54 
A NEW FORM OF OPHTHALMOSCOPE 
HE principal steps that have been made during the 
last twenty years in the knowledge of the healthy 
and of the diseased conditions of the eye have been effected 
by the employment of the ophthalmoscope, an instrument 
so simple, and yet so valuable, that, like other discoveries, 
it is only remarkable that the knowledge of the facts on 
which its construction depends should have so long re- 
mained unfruitful. Under all ordinary circumstances, when 
we look into the pupil of the eye of another person, how- 
ever widely dilated it may be, it appears of an intense 
black hue, because the degree of illumination is insufficient 
to render parts so deeply seated visible, the principal 
portion of the light being intercepted by the head of the 
observer. An exceptional instance, however, is sufficiently 
familiar to every one, in which a brilliant reflection may 
be observed to occur from the back of the eye. It is that 
of an animal crouching in the corner ofa cellar, whilst the 
observer is standing at the door, or looking towards a 
window, to which the back of the observer is turned. 
The principle on which the ophthalmoscope is founded 
is identical with this, the eye under observation being 
illuminated by a pencil of light proceeding, as it were, from 
the eye of the observer. ‘This is accomplished by placing 
a steady source of light at the side of or above and some- 
what behind the head of the person under observation, 
whilst the observer reflects its rays into the eye of the 
subject by means of a plane or concave mirror, the 
centre of which is perforated by a small opening through 
which he looks. The back, or fundus of the globe, 
then comes into view, presenting a red, or greyish 
red glare, tke illumination being greatly increased by the 
use of a lens at L, as shown in the accompanying little 
woodcut, from the recent work of Dr. Williams of Boston, 
where the rays of light emanating from the star are 
reflected from the concave mirror DE, and rendered con- 
vergent by the lens L, lighting up the whole of the poste- 
rior surface of the globe ; some of the rays returning from 
this pass through the opening in the mirror, and are 
seen by the observer at O. The precise mode in which the 
image is formed is shown in the following cut, borrowed 
from the same work. The rays returning from N, N’ N” 
representing a portion of illuminated fundus, are brought 
to a focus by the conyex lens L, at A, A’ A", and then form, 
NATURE 
| May 19, 1870 
the inverted aérial image of the fundus, which is seen by 
the observer. 
The image which comes into view under these circum- 
stances, especially if, as is usual, the pupil be dilated by 
the employment of a little belladonna or solution of 
atropine, is represented in the following woodcut, which 
we have carefully drawn from a child of twelve years 
of age. The reader must imagine the general surface 
to be of an orange vermilion, or scarlet vermilion tint, 
though in the negro it is ofavery dark vermilion ; the colour 
being produced by the reflection of the light from the 
capillary blood-vessels of the choroid. 
In the centre is a yellowish white spot, which is the optic 
disc, or point of entrance of the optic nerve. This is 
perforated by the branches of the central artery and veins 
of the retina, the lighter double lines representing the 
arteries and the darker the veins. Above and a little to 
the right is a spot whichis the true centre, and at the same 
time the most sensitive part of the eye ; from its colour itis 
sometimes called the macula lutea, or from its being 
slightly depressed below the surface it is termed the fovea 
centralis. The changes which the optic disc, the blood- 
vessels, and the retina undergo in disease can of course 
be readily followed, and may thus enable a positive opinion 
to be pronounced on cases which were formerly incapable 
of being distinguished even by the most acute observer. 
Nay, it has recently been suggested by M. Poncet to 
employ it as one of the mest reliable means of ascer- 
taining that death has really taken place. A great variety 
of forms of the instrument have been suggested, but the 
ordinary hand ophthalmoscope has proved the most 
convenient in practice, requiring only that the room 
should be darkened, and that there should be some 
steady source of light. Dr. Beale, however, has lately 
suggested a form of self-illuminating ophthalmoscope, 
which, although by no means new in principle, is yet 
convenient in application, doing away with the neces- 
sity of a dark room, and furnishing a very steady and 
good light. It is represented in the preceding figure, 
