44-6 



HISTORY OF SCIENCE. 



observations relative to the " Mechanism of the Eye " (the title of the 

 Bakerian Lecture for 1800) may here be made. And as certain dis- 

 coveries relative to vision may also be referred to in subsequent pages, 

 the diagram in Fig. 199 will serve to illustrate not only Young's obser- 

 vations, but others to which allusion may be made hereafter. 



FIG. 199. VERTICAL SECTION OF THE EYE. 



Fig. 199 represents in section the eyeball, which is nearly globular 

 in form, its outer covering being affirm tough case. This covering, 

 called the sclerotic coat, A, is white and opaque, except in the front, 

 where it becomes transparent, and forms the cornea, B. The first refrac- 

 tion of the rays of light entering the eye occurs at the outer surface of 

 the cornea, but the two surfaces of this coat being nearly parallel, its 

 refractive power has little or no effect, and the cornea serves only to 

 give a proper form to the aqueous humour, D, which fills and distends 

 its concavity. The aqueous humour is partly divided by the iris. F 

 (this is the part which gives different colours to the eyes of different 

 individuals). This is pierced in the centre by the circular opening 

 called the pupil, and immediately behind this is the crystalline lens, 

 G E. The substance of the crystalline lens is much more refractive 



