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.

    99/122 (page 93)
    or the subtraction of I. D. from the strength if convex, the amount of myopia or hyperopia in the second meridian. The difference between the two meridians is the amount of astigmatism. When this amount of astigmatism has thus been ascer- tained, the cylindrical lens correcting it is to be placed before the eye and with it, the spherical lens, which will bring the point of reversal to a distance of one metre. The trial is then repeated, and if the point of reversal be found at the surgeon's eye for all meridians of the pupil, the determination already made is accurate. If, however, there be found distinct movement in the visual zone in some one direction while movement in the principal merid- ian perpendicular thereto is abolished, the cylinder selected does not perfectly correct the astigmatism. If this movement be in one of the principal meridians as previously determined [in the direction of the axis of the cylinder placed before the eye, or at right angles to that axis] the cylinder has been placed in the proper direction, but is too strong or too weak; and its strength must be diminished or increased, according to the indications of the movement. If, however, the movement appears to be in a meridian different from either of the principal merid- ians as at first determined [different from the direction of the axis of the cylindrical lens before the eye, or the prin- cipal meridian at right angles to that axis] the axis has not been properly placed—does not conform exactly to the direction of the principal meridian. If this is the case and the cylindrical lens before the eye is of the right strength or too weak, its axis needs to be turned slightly toward the axis of a similar cylinder, which will correct the remaining astigmatism. If the cyl- indrical lens already before the eye is too strong, its axis needs to be turned toward the proper position for the axis of a cylindrical lens of the opposite kind that would correct