236 



SENSE OF SIGHT. 



If we suppose a luminous cone to proceed from a radiant point B, 

 Fig. 113, directly in the prolongation of the antero-posterior diameter 

 of the eye, the axis of this cone will also be the axis of the organ ; so 

 that a ray of light, impinging upon the humours in the direction of the 

 axis, as in the case of the lenses previously referred to, will pass through 

 the humours without undergoing deflection, and will fall upon the retina 

 at b. This, however, is not the case with the other rays composing the 

 cone. They do not fall perpendicularly upon the cornea ; and are, 

 consequently, variously refracted in their passage through the cornea, 

 aqueous humour, crystalline, and vitreous humour ; but in such a man- 

 ner that they join their axis in a focus at a point where it strikes the 

 retina. 



The transparent parts of the eye, as has been seen, are of different 

 densities, and consequently possessed of different refractive powers. 

 These powers it has been attempted to estimate ; and the following is 

 the result of the somewhat discordant evaluations of different experi- 

 menters; the power of air being 1-000295. 



A ray of light impinging obliquely on the surface of the transparent 

 cornea passes from a rarer to a denser medium. It will, consequently, 

 be refracted towards a perpendicular raised from the point of impact. 

 From this cause, as well as from the convexity of the cornea, it will 

 be rendered more convergent ; or, in other words, approach the axis of 

 the cone. In proceeding through the aqueous humour, little variation 

 will be produced, as the densities of it and the cornea differ but little ; 

 the latter is slightly more refractive, according to the table ; and there- 

 fore the tendency will be to render the ray less convergent. This 

 convergence gives occasion to the passage of a greater number of rays 

 through the pupil ; and necessarily adds to the intensity of the light 

 that impinges on the crystalline. Pursuing the ray through the two 

 chambers of the eye, we find it next impinging on the surface of the 

 crystalline, which possesses a much higher refractive power than the 

 cornea or aqueous humour; in the ratio of 1-384 to 1-336. From this 

 cause, and from the convexity of the anterior surface of the lens, the 

 ray is rendered still more convergent or approaches still more the axis 

 of the cone. It is probable, however, that even here some of the light 

 is reflected back ; and goes towards the formation of the image in the 



1 For the measurements of M. Vallee, see his Theorie de 1'CEil, p. 20, Paris, 1843; or 

 Longet, Traite de Physiologic, ii. 42, Paris, 1850. 



