594 PHYSIOLOGY 



to a distance on the retina between the two ends of the image of 

 00438 mm. Few people can distinguish two points of light the 

 line joining which subtends a smaller visual angle than sixty seconds. 

 If we measure the histological elements in the centre of the retina 

 which are responsible for distinct vision, viz. the cones, we find that 

 in the yellow spot each cone is about -002 to -005 mm. thick. The 

 limits of our power of distinguishing two luminous points is approxi- 

 mately in agreement with the diameter of each end-organ of vision. 

 In order that the images of the two points may give rise to distinct 

 sensations their images must fall upon different cones. This fine- 

 ness of vision exists only at the yellow spot, the accuracy of 

 vision in the peripheral parts of the retina being very much lower. 

 Not only is the image less perfectly focused, but the number of sensory 



FIG. 261. Path of the rays in the formation of an image on the retina. 



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 sensi- 

 tive plate. The same method is adopted in the eyes of certain animals. 

 In the mammalian eye, however, accommodation 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 therefore diminished. 

 Every eye possesses a certain definite range of vision, which, in a 



