366 VISION. LECT. XX. 



points A and B of the object, transverse the transparent cor- 

 nea c c, and enter the aqueous humour contained between 

 the cornea and the surface of the crystalline lens: they 

 thus undergo a first refraction, bending towards the rays 

 which enter parallel to the axis of the eye. It is easy to 

 calculate the convergence of these rays, when the convexity 

 of the cornea, and the refracting power of the aqueous 

 humour, have been ascertained. These rays arrive at the 

 crystalline, which is a real double convex lens, and suffer 

 here a further deviation and inclination towards the axis of 

 the eye. Lastly, they undergo a third refraction in the 

 same direction, when passing out of the crystalline lens, 

 and at the moment of their entrance into the vitreous hu- 

 mour. The route followed by the rays emanating from A 

 and B is indicated in figure 32: their focus is at a and b: and, if 

 F be the retina, a and b correspond to points A and B 

 of the object. If we suppose the retina at H or at G, the 

 points c and f, as well as e and o, will be the circles in 

 which the image of A and B will be diffused. Vision, 

 therefore, is distinct when the retina is exactly at such a 

 distance from the crystalline lens, that the focus of the rays 

 is formed upon it. But in order to obtain this result, it is 

 necessary to intercept all the rays which would fall near 

 the margin of the crystalline, and which would have their 

 focus at a different point from that of the rays which tra- 

 verse the central part of the lens. This is the most im- 

 portant function of the iris and the pupil, which produce 

 the effect of a diaphragm, provided with an opening sus- 

 ceptible of great variations in its diameter. For the same 

 purpose, the crystalline should be denser and more convex, 

 in proportion as the medium in which vision is effected is 

 denser ; as is the case with fishes. 



Lastly, the whole internal part of the eye, and espe- 

 cially the posterior surface of the retina, is covered with a 



