Abstract
THE 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 remained unfruitful. Under all ordinary circumstances, when we look into the pupil of the eye of another person, however 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 of a 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 somewhat 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, the 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 convergent by the lens L, lighting up the whole of the posterior 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 convex lens L, at A, A' A”, and then form, the inverted aërial image of the fundus, which is seen by the observer.
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POWER, H. A New Form of Ophthalmoscope. Nature 2, 54–55 (1870). https://doi.org/10.1038/002054a0
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DOI: https://doi.org/10.1038/002054a0