A practical guide to the study of the diseases of the eye : their medical and surgical treatment / by Henry W. Williams.

  • Williams, Henry W. (Henry Willard), 1821-1895.
Date:
1873
Catalogue details

Licence: Public Domain Mark

Credit: A practical guide to the study of the diseases of the eye : their medical and surgical treatment / by Henry W. Williams. Source: Wellcome Collection.

Provider: This material has been provided by the Harvey Cushing/John Hay Whitney Medical Library at Yale University, through the Medical Heritage Library. The original may be consulted at the Harvey Cushing/John Hay Whitney Medical Library at Yale University.

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    clearly seen, while those at right angles with the first will appear blurred or of double contours. On looking through a stenopajic disc, (a metal plate perforated with a narrow slit,) held in a proper direction, the confusion disappears. This forms, perhaps, the readiest method of deter- mining the direction of astigmatism. Its degree is ascertained by placing convex or concave glasses be- fore the slit, until we find with what number vision is clearest. The glasses adapted to neutralize this abnormal re- fraction are cylindrical in curvature, instead of being ground to a uniform convexity or concavity. Such a glass causes no deviation of rays in the plane of its axis, and this should therefore be placed to correspond with the meridian in which refraction is normal. Rays passing through other planes of the lens will be re- fracted,— those most of all which pass through the meridian at a right angle to the axis, — which is pre- cisely the line of greatest astigmatism. The glasses must be convex cylindrical or concave cylindrical, according as the astigmatism is hyperme- tropic or myopic. If we have no normal meridian, bi-cylindrical or sphero-cylindrical glasses must be employed, the curvature of the two meridians being adapted for those needing rectification in the eye in question. Bi-cylindrical glasses are appropriate to cases of mixed astigmatism, where one meridian is myopic, the other hypermetropic ; sphero-cylindrical