Skiascopy and its practical application to the study of refraction / by Edward Jackson, A.M.,M. D.

  • Jackson, Edward, 1856-1942.
Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: Skiascopy and its practical application to the study of refraction / by Edward Jackson, A.M.,M. D. Source: Wellcome Collection.

Provider: This material has been provided by University of Bristol Library. The original may be consulted at University of Bristol Library.

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    MYOPIA. distance at which the inverted movement is distinguishable, and takes a point midway between these to be the point of reversal. The distance of this point of reversal from the patient's eye is the focal distance of the lens that will be required to correct the myopia. To complete the test, however, a lens about i D. weaker than this is placed before the eye to bring the point of reversal to the distance of one metre ; and the test is repeated, the surgeon noting carefully the great- est distance at which the erect movement is visible, and the shortest distance at which the inverted movement is perceived, always in the visual zone. The distance of the point of reversal as thus determined is the focal distance of the lens required to correct the remaining myopia. The strength of such a lens, added to the strength of the lens already before the eye, gives the total amount of myopia present. Suppose the eye to be 6.5 D. myopic. With the first test the inverted movement will be perceived up to about eight inches from the patient's eye; and at five or six inches from the eye an erect movement will begin. From this, the surgeon may assume that the myopia is about 7 D. [focal distance 6}4, inches] and place before the eye, for the more accurate test, a concave 6 D. lens. On trying the movement of light in the pupil through this lens, it will be found at the distance of one metre to be with that of the light on the face. The surgeon then with- draws still farther from the patient until the direct move- ment becomes indistinguishable and at two metres is entirely lost. Drawing back still farther from the patient, he might in a favorable eye be able to distinguish the inverted move- ment in the pupil, and in this way fix the point of reversal at a distance of two metres, indicating with great accuracy an uncorrected myopia of 0.5 D. Often, however, the distance of two metres will be found