DIOPTRIC MECHANISMS OF THE EYEBALL 597 



of as the angle a. This angle in the horizontal meridian varies 

 between 3-5 and 7, and in the vertical meridian is about 3-5. 



Although this divergence of the axes causes a certain amount 

 of astigmatism (v. p. 598), it is too small to interfere appreciably with 

 the sharpness of vision. 



SPHERICAL ABERRATION. When a beam of parallel rays 

 falls on to the surface of a spherical lens those rays which pass through 

 the circumference are converged to a focus which lies nearer to the 

 lens than the focus of those rays which pass through its centre. In an 

 optical instrument the blurring of the image thus produced is counter- 

 acted in two ways : 



(1) By making the curvature in the middle of the lens greater than 

 at its periphery. 



(2) By stopping out the peripheral rays by means of a diaphragm, 

 or by using only a cylinder cut from the centre of the lens. It is 



familiar to every photographer that where sharpness of definition is 

 required it is necessary to use a small stop, giving a corresponding 

 increased exposure. 



In the eye spherical aberration is diminished by both these means. 

 The curvature of the lens is greater towards its centre than at its cir- 

 cumference, and the peripheral rays of light are shut out by a circular 

 diaphragm, the iris, the diameter of the aperture in which varies 

 according to the amount of light falling into the eye, and according to 

 the nearness of the object which is the point of regard. 



CHROMATIC ABERRATION. The refraction of light in passing 

 trom a lighter to a denser medium is due to the fact that in the latter 

 the velocity of propagation of the light is less than in the former. 

 This diminution of the velocity of the propagation affects rays of 

 various wave-lengths differently; so that of the various rays which 

 make up white light those at the red end with a long wave-length 

 are refracted least, and those at the violet end with a short wave- 

 length are refracted the most. On this account, when light passes 

 into a prism it is split up into its component rays with the produc- 

 tion of a spectrum. The same splitting up of rays occurs when 

 light passes through a simple lens. As is shown in Fig. 263, 

 the violet rays come to a focus at a point nearer the lens than the red 





