﻿Dr. R. E. Dudgeon on Subaqueous Vision. 351 



as 1*383 to 1*336, and its focus in this situation will therefore 

 be 24 millims. (nearly 1 inch). As, however, it receives the 

 rays of light already deflected by the more convex anterior 

 aqueous meniscus, its actual focus in the eye is 18 millims. 

 ( = *7 inch), or precisely the distance of the posterior surface of 

 the crystalline lens from the retina in the axis of vision. 



Listing (Dioptrik des Auges in Wagner's Handwbrterbuch der 

 Physiologie, quoted by Donders loc. cit.) makes the mean index 

 of refraction of the crystalline 1*455 ; and this index Donders ac- 

 cepts as the true one ; but a simple measurement according to Sir 

 D. Brewster's rule will show this to be too high an estimate; 

 and Donders himself (p. 62) admits that it may be too high. 



In the foregoing calculations I have assumed the surfaces of 

 the cornea and crystalline lens to be spherical ; but their actual 

 shapes, as determined by Chossat by careful measurements of 

 the eyes of oxen (Annates de Chimie et de Physique, vol. x.), are 

 not quite spherical. That of the cornea is an ellipsoid of revo- 

 lution round the major axis, whilst those of the crystalline lens 

 are ellipsoids of revolution round the minor axis of the ellipse. 

 The effect of these shapes will be to make the central portion 

 of the cornea more convex, and the central parts of the surfaces 

 of the crystalline less convex than if they were segments of 

 spheres. But for all practical purposes they may be regarded 

 as spherical. 



When the eye is immersed in fresh water, which has the 

 same index of refraction as the aqueous humour, the latter ceases 

 to refract the rays of light transmitted to it through the water, and 

 the crystalline becomes the only refracting medium in the dioptric 

 system of the eye. But, as has been stated, the crystalline sus- 

 pended in a watery medium has a focus of 24 millims. (nearly 

 1 inch), consequently 6 millims. ( = *24 inch) beyond the retina, 

 and distinct vision is destroyed. 



Obviously the restoration of distinct vision below water can 

 only be effected by supplying the loss of the anterior aqueous 

 meniscus of the eye by means of a lens of equal power. We 

 have found the aqueous humour to be a lens whose principal 

 focus is 46 millims. ( = 1*8 inch) ; so all we have to do is to 

 place before our immersed eye a lens having a similar focus in 

 water. A glass lens which has such a focus in air has a focus 

 of upwards of 7 inches in water, because the sine of the angle 

 of incidence of rays of light on a glass lens in water is to the 

 sine of the angle of refraction as 1*500 to 1*336, in place of being 

 as in air 1*500 to 1. So if we use glass we must take a lens 

 whose focus is only *45 inch in air, which will have a focus of 

 1*8 inch in water, and thus supply the place of the lost lens of 

 the aqueous humour. Practically we find this is so ; and with 



