VISION 



SECTION VI 

 DIOPTRIC MECHANISMS OF THE EYEBALL 



WHEN light falls on any object a certain proportion of it is reflected 

 and scattered, and will affect any organism in the neighbourhood 

 possessing sensibility to light. Mere sensibility of the surface to light 

 would not, however, suffice to arouse projected sensations, since the 

 rays of light from a number of different objects would interfere 

 with one another. An animal with such sensibility would be 

 aware of or be able to react to differences of light and darkness, but 

 could not direct its movements in accordance with the nature of the 

 objects from which the light proceeded. For this purpose there 

 must be not only a surface sensitive to light impressions but 

 also dioptric mechanisms, by means of which a real image of external 

 objects, in their proper spatial relationships, is thrown on to 

 an extended sensory surface. Each point in this surface will 

 correspond to a point lying outside the body and serving as a source 

 of light, and the sensations evoked, since they correspond to the rays 

 of light coming from external objects, can be projected, and referred 

 to the objects themselves lying outside and at some distance from 

 the body. 



The organ of vision, the eye, consists of two parts, viz. : 



(a) The sensory surface or retina, composed of a number of areas, 

 each of which can be separately stimulated by light. 



(6) A dioptric mechanism for projecting a real image of external 

 objects on this sensory surface. 



We have in fact an arrangement very analogous to a photo- 

 graphic camera, where a real image is thrown by a lens on to a sensitive 

 plate, each point of which undergoes chemical change in proportion 

 to the amount of incident light, so that a photographic record is the 

 result. 



THE FORMATION OF AN IMAGE BY A LENS 



We may confine our attention to the case of a bi-convex lens, 

 and we may assume in the first place that the thickness of the lens is 

 negligible. In Fig. 255 c and c' are the centres of the spherical 



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