M. F. Plateau on"the 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 Hel, 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, @ 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, 
