Considerations regarding the shining of the Eyes of the Cat, andseveral other Animals. By M. BENEDICT PREVOST.

EVERY body knows that the eyes of the cat shine in the dark.

Our domestic cats afford us so frequent opportunities of observing this phenomenon, that it seems peculiar to them; but there are several other animals which equally present it, and I have seen it in the dog, the sheep, the cow, the horse, the polecat, and even in several serpents, and in some insects, among others in the species of sphynx commonly known by the name of the Death's-head Moth.

Buffon says that "the eyes of the cat shine in the dark somewhat like diamonds, which throw out, during the night, the light with which they were in a manner impregnated during the day." Valmont de Bomare says, that "the pupil of the cat is during the night still deeply imbued with the light of the day," and some lines lower he adds, " the eyes of the cat are during the night so imbued with light, that they then appear very shining and luminous." Spallanzani says that "the eyes of cats, polecats, and several other animals, shine in the dark like two small tapers, and that this light is phosphoric." M. Dessaignes, in his memoir on phosphorescence, which was crowned by the Institute on the 5th April 1809, says that "the eyes of certain animals have the faculty of inflaming, and of appearing like a fire in the dark.

Thus the most eminent naturalists and philosophers are of one mind with the vulgar in regard to this fact, that the eyes of cats and some other animals shine in the dark with a light which is peculiar to them, or with which they have been impregnated during the day. I myself, also, was long in the habit of acquiescing in this opinion, taking the matter partly upon the authority of others, and partly observing the phenomenon for myself in the vague way in which every person sometimes observes things, and men of science as well as others; which would not, however, be productive of great inconveniences, were not more importance attached to the citing such observations than to the making them. "Every body," says Montaigne, "is subject to say foolish things; the misfortune is to say them curiously."

I am quite certain, that neither Buffon, nor Spallanzani, nor M. Dessaignes, ever observed on purpose the shining of the eyes of the cat, and that they never saw this phenomenon otherwise than cursorily, as one sees when he does not attend to a thing, or when one only partially attends to it; otherwise, they would immediately have perceived that the eyes of cats never shine in intense darkness, and that it is sufficient for them to shine, that too great a light does not prevent the pupil of the animal from dilating much; that, in reality, the phenomenon is only sensible to the observer, when his eye receives little light from surrounding objects.

The case, then, is the same with the eye which shines, as with the light which the pictures of a panorama reflect, and which appears to have all the intensity of that of the objects which they represent, although much inferior to it.

The less light the eye of the observer receives, the more is it sensible to that which the eye of the cat projects, and the less need has the latter of receiving any; it must receive more to produce an equal effect, if the former be situated in a lighter place. These are the conditions of the phenomenon. They appear to me to reduce this pretended phosphorescence to light reflected by a shining object. I shall give two examples, which I select from among the best adapted to render me understood.

1st, In a long and narrow passage, closed on all sides excepting the entrance, from which, during a very dark night, there could come but little light, I saw the eyes of a cat shine. They projected strongly upon the dark ground of a sort of deep nich, which made them appear like burning coals. The light which the eyes of the cat then received, and that which they sent back to me, was without doubt very weak; but to balance this, mine not being affected by any other light, would necessarily be very sensible to it. It was from a similar reason, that I once thought I saw from my bed something which shone like a star of second or third magnitude. It was nothing, however, but the back of a chair not very well smoothed, which reflected some rays of the moon; but having at the time my head almost entirely enveloped in my covering, and my eyes receiving no other light, these rays produced so much the more effect upon my retina, that they arrived the more isolated at it.

Ed, In the other example which I have to adduce, the circumstances were in some measure the reverse. It was in a room where the sun shone, but the head of the cat was turned toward one of the corners, and I looked at it myself in such a manner as not to receive either the direct rays of the sun, or the light directly reflected. Here the eye of the animal received much more light than in the other example, and transmitted more to me, but my eyes receiving more light from another direction, and being on this account less sensible to it, the eyes of the cat did not appear so shining.

Valmont de Bomare, in the article Chat of his Dictionary, (the edition in 15 vols.), says, after what we have already quoted, "it seems that the lustre, the splendour, which is observed in the day time in the eyes of the cat, comes from the shining part of the retina, at the place where it surrounds the optic nerve." This does not agree very well with what precedes; for in full day-light the retina of the cat is not visible, and if he means to speak of the lustre that is visible in a weak light, it is certainly of the same nature as that which is observed in darkness, and which Valmont de Bomare attributes to the imbibing of the light of day. Nor does this author speak here from his own observation: what he says of the eyes of the cat is taken almost word for word from Buffon's works, and from the first edition of the Encyclopedie. We also find in the Geneva edition of 39 vols. 4to, article Chat, the following words: " It appears that the lustre, the shining, the splendour, which are observed in the eyes of the cat, come from a sort of velvet, which lines the bot tom of the eye, or from the shining of the retina at the place where it surrounds the optic nerve." The phenomenon can be imitated with all its peculiarities, by placing bits of tinsel under suitable circumstances, or by other similar means. It is not therefore necessary to have recourse to phosphorescence for an explanation of it.

It is certain enough, that a great number of substances become luminous in the dark, after having been exposed some moments to the light of the sun, or only to the ordinary day-light, or to the light of a lamp, or of the moon. But it is not probable that the eyes of the cat are of this class; for, like those of other animals, they are filled with various humours; and there results from M.

Dessaigne's experiments, that neither the fluids nor the substances which have imbibed them manifest this property. 1,

Besides, as I have already insinuated, the eyes of the cat do not shine either in absolute darkness, or even in a very intense although imperfect darkness. A certain degree of light is always requisite, which may indeed be very feeble, but still quite perceptible. I have kept myself several times, thirty or forty minutes together, in dark places with cats, which mewed to each other, or devoured their prey in their usual grumbling manner, yet without their eyes manifesting any luminousness. I have caressed, provoked, tickled, pinched and frightened in the dark a very good natured cat, which has bitten and scratched me in frolic or in anger, but without its eyes having ever shone. Yet some instant before or after, the eyes of all these cats shone as usual, when they were suitably exposed to a certain degree of light. But what convinced me fully that the eyes which shine in the dark, owe this property only to the faculty of reflecting the light more strongly, is, that the eyes of all the animals that are susceptible of presenting this phenomenon, are evidently, and as appears to me exclusively, organized for this purpose.

It is known that "the inner layer of the choroid coat, which appears to be of a firmer texture than the rest of its thickness, and which bears the name of Ruyischian membrane, is lined in man and in several other animals, with a blackish, or even absolutely black and dull mucosity, which may be detached or wiped off with the finger or a pencil, and which serves to prevent the rays reflected by the internal walls of the eye from disturbing、 the vision. Now, the bottom of the Ruyischian membrane is only covered with a layer of that varnish through which its colour, which varies in a singular degree according to the species, is perceived. In man, and the monkey tribe, it is brown or blackish; in hares, rabbits, and hogs, of a chocolate brown; but the carnivora, the ruminantia, the pachydermata, the solipeda, and the cetacea, have bright and shining colours in this part. The ox has it of a beautiful gold green, changing into sky blue; the horse, the goat, the buffalo, the deer, of a silvery blue, changing into violet; the sheep of a pale gold green, sometimes bluish; the lion, the cat, the bear, and the dolphin, have it of a pale gold yellow; the dog, the wolf,

and the badger, of a pure white, edged with blue. This coloured part of the Ruyischian membrane is named the Tapis. Birds have it not

We see from this description, that the eyes of animals which do not shine like those of cats, have no tapis, or have it only of a dark colour. The eye of man does not shine, or only shines in a very slight degree †. I have often tried to ascertain whether it does, and in the most favourable circumstances have only at the most perceived an extremely feeble and doubtful light. I have never seen the eyes of hogs, rabbits, or hares, which have the tapis of a chocolate brown, emitting light; while I have very frequently seen the eyes of sheep, oxen, and horses, sparkle with the most lively colours. I have often had nocturnal birds at my disposal, and have often observed them, but without ever seeing their eyes shine like those of cats; and Spallanzani, who made numerous experiments upon these animals, and who examined them with reference to this subject, by night, by day, and during the twilight, both captive and in a state of liberty, never remarked either that their eyes were susceptible of shining in the dark, with that sort of lustre which he imagined to be phosphoric. It is true, that the tapis of the dog does not agree with the colour of the shining of its eyes; but this colour may be modified by that of the crystalline humour, or of some of the other humours of the

eye.

It is astonishing that M. Cuvier, after this description, does not say a word of the phenomenon of which the tapis appears to me to be the cause; but this celebrated anatomist, whose genius knows to subject itself to the laws of a rigorous accuracy, not having probably observed it himself with sufficient care, has rather chosen to say nothing, than to repeat the opinions of others respecting a subject which, at bottom, belongs much less to anatomy than to Natural History.

M. Dessaignes not only says that the eyes of certain animals kindle and appear as if on fire in the dark; but, according to him, they owe this faculty to the expansive effect of the lively passions with which the animal is affected. But he is certainly deceived,

Cuvier's Legons. d'Anat. Comp. t. ii. p. 397, 402.

+ This luminous property we have remarked in eyes of several individuals, principally females.-ED