I survey). With this instrument, the manner of using it, and its valuable applications, I am necessarily professionally much occupied in daily work; and as the editor of the "Popular Science Review" has requested me to give some plain account of the matter, I will endeavor to afford an untechnical statement of what the ophthalmoscope is, and what are some of the most useful results which have been obtained by its use. Let me first remind the general reader that in the human eye, behind the pupillary aperture of the colored iris, which presents to the unaided eye of the observer the mere aspect of black darkness, lies, first, a clear bi-convex lens; and behind this, filling the eye, and giving to it the character of a solid ball, a transparent globular mass, known as the vitreous body, or humor. It is into a depression in the front of this that the aforesaid lens is fitted, so that the whole space of the eye behind the iris is filled by the lens and vitreous body. The optic nerve, or nerve of sight, which pierces the tunics of the eye at the back and near the centre, spreads out and forms an expanded tunic of nerve-structure which enwraps the vitreous body as far as its most forward edge, where the colored iris descends in front of it. Enwrapping again this nerve-tunic or retina is a vestment, chiefly made of blood-vessels, connected by fine tissue and thickly coated with black pigment, having its own optical uses. This second outer pigmented vascular tunic is the choroid. This again is enclosed within the external strong fibrous membrane, which includes and protects all the sclerotic membrane [{120}] (
hard). These are the two humors and three tunics of the eye which can to a greater or less extent be examined during life by the aid of the ophthalmoscope. They can all be more or less investigated in the living eye by the aid of the ophthalmoscope, because by the aid of this instrument we are able to see through the pupillary space. If one considers what is the reason of the apparent darkness of the pupillary aperture and the chambers of the eye behind it, it is not difficult to gain an idea of the means by which this optical condition may be altered so as to enable us to see where all seem to the unaided vision obscure.
Doctor looking through ophthalmoscope.
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, such as one of those beautiful little white-furred rabbits, whose rosy eyes look like fiery opals edged with swan's down, 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 eye 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 vitreous body and lens, just as they were in entering the eye, with the object then of causing them so to converge as to produce upon the retina a clear and definite image of whatever external object they started from. Similarly, 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 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 rays. The mirror becomes the source of light to the observed eye; the rays which it flashed into the eye emerge [{122}] 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: r a 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 r' 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.
Diagram of preceding discussion.