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

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 Leçons. d'Anat. Comp. t. ii. p. 397, 402.

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

or at least this is subject to numerous exceptions. Besides what I have said of the cat in which I certainly excited lively passions, and whose eyes yet gave no sign of luminosity, it is easy to prove directly that the phenomenon may take place independently of the passions; for the animals whose eyes shine in the dark do not lose this property with life, and are susceptible of it even long after they are dead. I have seen two polecats that had been dead fifteen or twenty hours, whose eyes shone nearly like those of living cats. I have remarked the same thing in serpents and insects. I have also seen the eyes shine in some collared snakes which I extracted from the egg a considerable time before the period when they would naturally have come forth. There was no appearance then of their being susceptible of lively passions. It may be added that the animals whose eyes shine most, are often very tranquil at the moment when the phenomenon is most striking.

It was not enough to consider the shining of the eyes as phosphoric; it has also been pretended that it serves as a light to the animals which possess it, and that it assists them in seeing and guiding themselves in the dark. But the place which the reflectors occupy is reasonably a matter of astonishment, for it is not the light which proceeds from the eye to an object that enables the eye to perceive that object, but the light which arrives in the eye from it.

Spallanzani thought that cats, polecats, and some other animals, move with promptitude and certainty in a medium totally deprived of light, and this is also a subject of pretty general belief. I cannot help doubting it however. But should this really be the case, it ought not to be attributed to the shining of their eyes, since this aid, as we have seen, fails them when they have most need of it. Animals in the state of nature are never placed in such circumstances. Nor is it even probable that such an occurrence takes place in a state of domesticity. In whatever part they may happen to be, there is always a little light, and in order to see, they only require to have their pupil susceptible of great dilatation, and their retina of an extreme sensibility. It is said that a man shut up for a long time in a very dark dungeon becomes at length able to read. The nocturnal birds which Spallanzani reared, saw very well in a place in which he

1

himself could distinguish no object, and he admits that the eyes of these birds do not shine in the dark. Besides sheep, cows, horses, and several other animals which have the eyes shining, would no doubt find themselves much embarrassed in absolute darkness. If some quadrupeds, in fact, move with promptitude and security in complete darkness, it is certainly not to their eyes that they are indebted for it, but to some other sense. The bats in which Spallanzani discovered this faculty, owe it, according to him, to a sixth sense, of which we have no idea; and, according to Cuvier, to the extent of the membrane which their wing presents to the air, and which renders it capable of feeling its resistance, motion and temperature.

It is true that the animals whose eyes shine in the dark are all of the number of those whose motions the night rather favours than impedes, when its shades are not too thick, and although several others which feed, take their diversion, or provide for their subsistence, during the night, have not the eyes shining, one is yet tempted to search the cause of the agreement or concurrence of these two circumstances, which we observe so frequently to take plaće.

The light does not act upon the retina by impulsion, as some physiologists seem to think; its action, although its nature is not very well known, appears to be purely chemical; and the sensibility of the eye to the light, being on this account susceptible of a sort of saturation, it was necessary, in order to let it have all the delicacy which it would require to serve the animal in profound darkness, either to take care that the eye should receive but very little light during the day, or that this light, at least what was superabundant, should be immediately sent off by some reflector, which would not allow it to enter into combination. If, on the contrary, it were useful for the cat, that its eye should be filled with light in the night-time, nature would take care to prevent it from entering the light during the day, or provide that the little which its Ruyischian membrane might receive through a contracted pupil, should be instantly thrown out.

To conclude, the preceding observations seem to me sufficiently to prove, 1st, That the shining of the eyes of the cat and of other animals, which present the same phenomenon, does not arise from a phosphoric light, but only from a reflected light;

that, consequently, 2d, It is not by an effect of the will of the animal or by that of certain passions, that this light emanates from its eyes; 3d, That this shining does not manifest itself in absolute or too profound darkness; 4th, That it cannot enable the animal to move with security in the dark.-Biblioth. Britannique, T. 45.

Remarks on the Rhubarb of Commerce, the Purple-coned Fir of Nepal, and the Mustard Tree. By Mr DAVID DON, Librarian of the Linnean Society, Member of the Imperial Academy Naturæ Curiosorum, of the Wernerian Society, &c. Communicated by the Author.

1. On the Rhubarb of Commerce.

IT is well known that the plant which yields the rhubarb of commerce has been hitherto involved in much obscurity, and hence there have arisen many discordant opinions, both among botanists and pharmacologists, respecting the species of Rheum which affords this valuable medicinal root. They judged it rightly to be the produce of a species of Rheum, but of what particular species, without authentic materials it was impossible for them to decide. Linnæus considered it at first as the produce of his Rheum rhabarbarum or undulatum, but he afterwards appears to have altered his opinion in favour of Rheum palmatum; while Pallas, who certainly had better opportunities of gaining correct information on the subject, regarded it as composed chiefly of the roots of Rheum undulatum and compactum. Mr Sievers, an enterprising assistant of Professor Pallas, and well known by his interesting Letters on Siberia, published in the Nordische Beyträge, was sent by the Empress Catharine II. purposely to try to obtain the true rhubarb plant from its native country; and although, after travelling for seven years in the countries adjacent to that in which it is found, he was unable to effect the object of his mission, yet he obtained sufficient information to convince him that the plant was then unknown to botanists. But it was reserved for Dr Wallich, the zealous superintendent of the Calcutta Botanic Garden, to set this long agitated question at rest, by the transmission of seeds and dried specimens of the true rhubarb plant