Many of my friends, to whom I mentioned my experiments, and the result of them, objected to me, that I could not possibly be correct, for they themselves had observed cats' eyes shining in the very darkest places, as, for example, in a cellar. I have had frequent opportunities since of making observations to the same effect; but every time has, nevertheless, convinced me, that, even in such places, the rays of light having passed through a window or some other aperture, fell upon the eyes of the animals as they turned towards the opening, and were placed in a proper position in regard to the observer. Gruithuisen likewise mentions a case in which he could produce light from the eyes of a cat at pleasure; in places absolutely dark, however, he never observed any light.

To ascertain what appearance the eyes of a cat exhibit after death, I cut off the head of one, placed it opposite to the window, at the distance of some yards, so that the rays of light passing through the window might fall on the eyes. I now observed, that the eyes of the dead cat shone far more vividly than those of a living one. By illuminating the apartment, the light was not, as in living cats, weaker but stronger, and was so powerful when I completely illuminated the room, and allowed the sun's rays to fall immediately on the eyes of the cat, that it resembled the most beautiful green fire, which lost its intensity however, and exhibited only a clear greenish brightness, when the rays of the sun, as in the open air, fell on all sides. In places perfectly dark, the light of the eyes, as in all my former experiments, completely vanished. All my subsequent experiments, in which many a cat lost its life, were uniformly followed by the same result.

The light of the cat's eyes being brighter after death than when in life, may probably be owing to this, that after death the pupil is so much dilated as almost entirely to hide the iris ; and that the pupil, being now insensible to the rays of light falling on it, is never closed again, and does not obstruct the passage of the penetrating rays of light, as is well known to be the case during life.

Farther, the light of the eyes of those animals, that after death were subjected to experiment, became gradually weaker as the cornea grew duller. When that part of the eye was

moistened, the intensity of the light was increased to a certain extent, but faded away with the decomposition of the eye itself, and eternal darkness reigned in the stead of glorious light. I believe I observed a difference in the degree of light, according to the colour of the cat; and it is certain, that black and fox-coloured cats evolve a brighter and more conspicuous light from their eyes, than grey and white ones, though there may undoubtedly be exceptions to this distinction *.

Besides cats, many domestic as well as other animals, are furnished with luminous eyes.

Under similar circumstances as above, I observed that the light of a dog's eye, as was the case in my experiments on cats, vanished suddenly as soon as I had completely darkened the room where the dog was. I observed the eyes of another dog sparkle when he was irritated, and in the corner of a room that was faintly lighted. The eyes of the animal protruded very much, glittered brilliantly, and the pupils were dilated to an unusual degree. The colour of the light, which was commonly yellow, changed more or less as the rays of light fell on the eyes of the animal, and exhibited the following appearances. When a small body of rays of light fell on the eye, the light was of a fiery redness, and sometimes so strong, that after I looked a long time attentively at it, my own eyes experienced a disagreeable sensation. When there was a great body of rays, the light was green or yellow, sometimes bluish. In respect to this change in the colour of the light, I was inclined to think, that it might be owing as much to the motions of the animal's eyes as to the body of light that fell upon them. This change was different in different dogs, and in some it was not at all observable. Further, the eyes of every dog placed in the same situa tion, shone, but the intensity of the light varied with the individuals.

of

I have observed luminousness in the eyes of horses, sheep, and hares, which was different, however, in colour and strength. Many appearances of light have been observed in the eyes human beings. Treviranus mentions, that G. T. L. Sachs, and his sister, both belonging to albinoes, had phosphorescent

• Bened. Prevost's Memoir on the Luminousness of the Eyes of animals, will be given in our next number.

eyes. Late in the evening there appeared in them a lively yellowish brightness, which darted forth in fiery coruscations or globules, from the interior of the eyes. The balls rolled hither and thither, and frequently ejected rays, at least an inch in length. In these two relatives the light was liveliest and strongest after their birth, and during infancy: in their more advanced years the light was strongest when they were in deep meditation; at this time, also, the oscillation, which they had in common with other albinoes, was liveliest.

A rather remarkable observation, and similar to the case of the Sachs, is that of Michaelis, who, many years before his death, during the interval between day and night, and during the night itself, observed irradiations of light issuing from his eyes; sometimes so strong that he could read the smallest print. (Schlichtegroll's Necrolog, des 19. Jahrhunderts, B. 3. s. 337).

In a boy, who belonged to the Albino variety, I observed a similar case, though not accompanied with irradiation. In this boy, who suffered so much from the dread of light that he never ventured abroad except in twilight, I frequently observed the same fiery eyes, yet were they very different, both in the strength and colour of their light, from the luminous eyes of animals which I had observed, partly from design and partly from accident; for this boy's eyes might be called glassy rather than luminous. Some years ago I was assured by (Hr Geheimenrath W), that his sister had often observed the eyes of her children, who were also albinoes, to be luminous. These last two cases could be traced to rays of light falling on the eyes.

It now remained for me to search out the cause, which, by means of the incident rays of light, gave rise to the shining appearance in the eyes of human beings and inferior animals. The explanation seemed to me no easy matter, yet, from the beginning, I expected to be able to search out the cause of this phenomenon, in a reflection of rays of light penetrating into the eye. The colour of the light, however, and particularly its changes in dogs, appeared to me very difficult to explain, and to be rather at variance with my own opinion.

To discover the cause of the shining in the eyes of human beings and inferior animals, I came to the resolution of undertaking the extraction of the lens on a cat, from which I anticipated the best result, in so far as I might, by that means, best

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 towards 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.

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 erroneous. 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.