SENSE OF SIGHT. 



545 



to a focus upon the other side of it ; in such a manner that an inverted 

 picture of the object is formed upon a screen, placed in the proper posi- 

 tion to receive it. Thus in Fig. 162, A B is the object, and E F the 



Fig. 162. 



lens ; the rays issuing from the two extremities and the centre of the 

 object, are brought to a corresponding focus at a less distance on the 

 other side of it, so as to form a distinct picture ; but as the rays from 

 A are brought to a focus at D, and those from B at c, the picture will 

 be inverted. 



V. The further the object is removed from the lens, the nearer will 

 the picture be brought to it, and the smaller will it be. 



VI. If the screen be not held precisely in the focus of the lens, but 

 a little nearer, or further off, the picture will be indistinct ; for the 

 rays which form it will either not have met, or they will have crossed 

 each other. 



956. The Eye, in its most perfect form such as it possesses in Man 

 and the higher animals, is an optical instrument of wonderful com- 

 pleteness ; designed to form an exact picture of surrounding objects 

 upon the Retina or expanded surface of the Optic nerve, by which the 

 impression is conveyed to the brain. The rays of light, which diverge 

 from the several points of any object, and fall upon the front of the 

 cornea, are refracted by its convex surface, whilst passing through it 

 into the eye, and are made to converge slightly. They are brought 

 more closely together by the crystalline lens, which they reach after 

 passing through the pupil ; and its refracting influence, together with 

 that produced by the vitreous humour, is such as to cause the rays, that 

 issued from each point, to meet in a focus on the retina. In this 

 manner, a complete inverted image is formed, as shown in Fig. 163 ; 

 which represents a vertical section of the eye, and the general course 



Fig. 163. 



of the rays in its interior. As in the preceding figure, the rays which 

 issue from the point A are brought to a focus at D ; whilst those diverg- 

 ing from B are made to converge upon the retina at c. The Retina, 

 which is itself so thin as to be nearly transparent, is spread over the 



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