are refracted by the media of the eye and collected at P, a point in the retina of E. This point, P, in E's retina, is the image of the object 0; FIG.3.

and since the rays, when reflected from the eye, undergo a similar refraction but in an opposite direction, the rays from the retina at P will return only to the object 0. These reflected returning rays cannot therefore meet the eye of a person at A, but the pupil of E will appear black. And, if the observer's eye be placed in the line OE the illuminating rays will be intercepted. From this it is apparent that without some special contrivance, a person cannot bring his eye into the direction of the rays returning from the eye under examination, without at the same time intercepting the incident rays. This is effected by substituting reflected for direct light, the observer placing his eye behind and looking through the mirror into the illuminated eye.

The simplest form of an ophthalmoscope is represented in fig. 4; E is the eye under examination, F the flame of a lamp, and G a piece of plate glass. Rays of light diverging from F, in the direction of M, fall upon the plate glass G; some of these rays pass through the plate glass, but others are reflected from its surface into the eye E, by which if the eye is accommodated for the flame, they are converged to a focus on the retina at P. The rays that diverge from P, in the direction of the axis of vision, are so refracted by the ocular media that they leave the

Plate glass has the property of partly transmitting and partly reflecting rays of light that fall upon its surface.

cornea convergingly, and would, if not intercepted, meet in a focus at the point O, which is the same distance behind the plate glass that the flame F is in front of it. These converging rays however fall upon the plate glass, by which they are partly reflected and partly transmitted; those that are reflected are converged to a focus at the point F, and those rays that pass through the plate glass are converged to a focus at the point O. The eye of an observer placed at the point O, or in a line with O C, will be able to see a pinkish reflection from the fundus of the eye E, but as these rays that emanate from the eye are convergent, they can not be brought to a focus upon the retina of the observer's eye; hence he is not able, with his unassisted eye, to distinguish any details of the fundus of the eye under examination. To enable the observer to distinguish these details, it is necessary to give the converging rays that are reflected from the patient's eye, a degree of parallelism or slight divergence before falling upon the eye of the observer. This is effected by placing a concave lens of proper strength before the observer's eye. When the examination is conducted in this manner, the vessels of the retina, &c., &c., upon the fundus of the eye under examination, have their natural erect position, and the examination is called the direct method.

illustrated in fig. 4; L, a Both the eye of the patient

Let E (fig.5) represent the eye under examination,-the point P, on the retina, being illuminated by the method concave lens, and A, the eye of the observer. and the eye of the observer are represented in this figure, to be myopic; that is, adjusted for diverging rays. The eye E is adjusted for the point 0. Rays that are reflected from P, on the retina of E, would be converged to the point O, but being intercepted by L, these convergent rays are rendered divergent, which, falling upon the cornea of A, are converged to a focus on A's retina. Thus, the observer will be able to see the point P, on the retina of E. These diverging rays that fall upon A, after being refracted by L, have a direction as if they diverged from a point P, beyond E; the point P will, for this reason, appear to occupy the position of p, behind E, and will, consequently, appear larger than natural.

OPTICS OF THE INDIRECT METHOD.

ILLUMINATION.-When the fundus of the eye is examined by the indirect method, it may be illuminated by the light of a lamp reflected from plate glass as above described, or from a concave mirror, in the manner to be described hereafter.

When the light of a lamp is reflected into the eye, a small portion only of the retina becomes illuminated, the diverging rays falling upon the cornea being brought to a point on the retina, as is shown in fig. 3. In making ophthalmoscopic examinations, however, it is necessary to illuminate a larger field, so that a larger portion of the retina can be seen at the same time. This is effected by placing a 1 or 2 inch double convex lens about two inches from the cornea, between the mirror and the eye under examination.

Thus, in fig. 6, let F represent the flame of the lamp; B, a concave mirror with a central aperture; L, a 2 inch double convex lens, and E, the eye under examination. Diverging rays from F, falling upon B, are reflected convergingly towards L, by which they are refracted to a focus near the pupil of E; these rays, after crossing, proceed in nearly the same direction, and, as they diverge, a large portion of the fundus is illuminated at M N.

REFLECTION.-When the indirect method is employed, the mirror is held at a distance of from 12 to 18 inches from the eye to be examined. The eye of the observer looking throuh the sight-hole of B, will not see the erect image of the fundus of E at M N, but its inverted image will be seen instead in front of the convex lens at n m.

Fig. 7 explains this. Let E represent the eye illuminated as represented in fig. 6, and L a bi-convex lens of 2 inch focus, and at a distance

of two inches from the cornea of E. Rays of light reflected from M,* in the fundus of E, leave the cornea nearly parallel;† these parallel rays, falling upon the lower part of the lens at H, are brought to a focus at m, which is exactly at the principal focus of the lens, namely two inches from its optical centre. In the same way rays from the point N leave the cornea parallel and falling upon the upper part of the convex lens at K, are brought to a focus at n. There will then be formed in the position n m, an inverted image of the fundus of the eye E, which can be seen by the observer (see fig. 6) looking through the sight-hole of the mirror B.

LIEBREICH'S SMALL OPHTHALMOSCOPE.

I agree with Hulke in thinking that Liebriech's small ophthalmoscope is the best that can be recommended for ordinary examinations of the internal eye. It can be used for both the erect and the inverted image. This ophthalmoscope consists of a metallic concave mirror, two double convex lens 14 inches in diameter, and five lenses, (one convex and four concave) of an inch in diameter. The mirror is 1 inches in diameter and has a central aperture or sight-hole one-sixth of an inch in diameter. The principal focus of the mirror is seven inches from its surface. A small spring slip for carrying any of the small ocular lenses, is attached to the back of the mirror by a jointed limb. The large objective lenses are 11 inches in diameter and their focal length is 1 and 2 inches. The small convex (ocular) lens has a focal length of 12 inches and the four concave (ocular) lenses have a negative focus of 6, 8, 10 and 12 inches respectively.§

Rays of light diverge from M the same as they do from the flame of a lamp; the fundus of the eye does not reflect light as a mirror reflects it.

When the pupil of the normal eye is dilated with a strong solution of atropine, the eye is adjusted for parallel rays.

It will seen from fig. 7 that a line passing from M through the optical centre C, is continued, after leaving the cornea, in the same direction to H, and that this line M C H determines the direction that the parallel pencil will take after leaving the cornea. It will be seen also that the direction in which H m is refracted by the lens L, is determined by a line passing from c, the optical centre of L, to the principal focus m parallel to M C H.This is true also of N C K and Kя. || In Zander's treatises on the ophthalmoscope, translated from the German by Carter, there are no less than 37 varieties of the ophthalmoscope described; 2 of which are binocular; 5 stationary, and 30 portable. The binocular instruments are those of Giraud Teulon of Paris and J. Zachariah Laurence of London. Carter prefers the ophthalmoscope of Coccius for the inverted image. The ophthal moscope of Zehender is considered the best for examing the erect image.

§ Liebreich's small ophthalmoscope and the author's demonstrating and photographing ophthalmoscope can be procured of C Potter, optician, No. 20 King Street, Toronto.

THE OPHTHALMOSCOPIC EXAMINATION.

THE INDIRECT METHOD-The evening is the best time for making ophthalmoscopic examinations; if made during the day, the room must be darkened. The best light is that from the oil lamp, or from the argand gas burner. Before the examination, the pupil should be dilated with a solution of belladonna or atropine. For simply dilating the pupil, a solution of of a grain of atropine to the ounce will be strong enough, but in order to paralyze the accommodation of the eye, it is necessary to use a solution of 4 grains to the ounce. If the left eye, for example, is the one to be examined, the patient can either sit with his back against the side of a table and his left shoulder just in front of the lamp, or he can sit with his left side at the end of the table and the lamp placed opposite his left ear. The observer places his chair in front, and to the left of the patient, so that, when seated, he will be tête à tête with the patient, and facing the lamp. The mirror is taken in the right hand, and holding it about 6 inches before the patient's left eye, the light of the lamp is reflected upon it. Keeping the light upon the eye, the mirror is gradually withdrawn to a distance of 16 or 18 inches. The mirror, the eye of the patient, and the lamp should be nearly on the same level. The right eye of the observer is now brought to the sight-hole of the mirror, and the instrument kept steady by resting its upper part against the brow. The observer will now see the illuminated fundus of a pink colour and apparently the size of the dilated pupil, but no details can be distinguished. One of the large double convex lenses is now held by the thumb and fore finger of the left hand, between the mirror and the eye under examination, the hand being kept steady by resting the other fingers against the brow; the lens is at first placed about an inch from the eye of the patient, and gradually withdrawn until the pupil appears to be enlarged to the size of the lens, which will be about 2 inches from the eye, if the 2 inch convex ocular is the lens that is used, and about 1 inches from the eye when the 1 inch lens is used. If the eye of the patient is at this time looking directly at the mirror, the observer, at first, will, probably, see only the bright reflections of light from the two surfaces of the lens and from the anterior surface of the cornea. The patient should now be directed to fix his eye upon the observer's left eye; this will cause the eye under examination to be turned at the proper angle for the observer to search for the optic nerve entrance, which is always the first point to be discovered in making examinations of the internal living eye.

a very

diffi

It cannot be denied that for beginners, this examination is cult one; in fact few are able to see more than the pink reflection from