On the phenomena and mechanism of the focal adjustment of the eye to distinct vision at different distances / by Professor Allen Thomson.

  • Allen Thomson
Date:
[1858?]
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Licence: Public Domain Mark

Credit: On the phenomena and mechanism of the focal adjustment of the eye to distinct vision at different distances / by Professor Allen Thomson. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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    ON THE PHENOMKNA AND MECHANISM OP THE FOCAL ADJUSTMENT OF THE EYE TO DISTINCT VISION AT DIFfEEENT DISTANCES. Br Professor ALLEN THOMSON, M.D., F.R.SS. L. & E. The explanation of the mechanism by which the eye may undergo a change of its focal adjustment, or be accommodated to the dis- tinct vision of objects at different distances, has occasioned much difficulty and discussion among optical physiologists. Recent researches appear to have thrown considerable light upon this subject; and as I have been led to repeat and extend some of the experiments which bear upon it, I venture to lay before the reader some account of the present state of this interesting question. It appears strange that some should have doubted the existence ol any power of focal adjustment in the eye, and should have denied the necessity of the occurrence of such a change. The error ot this opinion, and the fact that an internal change in the refract- ing media is required in order to render distinct the vision of objects situated at different distances from the eye, may be satisfactorily proved by several very simple experiments. 1. If, for example, closing one eye, we fix the other alternately on a near and a distant object placed in the same line of vision it will be found that, when the distant object is viewed, the image of the near one is confused, and when, by an effort, we bring the eye to see the near one distinctly, we have no distinct perception, but only a general vision of the more distant object upticum, &c 1619), we view with one eye the head of a pin or some smal distinct body, through two minute holes in ab oard placed at a less distance from each other than the width of the pupil' whicttL '^'''''^ distance-whichis tK which the eye is most easily adjusted—at which the pin head an 111 I ^i^^'^i' ?P^^*^^ ^^^^^ tll^im^g^ appears l^irZ T ^^^^^ varies within a certSn Unge, IS obvt^fc f' ^J^^'T^ °f ^J^' The reason of this IS obvious from a consideration of the nature of the experiment at a certain amount of divergence, and at a certain distance the<,P may be brought to fall on the same part of the retina, so as to ft m a
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