An additional test for the diagnosis and correction of the optical defects of the eye / by William Thomson.

  • Thomson, William, 1833-1907.
Date:
1870
Catalogue details

Licence: Public Domain Mark

Credit: An additional test for the diagnosis and correction of the optical defects of the eye / by William Thomson. 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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    [Extracted from the American Journal of the Medical Sciences for January, 1870. J AN ADDITIONAL TEST FOR THE DIAGNOSIS AND CORRECTION OF THE OPTICAL DEFECTS OF THE EYE. Assist. Surg. to Wills Hospital for Diseases of the Eye, and late Assist. Surg. U. S. A. Before any object can be seen distinctly, the rays of light which are emit- ted or reflected from it, must be so refracted by the cornea, lens, and humours of the eye, as to cast upon the layer of the rods and cones of the retina its inverted and perfectly defined image. If the eye be regarded as an optical instrument composed of a refracting apparatus, the cornea and lens, and a receptive screen, the retina, it needs no argument to prove that there must exist a harmony between the refrac- tive power and the antero-posterior axis, to secure perfect definition. The human eye may be thus described as a camera of one inch in length antero-posteriorly, with refracting surfaces which are equal to a lens of one inch in focal length : and only when the rods and cones of the retina are situated precisely at the principal focal point of the refracting combination, do the images of distant objects fall upon them, so as to be fully appre- ciated by the brain. Any variation in the length of the visual axis, with- out a corresponding change in the refractive apparatus, would render the retinal images indistinct, and would result in imperfect vision. It may be considered as demonstrated by actual measurement, as well as by mathematical analysis, that the length of the visual axis does frequently depart from the normal standard, and that to this mechanical and anato- mical fact, the defective vision in such instances may be ascribed. To the eye whose refractive system and visual axis harmonize, Bonders has given the name Emmetropic, and to the eye which departs from this happy con- struction, Ametropic. The ametropic eye may have a visual axis either too long, when it becomes Myopic,or too short, when it is called Hypermetropic. In myo- pia, the retina is too remote from the cornea and lens, and the images of distant objects are formed before it, and are indistinctly seen : its point of perfect vision is at a finite distance, which may, in extreme cases, be placed at one or two inches only anterior to its cornea. In hypermetropia, the axis is too short, and the images would fall behind the retina; objects are clearly seen, only when the refracting power of the crystalline lens is increased by a contraction of the ciliary muscle, and the focus of the refracting combi- nation thereby shortened, to correspond with the diminished antero-poste- rior axis. The following table, for example, will express numerically these diversi- ties, as ascertained by the best authorities :— Emmetropic. Average leugtli of axis . . . 22.23 millimetres. Myopic. Possible “ “ ... 25 “ Hypermetropic. “ “ “ ... 20 “ Astigmatism is but a further subdivision of ametropia, in which one or both of the simpler forms may be found in the same eye, which may then By william THOMSON, M. D.,