chap, xxvii.] CHROMATIC ABERRATION. 



345 



different degrees to which these colours are refracted 

 by the prism. It is to be expected that the same 

 thing will happen in a lens, 

 and that the most refracted 

 rays will be brought to a 

 focus sooner than those least 

 refracted, the violet rays, 

 that is to say, will be 

 focussed nearer to the lens 

 than the red. This actually 

 happens, and is represented F'g. 154. 



155, where two 



n Fig. 



rays at extremities of the lens are shown to be dis- 

 persed, the violet rays forming a focus at b, the red at 

 r, and the rays of the spectrum between the red and 

 violet being disposed regularly between r and 6. If 

 the light has proceeded from an object, no proper white 

 image will be formed, but, instead, circles of colour will 



Fig. 153. Chromatic Aberration. 



surround the object, which, if placed at b, will have a 

 central circle of violet changing gradually till the 

 outer ring of the circle is red ; and, if placed at r, will 

 have a central circle of red, changing gradually through 

 the colours of the spectrum to violet. This is called 

 chromatic aberration. This property of lenses seemed 

 at first to offer an insuperable obstacle to the employ- 

 ment of lenses for magnifying purposes, since, owing to 

 it, no clear definition of an object could be obtained ; 

 and it seemed impossible to obtain, a lens which would 



