'SENSE OF SIGHT. 

 Fig. 14. 



207 



27. Thus the convergence of the rays begins in the aqueous 

 humour is perfected in the crystalline^ but when they strike 

 the vitreous humour, which is concave on its front surface, 

 and also less dense than the crystalline, they diverge or 

 spread out, and are thrown upon the back part of the eye. 

 The manner in which this is effected may easily be seen by 

 the following illustration. 



Fig. 15. 



o, the aqueous ; c, the crystalline ; t>, the vitreous humour. 



In passing through the crystalline, the rays cross each othei, 

 so that those rays which pass from the lower part of an 

 object are presented uppermost in the bottom of the eye and 

 the reverse, so that the images of objects are always invert, 

 ed, or bottom side upwards. Any one can see this by cut. 

 ting out a circular portion of the outer coat, at the back 

 part of the eye of an ox or sheep recently killed, and holding 

 it up at a window ; he will then see, on its posterior sur- 

 face, a perfect but inverted representation of objects whether 



