JO LETTEES ON NATUEAL MAGIC. 



vitreous humour, which resembles the transparent 

 white of an egg, and fills up the rest of the eye. 

 Behind the vitreous humour, there is spread out 

 on the inside of the eyeball a fine delicate mem- 

 brane, called the retina, which is an expansion 

 of the optic nerve, entering the back of the eye 

 and communicating with the brain. 



A perspective view and horizontal section of 

 the left eye, shown in the annexed figure, will 

 convey a popular idea of its structure. It is, as 

 it were, a small camera obscura, by means of 

 which the pictures of external objects are painted 

 on the retina, and, in a way of which we are 

 ignorant, it conveys the impression of them to 

 the brain. 



Fig. 1. 



This wonderful organ may be considered as 

 the sentinel which guards the pass between the 

 worlds of matter and of spirit, and through which 

 all their communications are interchanged. The 

 optic nerve is the channel by which the mind 

 peruses the hand- writing of Nature on the retina, 



