This darkness of the pupillary aperture is attributable partly to obvious causes, such as the natural contraction of the pupil or iris, which occurs under light-this contraction limiting the number of rays which can enter the eye. Then that black pigment which lines the iris absorbs a great deal of light; and thus, as in the case of albinos, whose eyes are deficient in pigment, or where the pupil is dilated, either through disease or by artificial agents, these obstacles for seeing into the living eye are removed. But still the main difficulties are not cleared away; and if you take for example an albino animal, as to produce upon the retina a clear and definite image of whatever external object they started from. Smilarly, then, on their emergence they are refracted chiefly by the lens and cornea, so as to form an image in the outer air, the emergent rays coinciding in their path with that which they took when entering, and the image formed in the air being conjugated with the retinal image; being formed, therefore, on the same side, varying with the position of the lens and object, and the accommodation of the eye. Thus, then, to perceive this aerial image, derived from the retinal reflection, the eye of the observer such as one of those beautiful little white-furred rabbits, whose rosy eyes bok like fiery opals edged with swan's dowa, and dilate the pupils with atropine, it is still not possible to see clearly the details of the structure within and at the back of the eye. This is by reason of the structure of the eve as an optical instrument, and because the rays of light in entering and in emerging from it undergo refraction, according to definite laws. The light which penetrates the eye traverses the transparent retina, producing the impression necessary for sight, and is partly absorbed by the black pigment of the choroid; but a great number of the rays are reflected; for here there is no exception to the general rule that some of the rays of light falling upon any substance are always reflected. These rays, in returning, are refracted through the vit reous body and lens, just as they were in entering the eye, with the object then of causing them so to converge needs to be placed in the axis of the converging rays; but since this is also the axis of the entering rays, he will of necessity in that position cut off those rays altogether of the light proceeding, say, from a lamp, or the source of light opposite to the eye to be illuminated. The problem to be solved consists, then, in the simple illumination of the eye to be observed by a source of light so arranged that the observer can be placed in the axis of the rays entering and emerging without intercepting those rays. This may be most conveniently effected by placing the source of light aside of the eye to be observed, and observing through a pierced concave mirror, which reflects that light into the eye. We can then, by looking through the central aperture of this mirror, place ourself in the path of the entering and emerging ravs. The mirror becomes the source of light to the observed eye; the rays which it flashed into the eye emerge in part, and return along the same path, forming the aerial image at a distance and under circumstances regulated by the optical conditions of the eye observed, and within view of the observer who is looking through the mirror. A very simple diagram will suffice to explain this: ra is the circle of diffusion of the retina, and the lines indicate how the reflected rays will pass through the media of the eye, and form at ra' a real enlarged but inverted image of the fundus of the eye. This will be placed at the distance of distinct vision of the subject, and has relation to the accommodation of the eye. As these are variable quantities, the practice of ophthalmoscopy demands a little address, which habit quickly gives. It is for want of understanding this, and from impatience of these preliminary difficulties, that many have been discouraged at the outset, and have abandoned unwisely the attempt to learn the use of the ophthalmoscope. The image obtained in the way mentioned is not so distinct as to give that full perception of details which is nccessary for scientific and medical purposes. A more defined image is obtained by interposing, for example, a bi-convex lens on the path of the luminous rays emerging from the eye observed. The effect of holding such a lens of short focus before the observed eye whilst examining it with a concave ophthalmoscopic mirror is to cause the rays emerging from the eye to undergo a further refraction, and to modify the actual image which they form, producing one which is smaller, more defined, but still inverted. This is the most simple and one of the most satisfactory methods of exploring the eye with the ophthalmoscope. It is that of the most general and easy application, and I will, therefore, add a few words to explain how it may most conveniently be practised. We will suppose that it is the human eye which is to be examined. The room is to be made dark; the person to be seated; a light-the white flame of an oil-lamp or an Argand gas-burner-to be placed near his head, on the side, and at the level of the eye to be observed. The ob server takes then the concave mirror in the hand of the side toward the lamp, and placing it against the front of his eye, so that the upper edge rests against his eyebrow, brings his head to the level of that of the person seated, looks through the central perforation at the eye to be observed, and by a little careful change in the direction of the mirror casts, by its aid, upon the eye examined the light of the lamp. He will now perceive that the pupillary aperture is illuminated, and, no longer black, shines with a silvery or reddened light. He takes now the bi-convex lens of short focus in the hand hitherto free, and places it in front of the examined eye, and at such a distance as to make the focus of the lens coincide with the pupil of that eye --a distance varying from two to three inches. He himself will usually need to be at a distance from twelve to eighteen inches. This is for normal eyes. The slight movements backward and forward necessary to adjust these distances correctly, are effected very easily and precisely after practice; but at first it is a little difficult to avoid changing the direction of the mirror while thus slightly advancing or retiring the head; and this is a point on which it is well to give a warning, for it is a frequent source of discouragement to beginners, who find that at every movement they interfere with the illumination of the eye, and so suffer from a series of little failures at the outset. The first thing, in fact, that every one sees amounts to a little more than a red, luminous disc; those who begin by seeing nothing more, therefore, need not to be discouraged; a little patience and time will enable them to see what more practised persons describe. The eye to be examined may be more fully observed by dilating the pupil and when the eye observed is directed upward and inward, we see the usually circular disc of the optic nerve, encircled by a double ring, creamcolored, or very faintly roseate or grey, and surrounded by the red choroid. The two rings are the apertures in the choroid and sclerotic, of which the former is the smaller. From out this disc we see springing the retinal artery and retinal veins, sometimes centric, at others excentric, in their passage. The artery is easily recognized as being somewhat smaller in calibre, and of a lighter red. The artery usually divides into a superior and inferior over the edge of the optic disc and bend to pierce the nerve. Fuller details of the ophthalmoscopic appearances of healthy eyes, both human and animal, will be found in Zander's treatise, excellently edited and translated by Mr. R. B. Carter, of Stroud. In the healthy eye the aqueous humor, lens, and vitreous humor are clear, and do not in any way_obstruct the passage of the light. It is otherwise in disease; and this brings us to the discussion of seme of the practical applications of the ophthalmoscope. Here, perhaps, I may be permitted to quote some of the para of the crystalline or converge toward the centre from the circumference. In order to see the latter, the pupil must be fully dilated with atropine; as, indeed, for the purposes of complete ophthalmoscopic examination it always needs to be; and then, just as the greatest expert cannot discover them except by ophthalmoscopic illumination, so, neither with its aid, can they be passed over with ordinary care. In order to be quite sure in any delicate case, it is well to lower the light a little, and use only a feebly illuminating power, as a very strong light may overpower a com |