File:A treatise on the diseases of the eye (1883) (14784103812).jpg

Identifier: treatiseondiseaseso00well (find matches)
Title: A treatise on the diseases of the eye
Year: 1883 (1880s)
Authors: Wells, J. Soelberg (John Soelberg), d. 1879 Bull, Charles Stedman, 1844-1912
Subjects: Eye Eye Diseases
Publisher: Philadelphia, C. Lea's son & co.
Contributing Library: Yale University, Cushing/Whitney Medical Library
Digitizing Sponsor: Open Knowledge Commons and Yale University, Cushing/Whitney Medical Library

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glass instead of a single slip. A still greater advance w^asmade when Helmholtz utilized Briickes experiment above referred to, andemployed a strong convex lens, held before the patients eye, to converge therays reflected from a large circle of diffusion formed upon the retina. Inthis way an enlarged and inverted image of the fundus was formed betweenthe lens and the observer. This constitutes the examination of the actualinverted image. Helmholtz placed the flame of a candle before the eye under observation,and a screen behind the flame, so that the observers eye could be broughtclose to the source of light, and thus catch the rays after they had beenunited by the convex lens, and formed an image of the fundus. This point 730 THE USE OF THE OPHTHALMOSCOPE. of union lies at the focal distance of the lens. This mode of examinationwas, however, troublesome and inconvenient, and hence Ruete had recourseto a concave mirror having a central aperture for the observers eye, and he Fio;. 227.
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After Zander. thus Still more increased the illuminating power. Since then different formsof mirror have completely superseded the plates of polished glass. The following description and illustration from Zander clearly explain the THE USE OF THE OPHTHALMOSCOPE. 731 action of the concave mirror in the inverted examination, i. e., the use ofa convex lens placed a short distance from the eye under observation, so as toconverge the rays emanating from the circle of diffusion formed upon itsretina. The patient is to accommodate for an infinite distance, so that therays issue parallel from this eye. Examination of the actual Inverted Image.—In Fig. 228, F is again theflame, S the mirror, L the convex lens, and B the eye observed. The raysaeb f, proceeding convergent from the mirror, and rendered more convergent Fig. 228

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