accompanying illustration. If a b, Fig. 5, be two points in the illumined fundus of A, pencils of rays will emerge parallel to the axial lines a L, b L to form at the anterior focal distance of A an image of a b. The concave lens L, however, being now inter

posed, these rays are diverged to d and f, and they appear to proceed from the prolongations of these lines-from a' b', at which place an enlarged upright image of a b will apparently be situated. The direct examination is principally of value in cases where it is desirable to obtain a more accurate observation of the minute details, such as individual vessels or nerve fibres, or in the determination of anomalies of refraction, such as myopia or hypermetropia. It is, however, attended with more difficulties than the indirect method of examination; it necessitates, also, a very close approximation of patient and examiner, which may, for various reasons, be undesirable or objectionable; the enlargement is very considerable, and therefore only a small portion of the fundus can be seen at a time; moreover, the concave mirrors of the ordinary ophthalmoscope do not suffice for this examination. I would, therefore, recommend beginners first to employ the indirect method of examination; and as soon as they may have become familiar with this method, and the reversed image, they may proceed to the direct examination. As might naturally be expected, there is a great variety in the form and construction of ophthalmoscopes; but no matter how they may apparently differ in outward shape and appearance, they are, one and all calculated to fulfil the same purpose-to illumine the background of the eye by dispersed light, and to allow of the parts so illumined being clearly seen at a convenient position and of a suitable size. The essential part of the ophthalmoscope is the perforated mirror; this may be either of glass, backed with a layer of amalgam, or of metal; the metal specula are preferable to the glass ones, inasmuch as they are less liable to injury, but VOL. XLIII., No. 85, n. s.

D

chiefly on account of the central aperture through which the observer looks; in the glass mirrors this orifice is more or less in the shape of a tube or canal, from the internal surface and edges of which there is very likely to be most disturbing and annoying reflection of light, which would obviously interfere with the reflected light from the fundus; in the metal mirrors, on the contrary, the walls of the central perforation are extremely thin and the margins sharp, and there can be, therefore, very little, if any, reflection from this part. The illumination afforded by glass mirrors is, moreover, in my opinion, too intense (provided the examiner's eye be normal); the details can be discerned more satisfactorily by a somewhat subdued illumination; hence I am in the habit, when using a glass mirror, of breathing on its surface, or otherwise damping its reflection. When I was in Berlin, some years ago, Liebreich, who has since then removed to Paris, employed a metallic mirror which he had dulled by means of sand-paper. All mirrors have necessarily an opening, and are usually attached to a suitable handle or frame.

Ophthalmoscopes may be divided into two great classes-in one the mirror is concave, and its focus calculated from its surface definite and fixed; such instruments are termed homocentric. In the other class the mirror is either plane or convex, and in combination with a double convex lens; in these the focus may be altered, is negative, and situate behind the surface of the mirror; such instruments are denominated heterocentric.

An enumeration and explanation of the several individual instruments constructed would be of little practical value. I will, therefore, merely adduce a few of the more important ones, commencing with the homocentric.

The simplest, most convenient, and, at the same time, most practical of all instruments is Liebreich's small or hand ophthalmoscope, and is the one I usually employ. It consists, as shown (the actual size) in Fig. 6, of a concave metallic mirror 14" in diameter, and of 8" focal length, which is fixed into a metallic frame, to which the handle is attached. The central aperture is a shallow, funnel-shaped opening, the base 3" wide in the metallic frame, the apex nearly" wide in the mirror itself; the sight-hole thus enlarging gradually from before backwards, and the parts adjoining it, and indeed the whole back and sides of the instrument being

The inch is, for brevity, expressed by (''), the foot by ('), and the line by (''').

thoroughly and effectually blackened there can be no reflection from these parts and no interference with the rays coming to the observer's eye. The handle, about 31" long, is screwed into the frame, to which is also attached a clip with a jointed movable arm;

Fig. 6.

In

the clip serves to hold a small convex or concave ocular lens, which can thus be brought immediately behind the opening in the mirror, and consequently in front of the examiner's eye. The instrument is fitted into a case 51" by 21", and little more than " thick; two large double convex object lenses, generally of 1 and 2′′ focus; four small concave ocular lenses, of 6, 8, 10, and 12 inch focus, and one small convex ocular lens of 10" focus are likewise fitted into the case. my own case I have substituted for the 2" convex lens one of 31′′, as I find in the general run of cases the enlargement produced by the latter much more satisfactory. One of the uses of the small ocular lenses is to neutralize any error of refraction or accommodation which may exist in the examiner; another use is to effect the examination in the erect image; the convex ocular may also be employed to magnify the reversed image. The large lens, whether convex or concave, which is held before the examined eye in order to produce the image, is styled the object lens, to distinguish it from the small one placed in the clip behind the mirror, which is called

the ocular lens, and the reflecting surface of the ophthalmoscope is regarded as the front of the instrument. Dr. N. C. Macnamara, professor of ophthalmic medicine and surgery in Calcutta, has lately published a most admirable and trustworthy work on ophthalmoscopy under the title Lectures on Diseases of the Eye; but in his description of Liebreich's instrument he states that "attached to the mirror is a clamp by which one of the object-glasses, to be found in the instrument case, may be fixed in front of the sight hole." This sentence, which to a beginner would be so confusing, shows the necessity for a strict adherence to a uniform nomenclature.

Messrs. Weiss manufacture an ophthalmoscope consisting simply of a concave mirror set in a circular frame, the principal recommendation of which is its portability, as it may, together with a convex lens, be carried in the waistcoat pocket or in a pocket instrument case.

The best of the fixed ophthalmoscopes belong to this division; such instruments are so constructed that when adjusted they and the patient's eye shall retain a definite, fixed, relative position, so that once the image is projected a number of persons may in succession, examine it without any trouble or difficulty on their part. Here, likewise, Liebreich's ophthalmoscope is undoubtedly the best. The accompanying diagram, Fig. 7, (about one-third the actual size), shows its construction. A and B are two tubes moving one within the other by means of a rack and pinion, I, and having their interior well blackened. At the end of the tube A a concave perforated mirror, M, is swung on trunnions, t t-and a portion of the walls of the tube cut away so as to allow the rays from a laterally placed lamp to fall on the mirror; the tube B carries the convex object lens L, of 2" focus, swung, like the mirror, on trunnions: this tube is provided with a stout encircling collar, having at one side a projecting clamp, capable of being widened or narrowed by means of the screw H; an upright rod, R, is affixed to the corner of a table by means of a strong clamp, D, and the instrument is then affixed to this rod by means of the perforated projection, and slides up and down on the rod, and may be fixed at any wished-for height by the screw H; on this rod slides also a stout horizontal jointed arm, C, having at its extremity an oval padded cup, which acts as chin-rest; this, of course, is put on the supporting rod beneath the ophthalmoscope. A horizontal bar, F, slides in a box projecting from the upper part of the collar: at the extremity of

this bar is a padded disk, which serves as a head-rest; a small jointed movable arm, K, carries a polished metal ball, for directing the patient's eye to any given position, and a shade, projecting laterally from each extremity of the instrument, at s and B, screens the observed and observer's eye from the lamp; there is also a clip behind

the mirror, M, for holding a convex lens, three of which are contained in the case. When in use the patient's chin rests in the cup, C, and his forehead against the pad, F; the instrument should usually be about two inches from the eye-the focal distance of the lens L, and a lamp placed towards the side, from which rays may fall on M, and so directed then through L into the eye; it will be seen that the anterior extremity remains stationary, while the tube A, carrying the mirror, is movable. The formation and position of the image is the same as that already described and illustrated by Fig. 4. When obtained, the image of the fundus is very clear