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.

    87/122 (page 81)
    Having determined this, the light is brought back as close to the mirror as possible, and the point of reversal for the 950 meridian is determined. To do this it may be advisable to change the convex spherical lens before the eye, but whatever lens is employed, from the results ob- tained with it, the surgeon deduces the fact that in that meridian the refraction of the eye is hyperopic I. D. He then proceeds to measure in the same manner the refraction of the eye in the other principal meridian, finding with the convex 3 D. lens that this point of reversal is at one metre, and its refraction, therefore, hyperopic 2. D. The differ- ence between these meridians will be 1. D., the amount of astigmatism present. To make the final determination, there should be placed before the patient's eye the 1. D. convex cylinder with its axis at 950 and a 2. D convex spherical lens; with which the point of reversal for all meridians will be found to lie one metre from the eye. If, in the placing of the cylinder, its axis is made to not correspond exactly with the meridian of least hyperopia, there will be found by this test a remaining astigmatism of low degree. Suppose, through carelessness or inaccuracy in the earlier observa- tion, the axis of the cylinder should be placed at 1050 in- stead of at 950, the remaining astigmatism then would be found to be such as would be corrected by a convex cylin- der with its axis at about 700. But on turning the cylin- der before the eye io° in that direction, that is, to its proper direction at 950, this remaining astigmatism would disap- pear. If, however, instead of the 1 D. cylindrical lens, a lens of 1.5 D. had been placed with its axis at 1050, there would remain an astigmatism which might be corrected by a concave cylinder with its axis at about 700, and the turn- ing of the cylinder before the eye io° in that direction [to 950] would cause the remaining astigmatism to so change that its meridians would be at 50 and 950, where a