Графични страници
PDF файл
ePub

determine to what extent the remoter parts of the eye

contributed to its luminousness.

I attempted to perform the above operation on a cat, but the utter restlessness of the animal rendered it extremely difficult, indeed almost impossible. Having ascertained that eyes of cats shine after death, I resolved to kill the cat, that I might have it in my power to dissect any part of the eye I thought proper.

First, by means of a pair of scissors I cut away the whole of the cornea, and completely destroyed the anterior chamber of the eye. I now observed, that the light of the eye was not in the least diminished, but somewhat weakened in regard to colour, which was changed from a yellow to a pale green. I then took away the iris, that lay exposed before me, without injuring the conformity of the hinder part of the eye, to discover whether the iris, as Treviranus maintained, really contributed to the light. This, however, was not the case ; for the light still continued. · The taking away of the lens was followed by a different result, which considerably weakened the intensity of the light, and the greenness of its colour. It now struck me that the tapetum in the hinder part of the eye must form a spot, which caused the reflection of the incident rays of light, and thus produced the shining. This was the more probable, as the light of the eye now seemed to emanate from a single spot. After taking away the vitreous humour, I observed, that, in reality, the entire want of the pigment in the hinder part of the choroid coat, where the optic nerve enters, formed a greenish silver coloured changeable oblong spot, which was not symmetrical, but surrounded the optic nerve in such a manner, that the greater part was above, and only a small part below it; and, therefore, the greater part lay beyond the axis of vision. It is this spot, therefore, that produces the reflection of the incident rays of light, and, beyond all doubt, according to its tint, contributes to the different colouring of the light, to which, nevertheless, the remaining parts of the eye, when conjoined, seem to be no less necessary.

The situation of this spot corresponds exactly with the position in which the shining of the eyes is seen to the greatest advantage. I have before remarked, that the shining is perceptible only in a certain position, and, in fact, when the eyes of the observer are almost opposite to the eyes of the animal on which he is performing the experiment. This is easily explained. Only those rays of light are reflected which fall on that part of the choroid where the pigmentum is wanting ; but as this spot occupies rather the upper wall of the concavity of the choroid, the reflection caused by it will not be perceived, if the eye of the observer is not in a nearly straight direction to the eye of the animal, and at some distance; and hence it is why, in living cats, we observe the light only when their eyes are directed to wards our own; in which case, the upper wall of the eye-ball becomes more the hinder and under, and the point of reflection stands in almost a straight line with our own eyes.

erroneous.

From these experiments, it is abundantly evident that there is no light or shining in the eyes in places absolutely dark, and that the opinion of many authors is, in this respect, completely

These experiments, at the same time, prove what has been doubted by some physiologists, the transparency of the retina ; for it must naturally be transparent, if reflection takes place from behind it. The transparency of the retina may also be proved from our seeing the image upon the choroid, or rather upon its pigment, while the retina has not the least share in producing the effect; since it appears when the retina on being taken away, brings the vitreous humour, or the lens, to the coats of the eye. 'I remarked above, that the light of the eyes of animals was stronger when they were irritated than when they were in a quiescent state ; and I attributed this phenomenon to the greater projection of the eyes, but particularly to the increased secretion of the lachrymal fluids. This was rendered still more probable by my last experiment, when I destroyed the convexity of the eye, by taking away the cornea and the lens. By this it appears, that a shining substance is better fitted for reflection than a dull one, which is proved by the gradual fading away of the light after death, from the cornea becoming duller, and by the gradual increase of light, when the cornea is moistened. I further remarked, that the different colours of animals, particularly of the cat, probably tend to strengthen or weaken the light; which may be thus explained, that, in beasts, as well as human beings, the greater or smaller size of the pigment may usually be in conformity with the colour of the hair, which is the common covering.

From this examination, it will now be more probable that the luminousness of the eyes of human beings, as well as of beasts, depends on the want of the pigment, and so much the more from being observed only in the albino. With this view of the matter, the two cases already quoted of Sachs and Michaelis are indeed at variance. I must confess that I have read and considered these cases with some degree of interest. Are they really fictions ? When we read of the shape of fiery coruscations, or balls in the eyes, of their rolling round, of their frequently darting forth rays an inch long, our suspicions are surely pardonable.

As to the different colours of the light in the eyes of dogs, it is owing to the different colouring of the place where the pigment is awanting in the choroid,-a fact of which anatomical experiments on the eye of these animals has convinced me; and hence the varied colour of the light of one and the same eye may be owing more to the motion of that part where the rays of light are reflected upon different coloured portions of the choroid, than to the quantity of the incident rays of light.

Finally, there is no question but the light observed in the eyes of some beasts of prey, as well as in those of birds, has the origin above ascribed to it; and its nature is neither phosphoric nor electrical, nor has it any psychological relation.

Account of the Habits of the Turkey Buzzard (Vultur aura),

particularly with the view of exploding the opinion generally entertained of its extraordinary power of Smelling. In a letter to Professor JAMESON, by John J. AUDUBON, a Citizen

of the United States *. As soon as,

me, you shall have seen the Turkey Buzzard follow, with arduous closeness of investigation, the skirts of the forests, the meanders of creeks and rivers, sweeping over the whole of extensive plains, glancing his quick eye in all directions, with as much intentness as ever did the noblest of falcons, to discover where below him lies the suitable prey ;-when, like

like

* This communication was originally intended to be sent to a friend unacquainted with the habits of birds. J. J. A.

eye,-then will

me, you have repeatedly seen that bird pass over objects calculated to glut his voracious appetite unnoticed, because unseen ; and when you have also observed the greedy vulture propelled by hunger, if not famine, moving like the wind suddenly round his course as the carrion attracts his

you

abandon the deeply-rooted notion that this bird possesses the faculty of discovering, by his sense of smell, his prey at an immense distance.

This power of smelling so acutely I adopted as a fact from my youth. I had read of this when a child ; and many of the theorists to whom I subsequently spoke of it, repeated the same with enthusiasm, the more particularly as they considered it an extraordinary gift of nature. But I had already observed, that Nature, although wonderfully bountiful, had not granted more to any one individual than was necessary, and that no one was possessed of any two of the senses in a very high state of perfection; that if it had a good scent, it needed not so much acuteness of sight, and vice versa. When I visited the Southern States, and had lived, as it were, amongst these vultures for several years, and discovered thousands of times that they did not smell me when I approached them covered by a tree, until within a few feet, and that when so near, or at a greater distance, I shewed myself to them, they instantly flew away much frightened, the idea evaporated, and I assiduously engaged in a series of experiments to prove, to myself at least, how far this acuteness of smell existed, or if it existed at all.

I sit down to communicate to you the results of those experiments, and leave for you to conclude how far, and how long, the world has been imposed on by the mere assertions of men who had never seen more than the skins of our vultures, or heard the accounts from men caring little about observing nature closely.

My first experiment was as follows:

I procured a skin of our common deer, entire to the hoofs, and stuffed it carefully with dried grass until filled rather above the natural size,—suffered the whole to become perfectly dry, and as hard as leather,—took it to the middle of a large open field, laid it down on its back with the legs up and apart, as if the animal

He ap

was dead and putrid. I then retired about a few hundred yards, and, in the lapse of some minutes, a vulture, coursing round the field, tolerably high, espied the skin, sailed directly towards it, and alighted within a few yards of it. I ran immediately, covered by a large tree, until within about forty yards, and from that place could spy the bird with ease. proached the skin-looked at it without apparent suspicionjumped on it-raised his tail, and voided itself freely (as, you well know, all birds of prey in a wild state generally do before feeding),then approaching the eyes, that were here solid globes of hard dried and painted clay, attacked first one and then the other, with, however, no further advantage than that of disarranging them. This part was abandoned ; the bird walked to the other extremity of the pretended animal, and there, with much exertion, tore the stitches apart, until much fodder and hay was pulled out, but no flesh could the bird find, or smell ; he was intent on discovering some where none existed, and, after reiterated efforts, all useless, he took flight, coursed about the field, when, suddenly rounding and falling, I saw him kill a small garter snake, and swallow it in an instant. The vulture rose again, sailed about, and passed several times quite low over my stuffed deer skin, as if loath to abandon'so good-looking a prey.

:.i; Judge of my feelings when I plainly saw that the vulture which could not discover, through its extraordinary sense of smell, that no flesh, either fresh or putrid, existed about that skin, could, at a glance, see a snake scarcely as large as a man's finger, alive and destitute of odour, hundreds of yards distant. I concluded that, at all events, his ocular powers were much better than his sense of smell.

Second Experiment. I had a large dead hog hauled some distance from the house, and put into a ravine, about twenty feet deeper than the surface of the earth around it, narrow and wind ing, much filled with briars and high cane. In this I made the negroes conceal the hog, by binding cane over it, until I thought it would puzzle either buzzards, carrion crows, or any other birds, to see it, and left it for two days. This was early in the month of July, when in this latitude a dead body

1

« ПредишнаНапред »