OF OPTICS. 



239 



The eye is nearly globular. It consists of three coats 

 and three humours. The part D H H G of the outer 



LECT. 

 vn. 



coat, is called the sclerotica, the rest D E F G the cornea. The eye 

 Next within this coat is that called the choroides, which 

 serves as it were for a lining to the other, and joins with 

 the ins in n, m n. The iris is composed of two sets of 

 muscular fibres ; the one of a circular form, which con- 

 tracts the whole in the middle called the pupil, when the 

 light would otherwise be too strong for the eye ; and 

 the other of radial fibres, tending every where from the 

 circumference of the iris towards the middle of the pu- 

 pil ; which fibres, by their contraction, dilate and enlarge 

 the pupil when the light is weak, in order to let in the 

 more of its rays. The third coat is only a fine expan- 

 sion of the optic nerve L, which spreads like net-work 

 all over the inside of the choroides, and is therefore 

 called the retina ; upon which are painted (as it were) the 

 images of all visible objects by the rays of light which 

 either flow to, 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 

 m m and n n, which communicate by the pupil P, aru* 

 has the same limpidity, specific gravity, and refractive 

 power as water. At the back of this lies the chrystalliiie 

 humour 1 1, which is shaped like a double convex glass ; 

 land is a little more convex on the back than the fore- 

 part. It converges the rays, which pass through it from 

 every visible object to its focus at the bottom of the eye. 



