THE SENSE OF VISION. 



315 



number of eyes. There are, however, a number of imperfections of the diop- 

 tric apparatus, many of which affect all eyes alike. Of these imperfections 

 some affect the eye in common with all optical instruments, while others are 

 peculiar to the eye and are not found in instruments of human construction. 

 The former class will be first considered. 



Spherical Aberration. — It has been stated that a pencil of rays falling 

 upon a spherical refracting surface will be refracted to a common focus. 

 Strictly speaking, however, the outer rays of the pencil — i. e. those which fall 

 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- 

 grammatically in Fig. 136. 



Fig. 136.— 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 maybe regarded :is insignificant in comparison with that caused 

 by the other optical imperfections of the eye. 



