424 Original Articles. | July, 
in it is the very germ of the subject ; light falling upon the concave 
bottom of the eye of these animals is reflected, and causes the luminous 
appearance by which the fundus of the eye, in certain positions of it, 
is seen: only place any other animal, man included, in favourable 
circumstances, and the same appearance will be observed. 
Dilate the pupil, so as to allow the rays of light freely to pass into 
and out of the eye; let the eye be placed in a suitable position for the 
rays of light from a luminous body to fall upon it, in a chamber from 
which all the light is excluded; and let the cbserver stand in a proper 
position, which is “as nearly as possible, in a direct line between the 
source of the light and the eye to be examined . . . . when the lumi- 
nosity of the interior of the eye will be immediately perceived ;” 
these, as stated by Mr. Cumming, are directions which really com- 
prise the principles upon which the ophthalmoscope must be con- 
structed and used. 
Helmholtz’s first instrument was a square box, with a darkened 
interior, containing three parallel plates of glass, placed obliquely at 
one end, and at the other, one or two bi-convex lenses, to concentrate 
the rays of light; but the image thus afforded was too faint to be of 
much value, as most of the light was lost by the intervening plates. 
A great improvement was shortly afterwards made by the introduction 
of a concave, or a plane-reflecting mirror, which, though it has been 
variously modified’in shape and mounting, or in the method of being 
held, constitutes in one form or cther the various forms of ophthal- 
moscope now used; the modifications being rather according to the 
fancy or the whim of the party using it, than involving any difference 
in principle. Suggestions have lately been made for a binocular 
ophthalmoscope, which it is asserted possesses advantages which the 
single reflector does not, but there is a difficulty in getting a correct 
focus with it, and the instrument has not obtained general adoption. 
Leibrich has invented a large and somewhat costly apparatus, with 
various tubes, rods, and supports for more accurately adjusting the 
focus, supporting the head of the patient, and fixing the eye under 
observation: this is said to accomplish its object satisfactorily, but 
from its being a fixture and cumbersome, is not much used. The 
instrument almost universally employed, at least in this country, 
is a circular, slightly concave, mirror about two inches in diameter, 
having a central aperture of } to } in. diameter, which, at the plea- 
sure of the observer, may be mounted on a stem, or simply held in the 
hand, and may be made of speculum metal, of polished steel, or, as 
is most common, of silvered glass. By this mirror the rays of 
light are received and reflected upon the patient’s eye. The mirror 
is held close before the observer’s eye, with its aperture correspond- 
ing with the centre of his own pupil; by this means his own eye is 
in a great degree protected from the light, while through the aperture 
he has a full view of the illuminated disk of the patient’s eye. The 
central aperture in the mirror should be of sufficient size to allow 
of this observation, but no larger, for of course, at this spot, there 
is no reflection of light: indeed, through this orifice, the bright 
light, which should all be reflected as nearly as possible, may find 
