An additional test for the diagnosis and correction of the optical defects of the eye / by William Thomson.

  • Thomson, William, 1833-1907.
Date:
1870
Catalogue details

Licence: Public Domain Mark

Credit: An additional test for the diagnosis and correction of the optical defects of the eye / by William Thomson. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    or myopia and hypermetropia may coexist in the same eye, in meridians at various angles to each other. To diagnosticate and correct the impaired vision caused by these forms of ametropia, the method usually employed clinically, is to place before the suspected eye various convex, or concave, spherical, or cylindrical, glasses of known focal length, and to direct the patient to examine with them cer- tain test-types, those of Snellen being usually preferred, which are com- posed of letters of such sizes as to present angles of 5' at the distances at which they are intended to be seen. As the power of refraction and length of axis are in the proper propor- tion in an emmetropic eye, any convex or concave glass must diminish its visual power: as the refraction is too great in myo})ia, its reduction by concave glasses improves vision : as the refraction is too low in hyperme- tropia for its short axis, convex glasses, by increasing it add to the visual power ; and in describing these defects the power of the glass, whether con- vex or concave, which gives the best definition, is used to designate the degree of ametropia. The ophthalmoscope is also usefully employed to ascertain the state of the refraction of the eye, by examinations which are fully described in the text-books on ophthalmic surgery. Usually, a full assortment of test-glasses are required, and with them, and the test-types, a series of careful observations must be made to ascer- tain the presence and degree of ametropia ; and as an addition to these methods, or when the apparatus above mentioned is not at hand, it is hoped that the sim})le test below descril)ed may be of service to those inter- ested in this branch of surgery. Let the reader take any strong convex lens, say one of six inches in focus, and standing at ten feet distance from a gas-light or lighted candle, project upon a piece of ground glass, held between the eye and the light, an image of the flame, which will be seen, inverted and distinct, when the screen is a little beyond six inches from the lens ; a circle of light replaces the image, increasing or diminishing in size, as the lens is moved to or from the screen. Now cover the lens with a |)iece of cardboard having two circular apertures in it, each ^ inch in diameter, and J inch apart, and two spots of light will appear on the screen when the lens is moved within or beyond its focus, becoming more distinct as this point is ap- proached, and uniting into one distinct image of the flame at the proper focal point. The least movement from this point, will cause the image to become double; and by this it is known that the screen is not placed at the point to coincide with the refractive power of the lens, or, in other words, that the axis or distance from screen to lens is not harmonious with the refracting power. This is precisely the condition of the ametropic eye, and by the simple test described below, its retina is enabled to perceive the double images it produces. For this purpose a disk of sheet brass inch in diameter may be placed directly before the eye, having in it, at a distance apart of J inch, two perforations, each 3^ i^^ch in diameter. When the eye regards a gas-light distant twenty feet through these perforations, two circles of diffused light are perceived at the disk, which overlap at their inner margins, and produce a bright elliptical space. Should the eye be emmetropic, the gas-light will be seen in the overlapping space, clearly defined and single; but if either