1893.] 



DE. G. L. JOHNSON ON THE SEAL's EYE. 



721 



like Ig in. focus' in air to see with under water. But this cannot be 

 made of glass, since the latter has nearly the same refractive index 

 as water. To meet this difficulty Dr. Dudgeon some years ago 

 had a pair of spectacles made consisting of portions of two hemi- 

 spheres of plain glass with parallel surfaces, mounted in a brass ring 

 with their convex surfaces nearly touching one another. • I have 

 brought the original pair to show you. In air they produce no 

 effect, neither magnifying nor reducing ; but in water the biconcave 

 air-lens, for such it is, becomes a biconvex one, the convex sur- 

 faces of the water in this case forming the lens. Dr. Dudgeon, who 

 has the credit of entirely solving this problem, made a niunber 

 of sketches under water, one of which I have reproduced. Were 

 it not for the distortion and astigmatism everything could be seen ; 

 even the date on a coin can be made out. 



Fig. 1. 



Fig. 2. 



TranBverse section of the Seal's eye ; natural size. (Diagrammatic.) 



.Fig. 1 shows the path of light-rays in the eye when uncorrected under water. 

 Fig. 2 shows the path of rays when the eye is corrected for air. 



In the Seal's case the problem is rather more complicated, 

 for the Seal has to see under four conditions. He has to see 

 out of water when out of water, in water when under water, out 

 of water when under the water, and in water when out of the 

 water. The first two conditions alone need be considered, as the 

 two latter follow by necessity. 



Now the Seal's eye is very much the same shape as our own, 

 excepting that the cornea is larger and more convex and the 

 anterior chamber very deep. This convexity of the cornea is very 

 curious, as one would expect to find it nearly flat as in the fish — 

 since the flatter the cornea and the nearer the lens to it, the less 



