DIOPTRIC MECHANISMS OF THE EYEBALL 539 



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 mechanism of accommodation becomes rapidly tired and 



FIG. 266. Diagram showing course of rays in 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 / A , 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 . 



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 of vision may be determined in any eye by finding out 

 the smallest distance from the eye at which small print can be easily dis- 

 tinguished. The distance is measured by means of a graduated rod between 

 the eye and the printed object. This ' accommodation,' by which the 

 eye is able to focus divergent rays on to the retina, implies either a change 

 in the distance of the refracting surfaces from the retina, or an increase 

 in the total refractive powers of the eye. 



In man and the higher animals it is by the latter means alone that 



