472 M.F. Plateau on the Vision of Fishes and Amphibia. 
is always flat, or at least much flattened in front of the crys- 
talline and over a space equal to the diameter of that lens, whilst 
the lateral portions may be much curved. As to the crystalline, 
I have always found it very nearly a sphere, as, indeed, is well 
known. Lastly, Cuvierand Monrolong since proved that in Fishes 
the humours of the eye may be assimilated to water. In other 
words, I prove, by a sufficient number of measurements detailed 
in one of the tables of my memoir, that the eye of Fishes is 
always constructed evidently on the plan of the ideal type which 
I have described above, even in those species which some authors 
have indicated as exceptional. 
I subject the eyes of the Amphibia (that is to say, of the 
animals which must make use of their organs of vision indif- 
ferently in water and in air) to the same investigations, and I show 
that in all, Mammalia, Birds, Reptiles, Batrachia, &c., the eyes, 
with the exception of some slight differences, exhibit a structure 
identical with that of these organs in Fishes. As regards 
Insects, whether terrestrial, aquatic, or amphibious, they all, 
according to modern researches, possess eyes with flattened 
corneze and the crystallines very convex, at least on the inner 
surface. Here the anatomical portion of my investigation ter- 
minates ; and then follows the experimental part, in which I de- 
termined the distances of distinct vision of ten species of 
Fishes of different genera, and of some Batrachia, in air and 
in water. 
The method which I have employed is, in few words, as follows, 
supposing we have to do with a fish, and the process is the samewith 
other animals. After rapidly killing the individual to be experi- 
mented on, one eye is carefully removed from its orbit without 
alteration of its shape, and then fixed by means of pins and of 
the fragments of the conjunctiva and muscles upon a plate of 
cork, in such a manner that the cornea shall be vertical. A suit- 
able aperture is then made at the bottom of the eye, by removing 
with fine scissors a portion of the sclerotic and retina, and in 
this aperture is fixed a small glass cup slightly roughened, upon 
which the image of an external object may be depicted as upon 
an artificial retina. The object employed is the extremity of a 
fine iron wire thrown out by the flame of a lamp. The experi- 
ment must of course be made at night, or in a room with closed 
shutters, and the image of the iron wire on the hinder part of 
the eye is observed by means of a lens. By varying the distance 
from the iron wire to the cornea, we may always at last attain a 
position in which the image is distinct. The experiment is re- 
peated a certain number of times to give a mean, and is per- 
formed successively in air and water ; in the latter case, of course, 
the iron wire is also immersed in the water. I need hardly add 
