An additional method to determine the degree of ametropia.

  • Thomson, William, 1833-1907.
Date:
[1871]
Catalogue details

Licence: Public Domain Mark

Credit: An additional method to determine the degree of ametropia. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    AN ADDITIONAL METHOD TO DETERMINE THE DEGREE OF AMETROPIA. In the American Journal of the Medical Sciences for January, 1870, will be found a description of a test for ametropia, based on the experiment of Scheiner. It was there shown that when- ever the visual axis is too long or too short, a point of light, used as a test object, will appear double to the eye of an ob- server when it is examined through two small perforations in an opaque screen. A short experience with the disk clinically, or with ametro- pia artificially produced, convinces one that the degree of ame- tropia influences the apparent distance apart of the double lights, and that if that distance could be computed with accu- racy, the degree of ametropia could be diagnosticated, and a correcting lens selected without a prolonged empirical exami- nation. After a sufficiently full demonstration of the laws of refrac- tion as applied to the human eye, and an exhaustive analysis of its refracting surfaces of different curvature and its media of varied density, Bonders gives his authority, in addition to that of Helmholtz, for the adoption, for purposes of calculation, of the simplified diagrammatic eye of Listing, which consists, as is well known, of a single refracting surface of 5 mm. radius in curvature, and a medium whose index of refraction is 1.833, whose posterior focus is 20 mm., and its anterior focus 15 mm. from its refracting surface. From a comprehension of these cardinal points, it becomes possible to trace, in this simple eye, the course of any given ray of light; to find the points of conjugate foci; to ascertain the dimensions of retinal images, or of circles of diffusion which fall upon the retina in ametropia; and these values thus found may be considered equal to those which would exist under the same circumstances in the human eye, with its complicated ap- paratus, composed of two complete dioptric systems.