﻿350 Dr. It. E. Dudgeon on Subaqueous Vision. 



laws which govern the expansion, the contraction, the compres- 

 sion or the rarefaction of gases, and hold true wherever they 

 may be properly applied ; so that, descending from great to 

 small, and from regular to irregular, we may fairly argue that 

 agencies which we can show regularly produce certain results on 

 the largest scales are capable also of producing similar effects, 

 even when the scale is limited or their mode of action constrained 

 or distorted. 



[To be continued.] 



XLV. On Subaqueous Vision, By R. E. Dudgeon, M.D* 



THE dioptrical apparatus of the human eye consists mainly 

 of two transparent media : — (1) the aqueous and vitreous 

 humours with nearly the same index of refraction, which differs 

 inappreciably from that of water; (2) the crystalline lens, 

 the mean index of refraction of which is variously stated to be 

 from 1*383 to 1*455. We may leave out of consideration the 

 cornea and the capsule of the crystalline lens, as, though the in- 

 dices of refraction of these membranes differ from those of the 

 other dioptrical parts of the eye, their respective surfaces being- 

 parallel prevents them having any material influence on the 

 passage of rays of light to the retina. 



The radius of curvature of the cornea, which bounds the ante- 

 rior surface of the aqueous humour, is (according to Donders, 

 ' Accommodation and Refraction of the Eye/ p. 67) 8 millims. 

 (=•32 inch). The radius of curvature of the anterior surface 

 of the crystalline lens (which forms the posterior concave surface 

 of the aqueous humour) is, according to the same authority, 10 

 millims. ( = '4 inch). The shape of the aqueous humour is con- 

 sequently a meniscus ; and as its thickness is 3 millims. ( = *12 

 inch) and its index of refraction 1*336, its absolute principal 

 focus, ascertained by Sir D. Brewster's rule (Optics, p. 23), 

 will be 46 millims. ( = 1*8 inch). 



The shape of the crystalline lens is a double-convex lens of 

 unequal convexities, the anterior surface (when accommodated for 

 distance, i. e. when the accommodation is at rest) having a radius 

 of curvature of 10 millims. ( = *4 inch), the posterior one of 6 

 millims. ( = *24 inch). The thickness of the lens being 5 mil- 

 lims. ( = *2 inch), and its mean index of refraction (taking Sir 

 D. Brewster's calculation) 1*383, its principal focus in air will 

 be 6 millims. (-= *24 inch) . But as it is suspended in a medium 

 whose index of refraction is 1*336, the sine of the angle of inci- 

 dence of the rays of light is to the sine of the angle of refraction 



* Communicated bv the Author. 



