Human Frame. 



vessels, and humours of three several 

 >, for the purpose of vision. The 

 first humour of the eye is called the aque- 

 ous humour, being in all respects like 

 water, hut of a spirituous nature ; for it 

 will not freeze in the greatest cold. The 

 second is called the crystalline humour^ 

 being transparent, and more solid than, 

 cither of the other ; its figure resembles 

 an optic lens, convex on both sides, and 

 its use in the eye is the same. Behind 

 this lies the vitreous or glassy humour ; 

 it is very much like the white of an egg ; 

 it is in greater abundance than either 

 of the other ; it gives the eye its spheri- 

 cal form ; and is thicker than the aque- 

 ous, but thinner than the crystalline hu- 

 mour. Next this humour, on the bot- 

 tom of the eye, is spread a fine curious 

 rnembrane, called the retina, through 

 which are expanded the medullary fibres 

 of the cpi'ic nerve, which come from the 

 brain. Now the rays of light, which 

 come from all parts of any object, falling 

 upon the aqueous humour of the eye, are 

 through it refracted to the crystalline hu- 

 humour, by which, as a double convex 

 lens (kept always at a proper distance by 



