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 necessary 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 observer 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 —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 [{123}] with atropine—a drop of a solution, one grain to a pint of water, or one of the atropized gelatines prepared for me by Savory and Moore, each of which contains one hundred thousandth of a grain of atropine, and will maintain dilation during several hours. This acts also perfectly well with rabbits or cats.
Doctor examining patient.
The first thing seen is the red reflection of the choroidal vessels showing through the transparent retina; 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, cream-colored, 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 branch, each of which subdivides forthwith into two secondary branches, and these again continue to subdivide, dichotomously, running forward to the anterior limits of the retina. The veins, which are somewhat larger and deeper colored, usually pierce the disc of the optic nerve in two trunks. Pulsation may occasionally be detected in the veins by watching carefully their color, which seems to change at each impulse just where they pass 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 some of the practical applications of the ophthalmoscope. Here, perhaps, I may be permitted to quote some of the [{124}] paragraphs of a paper which I read lately on the subject before the Hanveian Society:
Interior of eye.