PROPERTIES OF LENSES. 1 L 



is composed of rays of different colours, if it pass 

 through, a medium (such as a lens or prism), Fig. 9, 

 whose sides are not parallel, it is divided into its 

 component colours, OP dispersed. 



Now since all the different rays into which a pen- 

 cil of white light can be divided, have different foci, 

 that is, the point where the red rays come to a focus is 

 more distant than that of the violet, the intermediate 

 colours forming other foci between them, V 1 2 3 4 R 

 Fig. 9, the rays are unequally dispersed and the result- 

 ing image will be coloured at its edges. 



There are several methods of remedying these 

 aberrations, some of them diminish Spherical and 

 Chromatic aberration at the same time. 



Spherical Aberration may be destroyed by giving 

 the lens a decreasing curvature towards the edges, 

 in fact an ellipsoidal form, the difficulty of this in 

 practice was found so great that it was abandoned, 

 and if the form was such that its performance was 

 perfect for parallel rays, it would be inaccurate for 

 all others. It is much more easily corrected by altering 

 the curves of the two sides. For instance if the 

 plane side of a plano-convex lens be turned towards a 

 very near object, or the convex side towards a very dis- 

 tant one, the resulting aberration would be about one- 

 fourth of that which would occur if the respective 

 positions of the sides of the lens were reversed 



A method which has been frequently adopted for 

 photographic objectives deserves mention, namely 

 lessening the aperture, by placing a diaphragm in 

 front of or behind the lens which admits the rays at 



