' DIOPTRIC MECHANISMS OF THE EYEBALL 537 



points or strings of beads, which, since they alter their position with changes 

 in the direction of the eyes, are often spoken of as muscce volitantes. The 

 centring of the eye is also never perfect. In the horizontal meridian the 

 optic axes of the cornea only diverge about 03 from the axis of the lens, 

 but in the vertical meridian there is as much as 1-3 difference between the 

 two axes. Moreover the visual axis does not correspond exactly with the 

 optic axis of the eye. The fovea centralis, the point of distinct vision on 

 which the image of any object must be brought in order to see it as distinctly 

 as possible, always lies outside and somewhat below the point at which the 

 optic axis strikes the retina. The angle between the two axes is often spoken 

 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 astigma- 

 tism (v. p. 538), 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 circum- 

 ference 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 counteracted 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 circumference, 

 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 from 

 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 production 

 of a spectrum. The same splitting up of rays occurs when light passes 

 through a simple lens. As is shown in Fig. 265, the violet rays come to a 

 focus at a point nearer the lens than the red rays. A screen held at v will 

 therefore show a bluish- violet centre with a red margin ; at R the centre will 

 be reddish and the margin violet. 



In optical instruments this chromatic aberration is corrected by 



