DIOPTRIC MECHANISMS OF THE EYEBALL 



599 



meridian, the other for the horizontal meridian. The manner in 

 which such rays are diverged is shown in Fig. 264. 



The effects of astigmatism are especially noticeable when the patient 

 is trying to see clearly objects composed of horizontal and vertical 

 lines, such as print. A vertical line can be conceived as made up of a 

 series of points each of which sends out a horizontal sheaf of rays. In 

 the same way a horizontal line is distinguished as a series of points 

 sending out flat sheafs of vertical rays. If the curvature of the cornea 

 in the vertical meridian is greater than in the horizontal it will need 

 a greater effort of accommodation to bring the vertical lines to a focus 

 on the retina than it does to bring the horizontal lines. In reading 

 therefore there is a constant shifting of the focus of the eye, and the 



FIG. 264. Diagram showing course of rays n an astigmatic eye. (WALLER.) 



The curvature of the cornea is greater in the vertical meridian 

 vvv than in the horizontal meridian hhh. Hence the rays of light 

 coming from the point P and passing through the vertical meridian 

 come to a focus at f l , while those through the horizontal meridian come 

 to a focus at / 2 . There is thus no point behind the cornea at which all 

 the rays from p will come to a focus, and the image of the point must 

 be blurred, being elongated in a horizontal direction at /*, and in a 

 vertical direction at / 2 . 



mechanism of accommodation becomes rapidly tired and strained, 

 with the production of pain in the eyes or of headache. In order to 

 correct astigmatism it is necessary to find out first the curvature of the 

 cornea in the different meridians, and then to reinforce the curvature 

 of the weaker meridian by means of a cylindrical lens. If the eye is 

 myopic the cylindrical lens may be concave and placed so that its 

 curvature counteracts that of the cornea in the meridian in which the 

 curvature is greatest. 



ACCOMMODATION 



The rays falling on the eye from a point of light at a distance 

 greater than five metres from the eye may be regarded as practically 

 parallel, and are converged in the normal eye to a focus on the retina. 

 As the point of light is moved nearer to the eye the latter is still able 

 to focus the rays on the retina through a considerable range. On 

 approximating the points of light to a distance which is less than the 

 near point of distinct vision, the rays are no longer converged to a 

 point on the retina, and a blurred image is the result. This near point 



