216 RECENT PHOGRESS OF THE THEORY OP VISIOJT. 



thought he had discovered a relation between the re- 

 fractive and dispersive powers of various transparent 

 materials, from which it followed that no achromatic 

 refraction was possible. Euler,' on the other hand, con- 

 cluded that, since the eye is achromatic, the relation 

 discovered by Newton could not be correct. Keasoning 

 from this assumption, he constructed theoretical rules 

 for making achromatic instruments, and Dolland ^ carried 

 them out. But Dolland himself observed that the eye 

 could not be achromatic, because its construction did not 

 answer to Euler's rules ; and at last Fraunhofer^ actually 

 measured the degree of chromatic aberration of the eye. 

 An eye constructed to bring red light from infinite dis- 

 tance to a focus on the retina can only do the same with 

 violet rays from a distance of two feet. AYith ordinary 

 light this is not noticed because these extreme colours are 

 the least luminous of all, and so the images they produce 

 are scarcely observed beside the more intense images of 

 the intermediate yellow, green, and blue rays. But the 

 effect is very striking when we isolate the extreme rays 

 of the spectrum by means of violet glass. Glasses 

 coloured with cobalt oxide allow the red and blue rays 

 to pass, but stop the green and yellow ones, that is, the 

 brightest rays of the spectrum. If those of my readers 

 who have eyes of ordinary focal distance will look at 

 lighted street lamps from a distance with this violet 

 glass, they will see a red flame surrounded by a broad 

 bluish violet halo. This is the dispersive image of the 

 flame thrown by its blue and violet light. The phe- 

 nomenon is a simple and complete proof of the fact of 

 chromatic aberration in the eye. 



Now the reason why this defect is so little noticed 



' Leonard Euler born at Basel, 1707 ; died at St. Petersburgh, 1783. 



2 John Dolland, F.R.S. born 1706 ; died in London, 1761. 



' Joseph Fraunhofer born in Bararia, 1787 ; died at Munich, 1826. 



