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 diflferent 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 crys- 

 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 /rom 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- 



