The means by which the eye adapts itself to distinct vision, at different distances / by Charles W. Wright.

  • Wright, Charles W., M.D.
Date:
1859
Catalogue details

Licence: Public Domain Mark

Credit: The means by which the eye adapts itself to distinct vision, at different distances / by Charles W. Wright. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    by the iris alone, without moving the glasses either forward or backward, in viewing objects at different distances. If the lens adjusted the eye for variable focal distances, glasses, which serve the purpose of lenses, would have to be shifted backwards and forwards, like the tube in a camera obscura, accordingly as a distant or near object was brought into focus. In fact an indi- vidual who has been operated upon for cataract can enjoy a considerable range of distinct vision by the lenticular properties of the iris alone. From its mechanical relations to the lens, the ciliary muscle cannot change the position of that medium, and the only office of that muscle seems to be that of regivlating the quantity of blood sent to the iris and choroid coat, by which the sensibility of the latter, and the retina, to the rays of the light, is increased or diminished. It is stated by some writers that the adapting power of the ciliary muscle, is destroyed by belladonna, which paralyzes it. Now, belladonna paralyzes the circular fibres of the irisT and does not affect the radiating fibres of that structure, whereby presbyopia is temporarily produced. In such cases an artificial iris, constructed of perforated card, will enable a person whose pupil is dilated by belladonna to see distinctly at the usual focal distance. On the other hand, opium, by paralyzing the radiating fibres of the iris, and not exerting any effect on the circular ones, induces temporary myopia.