THE EYE. 70 



called the aqueous humour, from its liquidity and resem- 

 blance to water. Beyond this is situated the crystalline 

 humour, so called fram its clearness and transparency ; it 

 has the form of a lens, and refracts the rays of light in a 

 greater degree of perfection than any that have been con- 

 structed by art. The back part of the eye, between the 

 crystalline humour and the retina, is filled by the vitreous 

 humour, which derives its name from its supposed resem- 

 blance to glass. The most important part of the eye is the 

 retina ; for it is that which receives the impression of the 

 objects of sight, and conveys it to the mind. It consists of 

 an expansion of the optic nerve of the most perfect AVhite- 

 ness : it proceeds from the brain, enters the eye and is finally 

 spread over the interior surface of the choroid. The refrac- 

 tion occasioned by the several humours unites the v.'hole of 

 a pencil of rays, proceeding from any one point of an object, 

 to a corresponding point on the retina, and the image is thus 

 rendered distinct and strong. Tlie muscles of the eye are 

 six, and by the excellence of their arrangement it is enabled 

 to move in all directions. 



All three of the humours of the eye have some effect in 

 refracting the rays of light, but the crystalline is the most 

 powerful : it is a complete double convex lens ; and as every 

 point of an object sends out rays in all directions, some 

 rays from each point on the side next the eye will be con- 

 verged and brought to as many points on the retina, and will 

 form on it a distinct inverted picture of the object, which is 

 seen erect by the habit of the mind. Although an image 

 must be formed on the retina of each of our eyes, yet we 

 do not see objects double ; for when an object is seen dis- 

 tinctly with both eyes, the axis of each is directed to it, and 

 the object appears single ; but if the axes of both eyes are 

 not directed to the object, it always appears double. If you 

 look at any object, and then by pressing upon the under or 

 upper side of one eye, remove it out of its natural place, you 

 will see two objects, whose distance from each other will 

 vary as the eye is more or less turned from its natural 

 position. 



It is well known that an object at a distance appears 

 smaller than when it is near. The reason is, that the nearer 

 any object can be brought to the eye, the larger will be the 

 angle under which it appears ; for the rays fall more diver* 



