470 M.F. Plateau on the Vision of Fishes and Amphibia. 
vitreous humours of the same density as water and in small 
quantity. Let us place this eye successively in water and in the 
air, and examine what will be the course of the rays traversing 
the organ in these two different media. In water, whatever be 
the form of the cornea, as the aqueous and vitreous humours 
have, by hypothesis, the same density as this fluid, the cornea 
will play the part of a transparent lamina with parallel faces 
bathed with water on both sides; it will therefore by no means 
serve to render the luminous rays convergent or less divergent, 
and the crystalline alone will remain to combine in one point 
upon the retina the rays of each bundle. It must therefore be 
very convex and of relatively considerable density. 
Will this eye, organized for distinct vision in water, be un- 
fitted for distinct vision in the air? By no means. Let us 
suppose, in the first place, a bundle of parallel rays falling upon 
the anterior surface of the eye; these rays will arrive at the erys- 
talline retaining their parallelism, as the two surfaces of the cornea 
are flat and parallel, and it will be seen that, both in air and water, 
it is solely to the crystalline that is deputed the function of 
picturing the image at the bottom of the globe of the eye. 
Let us suppose, further, that the axis of the eye is of the proper 
length for the vision of objects sufficiently distant to allow the 
rays composing each bundle to be regarded as parallel. A fish 
furnished with a visual apparatus constructed on the above plan 
would see as distinctly in air as in water objects situated at a 
great distance, of course assuming the water to be of perfect 
transparency. 
Let us now examine the case of near objects. Although Fishes 
in general have very large eyes, the orifice of the pupil never 
presents a very great diameter; hence, supposing the object 
looked at to be near the eye (a few centimetres from it for 
example), the cones of rays emanating from each point of this 
object would still present a very small base in comparison to their 
length, and the rays constituting them will form but very small 
angles with the axes of these same cones. From this it follows 
that, even if we ascribe to the axis of our typical eye a length 
corresponding with the distinct vision of objects at a few centi- 
metres’ distance, this vision will still be as distinct in air as in 
water, except that the distance of the object must be rather less 
in the air. There, in fact, the slight divergence of the rays 
emanating from one point of the object will be necessarily some- 
what diminished as they penetrate into the aqueous humour, and 
consequently, after their refraction by the crystalline, they will 
converge at a point somewhat nearer the cornea than if the object 
were in water. It will therefore be necessary to diminish slightly 
the distance of the object in order to give the rays a greater di- 
