The ophthalmoscope : its varieties and its use / translated from the German of Adolf Zander by Robert Brudenell Carter ; with notes and additions by the translator.

Date:: 1864

Credit: The ophthalmoscope : its varieties and its use / translated from the German of Adolf Zander by Robert Brudenell Carter ; with notes and additions by the translator. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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$circle on the retina, to include a larger portion within the field, the lamp must be moved forwards or backwards; and if it be desired to diminish the illumination, a third or a half of the mirror may be covered by the screen. For examination of a virtual, erect image, it is necessary to place in the stem / a concave lens of from 8 to 9 focal length. This should be distant from 1 to 3 from a short-sighted, and from 3 to o from a far- sighted eye; but by sliding it backwards and forwards the exact distance required by the accommodation of any particular eye may be found. A short-sighted observer must use his accustomed spectacle glass. The inspection of the erect image is more especially useful in order to examine in detail the objects of the fundus oculi, and to determine their peculiar colours, as well as the transparency of the whole refract- ing media. For examination of an actual inverted image, it is necessary to substitute a convex for the concave lens. If the stem I carry a convex lens of about 1^ focal length, the observer will obtain, when this lens is distant nine or ten inches from his own eye, and about one inch from the eye of the patient, an inverted picture of the retina, magnified from 2 to 3J diameters, and showing the finest branches of the central vessels, small aneurisms, extravasations, and so forth. If two convex lenses be used, the first of 1£, about one inch from the eye of the patient,—the second of 4^, about 5J'' distant; this combination will afford an inverted image about three times magnified, but apparently filling a larger field than the former. The annoyance of reflected mirror images may be removed by turning the lenses on their vertical axes, so that they stand somewhat obliquely to the optical axis of the eye. An image magnified nine or ten times is afforded by two convex lenses, one of 1J focal length, the other of 3, the first about two inches, the second about six inches, from the eye of the patient; but the steadiness of the eye required by this combination can seldom be obtained. This, and all other concave mirrors, produce their effects according to the following optical principles:— In Examination of the virtual, erect Image.—In Fig. 9, F represents the flame, S the mirror, L the concave lens, B the eye exam- ined. From the flame F, rays of light F a and F b fall upon the mirror, and are reflected from it in directions converging towards its focus. They are intercepted before they meet by the concave lens L} and rendered divergent. Proceeding, they strike divergent upon B's cornea, and form upon its retina the dispersion image a ]3. Returning in their course of entrance, the rays a d e and (5 c f would unite in g, were they not again rendered divergent by L, so that the observer at A sees a magnified erect image a /3'', apparently situated behind a (5. c 2$