Eyesight, and how to care for it / edited by George Black.

Date:
[1888]
Catalogue details

Licence: Public Domain Mark

Credit: Eyesight, and how to care for it / edited by George Black. Source: Wellcome Collection.

    51/168 (page 47)
    of the eye visible, having once been demonstrated, the problem enlisted many distinguished scientists in its elucidation, and their efforts culminated in the invention of the Ophthalmoscope by Helmholtz, in 1851. When we speak of seeing anything, we mean that rays of light proceeding from it reach our retina, and the pupil ordinarily appears black, because the eye of the observer is not placed in a position to receive the rays of light coming from the interior of the observed eye. If, for instance, we hold a candle in front of an eye to illuminate the pupil, the light thrown back from the bottom of the eye through the pupil, and towards us, will be intercepted by the flame; and, of course, we cannot place our own eye in the course of the rays, between the flame and the observed eye, without shutting off the light altogether. In either case, the pupil of the eye examined will appear black, simply because no light passing out through it can reach our retina. This part of the problem is solved by using, instead of the candle, a small mirror, usually concave, with a hole in the centre. Light is thrown by the mirror through the pupil, and the observer, by placing his own eye behind the central opening, has it directly in the line of the rays reflected from the interior of the eye examined, a bright red reflection from the blood in the vessels of the choroid immedi- ately takes the place of the usual blackness of the pupil.