Determination of the refraction of the eye by means of the ophthalmoscope / by Edward G. Loring.

  • Loring, Edward G. (Edward Greeley), 1837-1888
Date:
1876
Catalogue details

Licence: Public Domain Mark

Credit: Determination of the refraction of the eye by means of the ophthalmoscope / by Edward G. Loring. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    which consists of two iindetachable but revolving disks, one of which contains concave, the other convex glasses. These are superimposed in such a manner that they rotate past each other, so that the focal value of each glass can be lessened to a greater or less degree (but not increased) by adding to it the various neu- tralizing glasses of the other disk. As there are twelve glasses in each disk, the focal value between the glasses is in itself small, but this can be made much smaller by tlie use of the other disk. But the presence of both disks at the same time necessitates (unless, indeed, one is on each occasion turned to zero) an elaborate calculation before the real focal value of the combination before the observer's eye can be determined. To facilitate this, a table has been prepared by Dr. Knapp, and published in the Archives of Ojphthalmology and Otology^ Vol. III., No. 2, 1874. Perhaps a simpler form of obtaining the many combinations afforded by the use of two disks was that employed some time ago by the writer. This consisted of two disks placed one over the other. The brass rim of the lower disk projected just enough beyond the edge of the upper to allow the former to be rotated without affecting the latter, which had an independent move- ment of its own. By arranging the glasses so that each disk shall contain concave as well as convex, the focal power of each glass can be increased as well as diminished, which is not the case Avhere one disk contains, as in Dr. Knapp's instrument, all the convex, and the other all the concave lenses. These instru- ments are, however, of little or no practical value, as the calcu- lation in case of all the weaker numbers, and even of the mod- erately strong ones, can only be carried out, even by an adept, on paper, or by a prepared table, and the result, when obtained, gives a focal interval of so small a degree as in most cases to be entirely inappreciable even to the most proficient expert. In order to use any of the numbers that more commonly occur without going into this calculation, one disk must be always turned to zero, while the required number is sought by revolving the other. Dr. Knapp's instrument, like that of Wecker's,^ has the advan- ' For a description and drawing of this ophthalmoscope see Klin. Monatsblat- ter, Zehender, Sept., 1873.
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