Jeffries.] 226 [December 16, 



density, water and the aqueous humor. The lens alone being denser, 

 produces the necessary refraction, hence its very spherical shape to 

 refract parallel or diverging rays to a focus on the retina. Out of the 

 water this e3-e wouM converge parallel or diverging rays to a focus 

 before they reached the retina, so that the latter would only receive a 

 circle of dispei-sion, instead of a perfect picture, were it not that, as 

 M. Plateau has shown, the coi'nea is flattened at its centre over the 

 small pupil of these animals. A large number of fishes were exam- 

 ined, and this was found to be invariably the case. His method of as- 

 certaining this was first to observe a reflected image from the cornea, 

 and more positively by removing the globe, carefully dissecting the 

 muscles, fat, etc., and placing the eye so as to bring the plane of the 

 iris vertical, pour round It thin plaster of Paris a little more than 

 half the depth of the globe, thus obtaining a cast of the meridian 

 curve of the cornea and sclerotic. He obtained such results as are 

 shown by these diagrams, the cornea being always flattened in its 

 central portion. An eye thus constructed would see nearly at the 

 same distance out of, as In the water. A very few of the fishes, how- 

 ever, leave the water, either in search of food, or in migration. 

 But there are a large number of amphibians who have a double exist- 

 ence, and seek their food in the water. When in this element, all 

 refi'active power.of the cornea, as in man, is eliminated; do they then 

 possess any power of accommodation by which they can render their 

 crystalline lens spherical? None such has apparently yet been shown. 

 If, however, their eyes are like those of fishes, then they will see 

 equally Avell under as out of the water. The comparative anatomists 

 who have examined the eyes of these animals, all report a large and 

 spherical crystalline lens, the purpose of which was well understood, 

 but apparently simply fi-oni lack of attention being called to it, the 

 exact shape of the cornea was undetermined, it being assumed to be 

 convex because the outer portion of it evidently was. It must of 

 course be admitted that there are no true amphibians excej^t a few 

 batrachians. Yet amongst nearly all classes of the animal king- 

 dom there are found species with respiratory apparatus adapted to life 

 in air, Avhich by slight modification of this and of the circulation, are 

 enabled to live a considerable time under the water, and in the major- 

 ity of cases seek their prey there. Now the presence of the ciliary 

 muscle, the principal seat of the power of accommodation, has been 

 determined in these animals, showing their power of vision in the air 

 at varying distances. Plateau quotes authority to show the resem- 

 blance to the typical fish's eye amongst mammals in the seal, whale, 



