192 RECENT PROGRESS OF THE THEORY OF VISION. 



observed that the eye could not be achromatic, because its 

 construction did not answer to Euler's rules; and at last 

 Fraunhofer l actually measured the degree of chromatic aberra- 

 tion of the eye. An eye constructed to bring red light from, 

 infinite distance to a focus on the retina can only do the same 

 with violet rays from a distance of two feet. With 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 inter- 

 mediate 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 phenomenon 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 under 

 ordinary circumstances, and why it is in fact somewhat less 

 than a glass instrument of the same construction would have, 

 is that the chief refractive medium of the eye is water, which 

 possesses a less dispersive power than glass. 2 Hence it is that 

 the chromatic aberration of the eye, though present, does not 

 materially affect vision with ordinary white illumination. 



A second defect which is of great importance in optical 

 instruments of high magnifying power is 'what is known as 

 spherical aberration. Spherical refracting surfaces approximately 

 unite the rays which proceed from a luminous point into a 

 single focus, only when each ray falls nearly perpendicularly 

 upon the corresponding part of the refracting surface. If all 

 those rays which form the centre of the image are to be exactly 



1 Joseph Fraunhofer born in Bavaria, 1787; died at Munich, 182G. 



2 But still the diffraction in the eye is rather greater than an instrument 

 made with water would produce under the same conditions. 



