INTRODUCTION TO OPTICS. xcv 



which derives its name from its clearness and transparency : it has the 

 form of a lens, and refracts the rays of light in a greater degree of per- 

 fection than any that have been constructed by art : it is attached by fibres, 

 mm, to each side of the choroid. The back part of the eye, between the 

 crystalline humour and the retina, is filled by the vitreous humour, h h, 

 which derives its name from a resemblance it is supposed to bear to glass 

 or vitrified substances. The membranous coverings of the eye are 

 intended chiefly for the preservation of the retina, i i, which is by far the 

 most important part of the eye, as it is that which receives the impression 

 of the objects of sight. The retina consists of an expansion of the optic 

 nerve, of perfect whiteness : it proceeds from the brain, enters the eye 

 at n on the side next the nose, and is finely spread over the interior 

 surface of the choroid. The rays of light which enter the eye by the 

 pupil, are refracted by the several humours in their passage through 

 them, and unite in a focus on the retina. 



Rays proceed from bodies in all possible directions; we must, therefore, 

 consider every part of an object which sends rays to our eyes as points 

 from which the rays diverge, as from a centre. Divergent rays, on entering 

 the pupil, do not cross each other ; the pupil, however, is sufficiently 

 large to admit a small pencil of them ; and these, if not refracted to a 

 focus by the humours, would continue diverging after they had passed 

 the pupil, would fall dispersed upon the retina, and thus the image of a 

 single point would be expanded over a large portion of the retina. The 

 divergent rays from every other point of the object would be spread over 

 a similar extent of space, and would interfere and be confounded with the 

 first, so that no distinct image could be formed on the retina. 



Fig. 32 represents two pencils of rays issuing from two points of the 

 tree A, B, and entering the pupil, refracted by the crystalline humour D, 

 and forming distinct images of the spot they proceed from on the retina, 

 at , b. Fig. 33 differs from the preceding, merely from not being sup- 

 plied with a lens : in consequence of which the pencils of rays are not 

 refracted to a focus, and no distinct image is formed on the retina. The 

 rays issuing from two points of an object are alone delineated, and 

 the two pencils in Jig. 33 distinguished by describing one of them 

 with dotted lines. The interference of these two pencils of rays will 



Fig. 32. 



enable you to form an idea of the confusion which would arise from 

 thousands and millions of points at the same instant pouring their 

 divergent rays upon the retina. The refraction of the several humours 

 unites the whole of a pencil of rays, proceeding from any one point of an 

 object, in a corresponding point on the retina, and the image is thus ren- 



