M. F. Plateau orij he Vision of Fishes and Amphibia, 471 



vergence and thus compensate the slight refraction produced at 

 their entrance into the eye. 



As I shall show hereafter, the actual eye of Fishes closely 

 approaches our ideal type, so that we are entitled to conclude 

 theoretically that these animals can see distinctly in the air, and 

 that their distance of distinct vision must be nearly the same in 

 this medium and in water. Although Fishes, with the excep- 

 tion of some privileged species, such as the Eel, the Chironectes, 

 and the climbing Perch, have hardly any need for combining 

 the faculty of seeing distinctly in water with that of seeing dis- 

 tinctly in the air, this double faculty is evidently indispensable 

 to the Amphibia. 



It will be easily understood that if we suppose the eye of 

 these latter animals to be constructed exactly like that of animals 

 living exclusively in the air, their vision in water will be con- 

 fused. In fact, as I have already said, when once the eye is im- 

 mersed in water neither the cornea nor the aqueous humour has 

 any action, and the crystalline remains alone ; but, as in the sup- 

 position which we have just made its curvature would be slight, 

 it would no longer suffice to cause the rays to converge upon the 

 retina, or, in other words, its focus would be far behind this. 

 This, as is well known, is what happens in the eye of a man, for 

 example, when diving in the water. 



Have the Amphibia so great a power of adaptation as to render 

 their crystalline spherical? This appears, a priori, to be 

 doubtful. 



It is, on the other hand, very easy to assume that the eye of 

 the Amphibia is organized exactly, or very nearly, like that of 

 creatures living exclusively in water, since in that case the 

 distance at which the animal sees distinctly without effort of the 

 eye must be pretty nearly the same in water and in air. 



The purpose of my investigations is to show that the eye of 

 Fishes closely approaches our ideal type, and that that of the 

 Amphibia is almost exactly like it, and, finally, to prove experi- 

 mentally that distinct vision takes place at sensibly equal 

 distances in air and in water, and with the same perfection in 

 both media, in all the animals under consideration. 



I therefore, in the first place, examine what is the exact form 

 of the cornea in Fishes. By simple examination, by the reflec- 

 tion upon this membrane of a dark rectilinear object standing 

 out from a luminous ground, and the image of which, when the 

 eye is looked at from the side, is incurved by the curvature, and, 

 finally, by the actual measurement of the radius of this curve 

 upon a model of the eye taken immediately after the death of 

 the animal, I find that the cornea of Fishes, although rather 

 variable as regards its projection upon the surface of the head. 



