ffii: EYEi 97 



of the glass, in a perpendicular direction, as it approached it ; and, obey- 

 ing both forces, it took a new path, f, between the two forces, and more 

 towards a line, c, drawn perpendicular to the surface of the glass. This new 

 path it continued to pursue, until it had penetrated through the glass ; and 

 then, being about to quit the glass, it was once more acted upon by two forces ; 

 — this combined impulse in an oblique direction, and the attraction of the 

 lower surface of the glass in a perpendicular one ; — and as before, obeying 

 both, it again traversed a new path, e, between both forces, and in a direc- 

 tion from the perpendicular. 



The degree and kind of deviation from the original line will depend on 

 the difference in density between the air and the glass or water, or whatever 

 substance may be used, and likewise on the surface of the refracting body. 

 Passing through a transparent substance, with a plain and level surface 

 above and below, the rays will be bent out of their first direction, but will 

 continue parallel to each other. Passing through a concave glass (a glass 

 hollowed on one or both sides), the rays will diverge or separate ; and tra- 

 versing a convex one (rounded on one or both sides), they will converge 

 or approach each other, and tend to a point ; and the degree of convergence 

 or divergence will depend on the degree of convexity or concavity. 



Let us apply this to the mechanism of the eye of the horse (vide p. 89). 

 We have spoken of the cornea, f, and the aqueous humour, q, and the crys- 

 talline lens, g, and the vitreous humour, h ; but although possessed of dif- 

 ferent refractive powers, according to their form and density, (and the 

 cornea from its convexity, and the crystalline lens from its density, being 

 the principal agents,) they are so fitted to each other, that we may consider 

 them as composing one exceedingly convex lens, and of such power, that 

 the rays entering the pupil, m, are brought to a point within the very sub- 

 stance of the lens. 



The place of distinct vision, however, will not be at this point, but a little 

 way behind. If the glass of a spectacle, such as those generally worn by 

 old people, be held between a candle and a piece of paper, the rays of 

 light will converge by the convexity of the glass, and be brought to a very 

 small surface or point on the paper ; but on that point, there will be no 

 distinct picture of the candle, and the paper must be gradually removed 

 from the light, until a distance be found at which the image of the candle 

 will be seen most vivid and distinct, although inverted. So (see the cut, 

 p. 89) the retina which is spread over the internal coat of the eye is placed 

 at a little distance behind the point where the rays meet and cross. If the 

 eye be too convex, and its converging power too great, the rays will cross 

 too soon, and the image will be formed, brightest and best, before they 

 reach the retina, and the vision or sight will be imperfect and obscure. If 

 the eye is not sufficiently convex, and consequently does not possess con- 

 verging power enough, the rays will not cross until they are too near the 

 retina, and the picture would be most luminous and distinct behind 

 the retina; and thus, likewise, the sight would be imperfect and ob- 

 scure *. 



* " In considering vision as achieved by means of an image formed at the bottom of the 

 eye, we can never reflect without wonder on the smalhiess, yet correctness of the picture, 

 the subtilty of the touch, and the fineness of the hnes. A landscape of five or six square leagues 

 is brought into a space of half an inch in diameter ; yet the muUitude of objects which it 

 contains are all preserved, are all discriminated in their magnitudes, positions, figures, 

 colours. A stage coach passing at its ordinary speed, for several minutes, passes in the 

 eye only over one-twelfth of an inch, yet is the change of place in the image distinctly per- 

 ceived throughout its whole progress." — Paleys Natural Theology, p. 32. 



H 



