and choroid, is the only obstacle to the success of this experiment; since it is the opacity of these coats, which makes their removal necessary.-Easy indeed would the experiment become, if we could operate upon eyes, whose exterior membranes were transparent to a certain degree: thus permitting us to see the images formed on the retina.

Chance has thrown in my way an opportunity of remarking, that the eyes of many domestic animals possess this advantage; for instance rabbits, guinea pigs, young dogs, kittens, and pigeons; and this becomes the more apparent as the animals are younger. The eyes of owls and other nocturnal birds of prey, present a similar structure. If these birds were more frequently to be met with, and if their eyes resembled more those of the mammiferæ, by their size and degree of transparency of their exterior membranes, perhaps they ought to be preferred to those of the animals enumerated above. However, we have only to take the eye of one of the abovementioned animals, to separate the sclerotica from the fat and surrounding muscles, and to direct the pupil towards illuminated objects; the images will be then seen distinctly represented through the sclerotica and choroid on the retina. As the experiment succeeds without the dark room, it is unnecessary to have recourse to it.

Thus this important experiment, which required a great deal of precaution and skill, and which could be repeated only by a very small number of persons, can henceforth be performed by any one, and without fear of failure.

I will however add, that when recourse is had to this mode, images are distinct only where the objects are very bright. The image of an object is much better seen, when the object is exposed to the sun, than when it is placed in the shade. I shall also state, that the black matter with which the choroid is covered, prevents the distinguishing, perfectly, the genuine colours of the image; for instance, the image of a flame of a taper appears to have a reddish tinge, which, in reality, it does not possess.

I sought, for some time, the means of obviating this inconvenience. It finally struck me, that among the species, the eyes of which I had employed, there were some that were albinos. It is well known that one of the characteristic marks of this abasement of animals, is the absence of the coloured coat of the choroid and iris.

I examined the eyes of white rabbits and albinos pigeons; I was much gratified to find, that they were in the most favourable condition. The sclerotica is thin and nearly transparent; the choroid is likewise thin, and as soon as the vital spark is extinguished in the animal, the blood which coloured this part disappearing, it becomes incapable of offering any sensible obstacle to the passage of light. Indeed the eyes of these animals appear to have been formed for the express purpose of showing the images that form on the retina. The outline is well defined, and all the colours distinctly perceived; and so perfectly are they represented, that the image of the sun hurts the eyes of the examiner, almost as much as if he were in reality viewing this planet.

A very convenient mode of making the experiment, consists in placing yourself at the window of the first story, during a fine day; to direct the transparent cornea of a large white rabbit towards the street: every person that passes, is then represented on the retina, by an image of about a millimêtre* in height; the colour of the dress and the slightest

The new French measures proceed in a decimal order; for instance, the millimêtre is the tenth part of the cêntimêtre; the latter is the tenth part of the decimêtre, the decimêtre is the tenth part of the mêtre, and so on. The numbers which are annexed to the following names of the French measures, express the number of English inches, or troy grains to which they are equivalent.

The mêtre is equal to the forty millionth part of the whole circumference of the earth.

motions of the individual, are represented with such surprising accuracy, that the spectator is at once amused and astonished.

The facility with which images are beheld by this method, has enabled me to make some remarks, and suggested some experiments; I will in a few words say something of both.

On a fresh eye, images always appear clearly defined and their size increased, on the object being brought closer; and their diminution by its being removed farther off has no effect upon their clearness. If we were to rely upon the first view, we should be led into the belief, that the eye is a perfect acromatic instrument.

I wished to determine what would be the effect of a circular pressure, made perpendicularly to the axis of the eye, from behind forwards, near the middle of this organ: I found, that whatever be the degree to which it is carried, the effects are not to be discovered in the dimensions of the image. If it has any influence, I presume it is perceived by the intensity of the light; at least it appeared to me, that the intensity of the light was greater after the pressure.

It must not be thought, that a deficiency in the clearness of the outlines of the image, cannot be detected at first sight; it is only necessary to make a small opening in the transparent cornea, with a point of a lancet, near to its circumference, so as to occasion the exit of a small portion of aqueous humour, as it thus becomes very apparent that the image has lost its clearness.

I afterwards sought to ascertain the rule, by which the image decreases in consequence of the increased distance of the object; I found that the size of the image is visibly proportioned to the distances. Mr. Biot was kind enough to assist me in ascertaining this; in other respects it corresponds with what has been published by Mr. Lecat in his Treatise on Sensations.

What alteration would take place in the form and dimen

sions of the image, if the refracting parts of the eye were removed? This could only be ascertained by experiments.

I made a small opening at the circumference of the transparent cornea, near its union with the sclerotica, and contrived so that the aqueous humour should escape through it. The image (it was that of the flame of a taper) appeared to me, cæteris paribus, to occupy a larger space on the retina; it was evidently not so well defined and less bright, as the image of the same object seen in the eye of the other animal, which I had just placed in the same position, with respect to the taper, but which had been preserved entire, in order that it might serve as a comparison.

Depriving afterwards the eye, from which I had extracted the aqueous humour, of the transparent cornea, by means of an incision made circularly at its junction with the sclerotica, the image did not appear to me to have altered its dimensions; but I thought that the light which formed it had lost much of its brilliancy.

This, as well as the preceding fact, become conspicuous, when the object, the image of which we examine, is situated at a considerable distance, five or six mêtres for instance.

An eye, from which the transparent cornea has been taken, presents the iris naked; it is then easy to enlarge the pupil, by means of a circular incision in the substance of the iris. And upon an eye whose pupil I had thus enlarged, it appeared to me that the image had increased by the very act of enlargement.

If we take the anterior lamella of the capsula of the crystalline lens, with all due precaution, from an eye whose transparent cornea has been removed, the image is surrounded by a circle of light, less bright and irregular at its circumfer

ence.

When we extract the crystalline lens, by a process similar to that of the operation of the cataract, the image is always formed at the bottom of the eye. But it has become considerably larger; and is at least four times the size of the image,

produced in an entire eye: It is besides imperfectly defined, and the light which produces it, very dull.

When, from the same eye, we take the aqueous humour, and the crystalline lens, and only leave, of the interior parts of the eye, the vitreous humour and the crystalline capsule, images are no longer seen formed at the bottom of the eye; light, it is true, penetrates, but causes no image to be represented correspondent with that from which it is reflected.

I have thus enumerated the small number of facts, which the facility with which images may be seen formed on the expansion of the optic nerve, has enabled me to ascertain: the greater part correspond with the theory of vision: some however form exceptions. To the philosopher it appertains to decide whether these last merit any degree of attention.

Case of a Wound of the Knee Joint.

BY DR. WILLIAM WATSON.

Hay Market, Prince William County, Va. July 4th, 1814.

To the Editors of the Eclectic Repertory,

GENTLEMEN,

THE following case was communicated to me, by Doctor William Watson, of Bedford, Pennsylvania, a physician of great respectability; and thinking it might be of practical importance to the public, I obtained his consent to publish it, with such remarks on it as I might think proper to make. These are few, and accompany the report of the case, both of which you will please to publish, if they should appear worthy of a place in the Eclectic Repertory,

Ánd oblige

THOMAS P. HEREFORD.