THE FLATNESS OF THE FIELD OF VIEW. 



75 



the colours, is situated to the right of the centre of curvature ; 

 but, since in this case the spherical aberration of the red pencils, 

 incident at a greater angle, increases with the distance of the two 

 lenses, there is a limit where it equalizes the chromatic aberration. 

 Hence the blue fringe of the field of view is changed to a red 

 one. As this limit is, with given curvatures, dependent solely upon 

 the dispersive power of the glass, a glass may be selected which 

 will admit of the production of aplanatism for red and violet 

 light at the same time. And if, for instance, crown-glass possesses 

 approximately this property for a particular combination of lenses, 

 it must be possible to reduce to a minimum the remaining aber- 

 rations by means of slight alterations in the distances and curva- 

 tures. We therefore arrive at the conclusion that the elimination 



of dispersion is compatible with the simultaneous elimination of 

 spherical aberration. 



As regards the curvature of the image-surface, the usual explana- 

 tion as to the field-lens image having its convex side downwards- 

 is erroneous. Exactly the opposite takes place. We may convince 

 ourselves of this by placing a flat plate of glass covered with fine 

 dust, or a micrometer plate, upon the diaphragm of the eye- 

 piece, and by comparing its margins in the microscopic image 

 with those of the field-lens image. We shall find that the 

 margin of the latter image requires a somewhat lower adjustment 

 of the eye-lens than the similarly-situated points of the glass 

 plate. The field-lens image is therefore curved upwards. On 

 theoretical principles it cannot be otherwise. If we suppose a 



