DIOPTRIC MECHANISMS OF THE EYEBALL 



535 



perfectly focused, but the number of sensory elements in a given area of 

 the retina diminishes steadily as we pass from centre to periphery. 



OPTICAL DEFECTS OF THE EYE 



The normal eye is so constructed that parallel rays come to a point on the 

 retina. Rays which are divergent when they fall on the anterior surface of 

 the cornea are brought therefore to a focus behind the retina. 



In such an eye any object lying at a distance nearer than five metres 

 would give rise to an image behind the retina, were it not for the fact that 

 the eye possesses means of altering its focus and so of bringing divergent rays 

 to a focus on the retina ; this means 

 is known as accommodation. In a 

 photographic camera the process of 

 focusing is carried out by altering the 

 distance between the lens and the 

 sensitive plate. The same method is 

 adopted in the eyes of certain animals. 

 In the mammalian eye, however, ac- 

 commodation for near objects, i.e. the 

 focusing of divergent rays on to the 

 retina, is accomplished by a change in 

 the curvature of the lens. The lens 

 becomes more convex on its anterior 

 surface, so that its refractive power is 

 increased and the hinder focus of the 

 dioptric mechanism of the eye is there- 

 fore diminished. Every eye possesses 

 a certain definite range of vision, which, 

 in a normal person, has its far point 

 at infinite distance and its near point 

 at a distance from the eye which depends on the elasticity of the lens 

 and the range through which this can alter its curvature. 



The normality of an eye is determined by the fact that parallel rays come 

 to a focus on the retina when the apparatus of accommodation is at rest. 

 Two classes of deviation from this normal, or emmetropic, eye are common. 

 In the first class parallel rays come to a focus in front of the retina. In such 

 eyes, which are designated myopic, it is impossible to get a clear image on the 

 retina of distant objects. In order that the rays may be focused on the 

 retina they must be divergent, so that even when accommodation is para- 

 lysed the far point of distant vision lies at some finite distance from the eye, 

 varying with the extent of the disorder. If in such' eyes the range of 

 accommodation is normal, the near point will be much nearer to the eye than 

 in the emmetropic eye (Fig. 264). 



The second class of abnormal eyes are known as hypermetropic. These 

 eyes can be regarded as too short for their refractive media. Parallel rays 



FIG. 264. Diagrams of course taken 

 by parallel rays in entering normal 

 (emmetropic) eye (A), hyper-metro- 

 pic eye (B), and myopic eye (c). 



