180 Experimental Philosophy. [Lecture 13. 



the images of all visible objects, by the rays of 

 light which either flow or are reflected from 

 them. 



Under the cornea is a fine transparent fluid, 

 like water, which is therefore called the aqueous 

 humour. It gives a protuberant figure to the 

 cornea, fills the two cavities mm and nn, which 

 communicate by the pupil P, and has the same 

 refractive power as water. At the back of this 

 lies the crystalline humour R, which is shaped 

 like a double convex glass, and is a little more 

 convex on the back than the forepart. It con- 

 verges the rays, which pass through it from every 

 visible object, to its focus at the bottom of the 

 eye. This humour is transparent, like crystal, 

 and is much of the consistence of hard jelly. 

 It is inclosed in a fine transparent membrane, 

 from which issue radial fibres, called the ligar 

 mentum 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 crystalline humour, and also shift it a 

 little backwards or forwards in the eye, so as to 

 adapt its focal distance at the bottom of the eye, 

 to the different 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 crystalline lies the vitreous 

 humour KK, which is transparent like glass, and 



