THE EYE. giji 



attraction of the glass, in a perpendicular direction, as it approached it j 

 and, obeying 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 

 tlie 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 direction 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 what- 

 ever 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 traversing 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 

 crystalline lens, g, and the vitreous humour, h; but although possessed of 

 different 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, thei'e 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 

 converging 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 obscure.* 



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

 the eye, we can never reflect without wonder on the smaUness, yet correctness of tlie 

 picture, the subtilty of the touch, and the fineness of the lines. A landscape of five or 

 six square legues is brought into a space of half an inch in diameter; yet the multitude 

 of objects which it contains arc all preserved, are all discriminated in their rnaarnitudes, 

 positions, figures, colours. A stage-coach passing at its ordinary speed, for several min- 

 «tee, passes in the eye only over one-twelfth of an inch, yet is the change of place in the 

 linage distinctly perceived throughout its whole progresa."— "Pa/ey'« Nat. Theology, p. 32. 



