THE SENSE OF VISION. 



761 



near the periphery of the refracting surface will be refracted more than those 

 which lie near the axis and will come to a focus sooner. This phenomenon, 

 which is called spherical aberration, is more marked with diverging than with 

 parallel rays, and tends, of course, to produce an indistinctness of the image 

 which will increase with the extent of the surface through which the rays 

 pass. The effect of a diaphragm used in many optical instruments to reduce 

 the amount of spherical aberration by cutting off the side rays is shown dia- 

 gramrnatically in Fig. 224. 



FIG. 224. Diagram showing the effect of a diaphragm in reducing the amount of spherical 



aberration. 



The role of the iris in the vision of near objects is now evident, for when 

 the eye is directed to a near object the spherical aberration is increased in con- 

 sequence of the rays becoming more divergent, but the contraction of the 

 pupil which accompanies accommodation tends, by cutting off the side rays, to 

 prevent a blurring of the image which otherwise would be produced. It must, 

 however, be remembered that the crystalline lens, unlike any lens of human 

 construction, has a greater index of refraction at the centre than at the periph- 

 ery. This, of course, tends to correct spherical aberration, and, in so far as it 

 does so, to render the cutting off of the side rays unnecessary. Indeed, the 

 total amount of possible spherical aberration in the eye is so small that its 

 effect on vision may be regarded as insignificant in comparison with that caused 

 by the other optical imperfections of the eye. 



Chromatic Aberration. In the above account of the dioptric apparatus 

 of the eye the phenomena have been described as they would occur with mono- 

 chromatic light i. e. with light having but one degree of refrangibility. But 

 the light of the sun is composed of an infinite number of rays of different 

 degrees of refrangibility. Hence when an image is formed by a simple lens 

 the more refrangible rays i. e. the violet rays of the spectrum are brought 

 to a focus sooner than the less refrangible red rays. The image therefore 



