Observations on vision : ... Read before the Royal Society of London, May 1, 1794, and published in their Transactions of the same year. Communicated to the Society by George Pearson.

  • Hosack, David, 1769-1835.
Date:
1813
Catalogue details

Licence: Public Domain Mark

Credit: Observations on vision : ... Read before the Royal Society of London, May 1, 1794, and published in their Transactions of the same year. Communicated to the Society by George Pearson. 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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    account for the phenomena in question. Accordingly, ana- tomists and philosophers, who have written upon this subject, have generally had recourse to this explanation. Amusing myself with these changes of the pupil, as a mat- ter of curiosity, by presenting to the eye different objects at different distances, I soon perceived that its contraction and dilatation were irregular and more limited than had been supposed: i. e. that approaching the object nearer the eye, within a certain distance, the pupil not only ceased to con- tract, but became again dilated; and that beyond a few yards distance, it also ceased to dilate : these circumstances im- mediately occurred as objections to the above explanation ; for were it from the contraction and dilatation of the iris alone that we see objects at different distances, I naturally concluded it should operate regularly to produce its effects; but if to view an object at a (ew yards distance it be enlarg- ed to the utmost extent, surely it must of itself be insufficient to view one at the distance of several miles ; for example, the heavenly bodies. Another difficulty here presents itself: in viewing the sun, instead of dilating, according to the distance, it contracts, obe\ ing rather the quantity, or intensity of the light, than the distance of the object. Knowing no other obvious pow- er in the eye itself of adapting it to the different distances of objects, it occurred to me to inquire, whether the combined action of the external muscles could not have this effect. I first proposed this query to an optician of eminence in Lon- don,* and who has written expressly on this subject. I re- peated the same question to a celebrated teacher of anato- my.f Encouraged by their replies, 1 have since attended more particularly to the subject, and hope my inquiries have * Mr. George Adams. t Dr. Andrew Marshall, of Thavies Inn.