240 FERGUSON'S LECTURES. 



This humour is transparent like crystal, is much of the 

 , consistence of hard jelly, and exceeds the specific gravity 

 of water in the proportion of 11 to 10. It is enclosed 

 in a fine transparent membrane, from which proceed 

 radial fibres o o, called the ligamentum ciliare, all around 

 its edge ; and join to the circumference of the iris. 

 These fibres have a power of contracting and dilating 

 occasionally, by which means they alter the shape or 

 convexity of the chrystalline humour, and also shift it a 

 little backward or forward in the eye, so as to adapt 

 its focal distance to the bottom of the eye to the differ- 

 ent distances of objects ; without which, provision, we 

 could only see those objects distinctly, that were all at 

 one distance from the eye. 



At the back of the chrystalline, lies the vitreous hu- 

 mour K K, which is transparent like glass and is largest 

 of all in quantity, filling the whole orb of the eye, 

 and giving it a globular shape. It is much of the con- 

 sistence of the white of an egg, and very little exceeds the 

 specific gravity and refractive power of water. 



As every point of an object ABC sends out rays in 

 all directions, some rays, from every point on the side 

 n-ext the eye, will fall upon the cornea between E and 

 F: and by passing 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 

 distinct 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 converged 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 op- 

 tic nerve to the common sensory in the brain, and there 

 discerned, is above the reach of our comprehension. 



