eye-lashes; and likewise by the diffraction which the eyelashes, in consequence of their fineness, exert upon the luminous rays. 228. The instrument of vision, so wonderful in man, is far from being confined to him. In bestowing it upon the different orders of animals, nature has varied the structure in certain particulars, either by suppressing some parts which would be useless in the situations in which the subject is placed, or by adding others more suitable to its condition. At other times, she has pursued a plan entirely new, the reasons of which we are totally unable to explain. In connexion with this topic, I will cite a few general results extracted from the lectures of M. Cuvier, on comparative anatomy.

229. All red-blooded animals have eyes constructed on the general plan of the human eye, but with more or less considerable modifications.

Among quadrupeds, many have the pupil during the day, in the form of an oblong opening; but, during the night, the iris contracts and the pupil enlarges into a circular form, having a much greater aperture. This is to be observed in the ox, the horse, the cat. Consequently, these animals, being able to concentrate a proportionably larger number of luminous rays than man, can see much better in the dark.

The same is true of those birds which seek their prey in the night, as owls, &c. In general, the eye of birds occupies a considerable portion of the bulk of the head. It is distinguished, likewise, by several other important particulars. The crystalline, almost gelatinous, is much flatter than that of man, and to make amends for this the cornea is much more convex; it is also proportionably smaller; the reason of which undoubtedly is, to prevent the increase of the aberration of sphericity, that would be produced by an increase of its curvature. This small cornea, very convex, is adjusted to the extremity of a very short cone, which projects out before the Fig. 147. globe of the eye, like the tube of a small telescope. Here, as in man, we are ignorant how the eye is adapted to different distances. Yet what extent must this variation have in the eye of a bird of prey, which, singling out from its aerial height the small animal which is to be its victim, encloses it in the vast circles of its flight, and gradually approaching, at length pounces upon it with the rapidity of lightning, and transfixes it with its

talons and its eye, almost at the same instant. The interior of the eye of birds contains a peculiar organ, which may be destined for this purpose. It is represented in figure 148. It is a kind of black veil, formed by a membrane of the same nature as the choroid, and plaited like a fan, except that the folds, instead of radiating from the same centre, come from different points of a common stem, which is the continuation of the optic nerve itself, prolonged to the interior of the eye, and applied along its concavity. The direction of this membrane is oblique to the axis of the eye, and floats in the interior of the vitreous humour, extending nearly to the back part of the crystalline, to which it sometimes appears attached as by a thread, which connects the extremities of its folds. Naturalists have given it the name of comb. From its vascular structure, and the number of blood vessels which chequer it, may it not be designed, by its swelling and collapsing, to push out and draw in the posterior cavity of the eye, in such a manner as to change, by pressure, the curvature of the crystalline, or its distance from the retina, in order to accommodate it to the different distances of objects?

230. The eye of fishes, represented in figure 149, exhibits different peculiarities. Destined to live in a medium, the refracting power of which is nearly equal to that of the aqueous humour in the eye of man, this fluid would not produce any refraction. Accordingly its place is supplied by a viscid liquid, probably of a greater refracting power than pure water, the quantity being very small at the same time. By a necessary consequence, the pupil is extremely near the cornea, and this circumstance deprives it in part of the properties which it possesses in man, but which are the less necessary here on account of the small refraction which the rays experience in entering the humours of the eye from a watery medium. Moreover the pupil of fishes is not susceptible. of dilatation, and the iris is usually tinged on the outside with the most lively colours, although on the inside it is always black. The crystalline, applied almost immediately to the back part of the cornea, is very nearly spherical. We are absolutely ignorant of the reason why nature has given it this form; but it appears to be adapted to vision in the liquid medium in which fishes live; since, in most birds which dive in the water, we find the crystalline in like manner spherical. In other respects, it is composed of concentric laminæ like that of other animals.

231. Lastly, to confound our penetration, there are some animals whose eyes, more or less in number, are formed entirely of a transparent and lenticular cornea, behind which the nervous filaments expand. We have an example of this kind in insects, whose eyes are sometimes smooth and continuous; at others they appear to be cut with facets. But it appears on dissection, that, in this last case, each facet is a real eye which has its cornea and particular nervous filament, proceeding from one common stock. Accordingly, this last sort of eyes has been denominated compound, and the other smooth; it would be more natural to divide them into multiple and simple. Each is always fixed in insects, and makes visible only the objects which are brought of themselves within its field of view. It is impossible to conceive how such a system can produce distinct images; but still it is very evident that it serves the essential purposes of vision; for if we cover the eyes of insects with a black cnvelope, but of such a nature as not to injure the eyes, they become entirely blind, and cannot avoid any of the obstacles which are in their way. While there are some which have multiple eyes, there are, on the other hand, whole classes of animals which have no eyes at all; such are the moluscæ acephala, zoophites, and worms.

232.. We close this account by remarking that there are many animals which see when it is too dark for us to perceive any thing. There can be no doubt that cats and screech-owls discover their prey in the night, since it is then that they seek and secure it. Fishes, which live at the depth of two or three thousand feet in the sea, are in a still greater degree of darkness; for through the immense body of water that covers them, they can receive only the most feeble portion of light. Yet it is certain that they see their prey and pursue it with impetuosity and effect. These, accordingly, as well as cats and owls, have very large eyes. Is it not possible that, in these animals, vision is produced by those rays, which, to our eyes, are only rays of heat? From the relations of light and heat, it would seem that this supposition is not without probability.

Reflections, Refractions, and Colours of thin transparent Bodies.

233. In all the phenomena hitherto examined, the thickness of bodies which act upon light, whether in the case of refraction, dispersion, or reflection, has been considered as infinite, compared with the distance to which this action sensibly extends. The refraction of rays, for example, would have taken place under no larger angle, if we had used thicker prisms; and the reflection produced at the surfaces of metallic mirrors or reflecting glasses, would have been exactly the same, if we had increased their thickness. But when the bodies are reduced to very thin plates, the results are different; they reflect less light at the first surface, and at the second, they reflect or transmit certain colours in preference to others, according to their chemical nature, and their degree of tenuity. This phenomenon, exceedingly important in its consequences, was analysed by Newton, in a course of experiments which are now to be described.

When we take two prisms of polished glass, and place one of them upon the other without pressing them, the small plate of air, which naturally adheres to their surface, ordinarily possesses all the thickness necessary to its complete action upon light; for the phenomena of reflection and refraction through these two prisms and through this plate of air, obey exactly the laws which we have established in the preceding sections; and if we separate further the two contiguous surfaces, the increased thickness of the plate of air will not lead to a different result. But if we rub the two prisms against each other, and press them together with so much force as to exclude a part of the air which separates them, we shall at once perceive an adhesion, which is generally closer in some parts than in others, either because their surfaces are almost always slightly curved, or because the powerful pressure exerted, bends them. We thus obtain a plate of air thinner than the preceding, and whose thickness goes on increasing in all directions from the point where the surfaces applied touch each other or come nearest to touching, to the points where they are farthest apart. Then if we present the prisms to full daylight, and turn them in such manner, that the eye may receive the light partially reflected at the plate of air

which separates them, we shall perceive a greater or less number of coloured rings, which, when the prisms are sufficiently pressed, will surround a dark spot corresponding to the point of con

tact.

234. In order to observe well the order of these rings and the succession of their colours, it is necessary to employ prisms of a small angle, so that the light which traverses them to form the rings, may not experience a sensible dispersion before it arrives at the plate of air. It is well to place these prisms over a dark body, in order that no foreign light from external objects may mingle, by transmission, with that of the rings which we wish to examine. We should, therefore, take our position before Fig. 150. an open window, turning the edge B of the upper prism inward, and placing the eye above this prism, in such a manner as to receive only the light reflected from the inclined surface BC, and avoid that which is reflected from the upper surface AB. The most advantageous position is that in which rays reflected at the plate of air between the two pristns, traverses perpendicularly the surface AB; while the incident light RI, which is partially reflected at the same surface, is directed towards R', without coming to the eye of the observer, which we suppose so far elevated above the plane AB, as not to receive any of the rays which it reflects obliquely. Moreover, if the inferior prism is placed over a black body, as mentioned above, the rings will reflect the most vivid colours; and the central spot, entirely dark, will appear like a hole pierced through the middle of them. But we shall distinguish them more clearly still, if we dilate them, as we may do, by viewing them through a simple lens, or small. compound magnifier of a short focus.

The rings thus formed are not produced by the light reflected from the superior surface of the prisms, since this light does not come to the eye of the observer in the position we have supposed. Neither are they formed, for the same reason, at the inferior surface CA', of the second prism; moreover, we might blacken this surface with India ink, or with any other substance capable of absorbing the light, and the rings would not suffer any diminution. It necessarily follows, therefore, that they are formed by the reflection of light between the contiguous surfaces of the two prisms.