Vision and Optical Glasses. 179 



considered as a convex lens, constructed with 

 such admirable skill by the great Author of 

 Nature, that the rays converge to a point exactly 

 in the proper place ; so that if the humours were 

 otherwise disposed, even to the breadth of a horse- 

 hair, the effect would be totally destroyed. But 

 you will understand the subject better, by con- 

 sidering the structure of this curious organ ; in 

 describing which, I shall adopt the simple, but 

 expressive language of Mr. Ferguson. 



The eye is nearly of a globular form. It con- 

 sists of three coats and three humours. (See 

 fig. 66.) The part DHHG of the outer coat is 

 called the sclerotica ; the rest, D E F G, the 

 cornea. Next within this coat, is the choroides, 

 which serves for a lining to the other, and joins 

 with the iris mn, mn. The iris is that coloured 

 circle which gives the character, as to colour, to 

 the eye, and is composed of two sets of muscular 

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

 tracts the hole 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 pupil ; which fibres, 

 by their contraction, dilate and enlarge the pupil 

 when the light is weak, in order to let in more of 

 its rays. The third coat is only a fine expansion 

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

 all over the inside of the choroides, and is there- 

 fore called the retina ; upon which are painted 



