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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    ing with the different distances of the objects ; both of which probably take place, as will hereafter appear.* Having then established these as our premises, we shall next examine the different principles which have been em- ployed for explaining vision at different distances. Most writers upon this subject refer this power of the eye to the contraction and dilatation of the iris. Within certain limits this would, upon first examination, as already observed, appear to be the case, since the pupil enlarges as the object is further removed from the eye, and again contracts as it is brought near. The extent of this principle I have already pointed out; but I suspect we also err in attributing to the difference of distance what are only effects of different quan- tities of light, a circumstance in which it is the more easy to commit error, as they are generally proportionate one to the other ; i. e. as the object is near we require a less degree of light, and to exclude what is superfluous the iris contracts ; but as it is more distant, a greater quantity of light becomes necessary, and the iris dilates : thus far we see the use of the enlargement or diminution of the pupil, as the object is more or less distant. But distinct vision does not consist in the quantity of light alone, though too much or too little would obscure the image. It is also necessary that the rays which flow from the ob- ject should fall upon the retina in a certain direction, to form a distinct picture ; but surely the greater or less quantity of light, the greater or less number of rays, which it is only * Facile enim intelligitur, quo longius radii advcniunt, eo migis esse paralle- ls; eo minus ergo differre ab axi, et eo minoribus viribus cornea et lentis crys- tal! inac in focum coji. Ut enim corpus magis distat, ita sub minori angulo radii adveniunt. Contra si corpus conspicuum valde vicinum fuerit, radiorum ab eo advenientium angulus est major, et adeo magis divergentes in oculum incidunt, et viribus egent refringentibus majoribus omnibus densioribus. Haller. Elcm. Phys. lib. xvi.