion is indicated by the devia- 

 tion of these ends inwards (Fig. 100). From the curvature of the 

 straight line m n the curvature of the virtual image which the 

 eye-lens forms of a flat surface, under the given circumstances, 

 may be estimated. 



The instruments we have examined in the manner indicated 

 (by Oberhteuser, Hartnack, Beneche, Merz, and Kellner) showed 

 but slight distortion of the objective-image, when examined either 

 with or without the field-lens. In the latter case the final virtual 

 image was more distorted, because the eye-lens was optically 

 effective in its whole diameter and with far greater aperture, and 

 thus the direction of the incident rays was more conducive to this 

 effect. The same result was also obtained by another method of 

 testing, in which the eye-piece being removed, the objective-images, 

 produced with or without the field-lens, were received at the upper 

 end of the body-tube on a screen of ground glass. 



In order to test the curvature of the image-surface, we examine 

 fine markings on a plane surface (for instance, the markings pro- 

 duced by pressing the tip of the finger on a glass slip), and observe 

 whether all points of the field of view can be clearly seen at the 

 same time. If we find, as we generally do, that the peripheral 

 points require a lower focal adjustment than the central ones, then 

 the image-surface is evidently convex to the upper side. The 

 curvature is, of course, the greater in proportion to the difference 

 of plane. 



The application of a cover-glass with similar markings on the 

 diaphragm of the eye-piece, will also exhibit the curvature of the 

 objective-image according to the same principles which determine 

 the distortion of the image. 



