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 photographic 

 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. 257 c and c' are the centres of the sperical surfaces bounding the lens ; 

 the line joining them is the optical axis of the lens. The surface at Q is 

 parallel to the surface at R, so that a ray of light, such as PQ falling on the 



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