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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    adapt itself to viewing objects distinctly which are situated at different distances from the ob- server, I think there cannot be a doubt. Thus, in a uniform light, the eye can be made to see objects distinctly at different distances, by view- ing them through apertures of varying diame- ters, the apertures being diminished as the ob- jects approach the eye. This operation may be carried to such a degree, by using apertures sufficiently small, that a person will be enabled to read printed matter which is held within a half inch of the cornea. A similar experiment can be made with the iris itself, by allowing the sun's rays to fall directly upon the printed page, when it may be brought in contact with the eye- lashes, and the words will be distinctly legible. In such an experiment the pupil is contracted to a diameter less than that of the head of a com- mon pin. Opium will also shorten the focal distance, by contracting the iris, and belladonna, pink-root, and hyosiamus, produce the opposite effect, by dilating the pupil. The pupil is always observed to expand when a distant object is viewed, and to contract as one near the eye is observed. Both eyes are never used simultaneously in the same individual, in the careful examina- tion of an object, but are employed alternately. If the object examined is situated to the left of the observer, the corresponding eye is used, if to the right, the right eye is employed. Hence the pupils of the eyes of the same individual