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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    CHAPTER IV. REGULAR ASTIGMATISM. The essential fact of regular astigmatism is that in two different directions, at right angles to each other [the prin- cipal meridians] the curvature of the dioptric surfaces dif- fers, so that they exert unequal refractive power; and that in all other directions, or meridians, the refractive power bears such a relation to the refractive power of these prin- cipal meridians, that it is only necessary to consider what happens in their direction. Two Points of Reversal.—In such an eye, the rays coming from the same point of the retina, and passing out through surfaces that refract unequally in different merid- ians, must leave the eye with different degrees of divergence, or convergence, in the directions of these different meridians. If after passing out the rays are convergent, or rendered so by passing through a convex spherical lens, they will be more convergent in one principal meridian than the other, and the point of reversal for one principal meridian will be nearer the eye, than the point of reversal for the other principal meridian. The position of each point of reversal gives the amount of myopia (either original or produced) in the principal meridian to which it belongs. The differ- ence between the amounts of myopia in the two principal meridians is the amount of astigmatism. The general plan of measuring astigmatism by skiascopy is to ascertain the point of reversal and measure the degree of myopia for each principal meridian separately; and then, by subtracting