EYE AS A CAMERA OBSCURA. 331 
« Let A be a flame, whose rays are caught at an angle on a glass-plate C, the 
rays will be thrown along the line CD, into the eye D, which will see an image of 
the flame along the line DB; but the rays reflected from 
the retina passing out in the same line DC, will again 
meet the plate C; they will be in part turned towards 
A, but in part also will traverse the glass-plate C, and go 
to form a picture at B of the image on the retina; but an 
eye G, placed behind the glass-plate and on the line CB, 
will meet these reflected rays, and will consequently see f 
the posterior chamber of the eye D illuminated.”* A 
The experiment is thus made :—‘“ In a dark room, 
with a single flame at the side of the experimenters, 
and on a level with their eyes, the person whose eye 
is to be observed holds a piece of glass (a microscope 
glass slip), so as to catch the image of the flame on 
it; he then, by inclining the glass, brings the image of 
the flame opposite the pupil of the observer’s eye; the 
latter will then see the pupil of the observed eye lumi- 
nous, of a reddish-yellow bright colour. . . . .A 
person may also see one of his own pupils luminous: standing before a looking- 
glass, and seeing the image of the flame in the reflector with his right eye, let him 
bring this image opposite the pupil of the left eye in the looking-glass ; the left 
eye will then perceive the right pupil in the mirror luminous.”+ 
The very simple arrangement which has been described, is all that is required 
to develop the phenomena to which I wish to refer; and the speculum differs from 
it only by employing four glass-plates instead of one, to increase the illumination 
of the observed eye, and adding a double concave glass, to increase the distinct- 
hess of the image on the observer's eye. 
The reflector of Coccrus, plane or better concave, is an ordinary glass mirror, 
with the silvering on the back scraped off, so as to leave in the centre a small 
transparent circular spot, or with a hole bored through the centre of the entire 
mirror. Through this transparent spot or aperture the observer looks from the 
unsilvered side, directing the light reflected from the opposite surface into the 
observed eye, while the pupil of the latter and of his own eye are in a straight line. 
The mirror is round, with a diameter of about two and a half inches; and a 
focal distance of about six inches. The aperture in the centre has a diameter of 
about two lines. The arrangement of the lamp is substantially the same as when 
HELMHOLTz’s speculum is used, and the mode of action, so far as illumination of 
the observed eye is concerned, similar. 
With either of the arrangements described, the interior of any eye can be 
B 
£ 
* Edinburgh Monthly Med. Journal, July 1852, p. 41. t Op. cit., p. 42. 
VOL. XXI, PART Il. 4u 
