optics. 25S 



The second humour of the eye is the crystalline, 

 which is as transparent as the purest crystal ; but 

 in consistence like a hard jelly, growing somewhat 

 softer from the middle towards the edges. Its form 

 is that of a double convex lens, but more convex 

 on the interior than on the exterior surface. This 

 humour is contained in a very strong and trans- 

 parent membrane, called the araclmoides, and is 

 suspended behind the aqueous humour by the liga- 

 mentum ciliare. 



The vitreous is the third humour of the eye : it 

 receives its name, like the others, from its appear- 

 ance, which is like melted glass. It is not so hard 

 as the crystalline, nor so liquid as the aqueous 

 humour. It fills all the interior chamber of the 

 eye, behind the crystalline humour. 



As every point of an object, ABC, (Plate 13. 

 fig. 6.) sends out rays in all directions, some rays 

 from every point on the side next the eye will fall 

 upon the cornea, between E and F; and bypassing 

 on through the humours and pupil of the eye, they 

 will be converged to as many points on the retina, 

 or bottom of the eye, and will thereon form a dis- 

 tinct inverted picture, c b a, of the object. Thus 

 the pencil of rays, q r s, that flows from the point 

 A of the object, will be converged to the point a 

 on the retina; those from the point B will be con- 

 verged to the point b; those from the point C 

 will be converged to the point c; and so of all the 

 intermediate points, by which means the whole 

 image a b c is formed, and the object made 

 visible; although it must be owned, that the 

 method by which this sensation is carried from 

 the eye by the optic nerve to the common 

 sensorium in the brain, is above the reach of our 

 conception. 



