The prescribing of spectacles / by Archibald Stanley Percival.

  • Percival, A. S. (Archibald Stanley), 1862-
Date:
1910
Catalogue details

Licence: In copyright

Credit: The prescribing of spectacles / by Archibald Stanley Percival. Source: Wellcome Collection.

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

    42/176 (page 30)
    C YLINDRICA L NO TA TION especially those at right angles to the distinct one. The distinct line gives the meridian of greatest curvature. Let us suppose it is the line at 80° from the horizontal. On now holding in front of the trial frames concave cylinders, with their plane axes at right angles to the 80° line (i.e., at an angle of 170°), of gradually increasing strength, we shall eventually find that cylinder with which the 170° line can be defined. This is the correction. On putting the cylinder at this angle into the trial frames we try the effect of a weak convex or a weak concave glass held in front of the combination, and make any slight alteration with which he can read more of the test types. If he sees better with his head tilted to the right, he will require the cylinder slightly rotated to the right, and vice versa. Astigmatism may be also tested with the stenopaic slit as described on ft, 27, but it should be remembered that no test for astigmatism is reliable without the instillation of a mydriatic, and it is well not to waste too much time on subjective tests, as a more accurate determination can be made in a far shorter time by the objective tests. Notation of Cylindrical Lenses. — This is still a vexed question; all, however, are unanimous on one point, that the meridians of astigmatism should be measured and represented as the observer looks at the patient. With regard to the angle notation, there is almost every possible diversity of opinion. Some, paying attention to the undoubted bilateral symmetry of the eyes towards the middle line, consider that the nasal extremities of the horizontal line of the trial frame should be the zero for each eye, and that the angles should be measured from this point by a radius vector rotating through an angle of 180° to the temple.* Here again there is room for * This was the method submitted by the Commission at the International Congress held at Naples, 1909.