Two essays : one upon single vision with two eyes ; the other on dew : a letter to the Right Hon. Lloyd, Lord Kenyon, and an account of a female of the white race of mankind, part of whose skin resembled that of a negro ; with some observations on the causes of the differences in colour and form between the white and negro races of men / by the late William Charles Wells, with a memoir of his life, written by himself.

Date:
1818
Catalogue details

Licence: Public Domain Mark

Credit: Two essays : one upon single vision with two eyes ; the other on dew : a letter to the Right Hon. Lloyd, Lord Kenyon, and an account of a female of the white race of mankind, part of whose skin resembled that of a negro ; with some observations on the causes of the differences in colour and form between the white and negro races of men / by the late William Charles Wells, with a memoir of his life, written by himself. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    know, only two theories offered respecting the apparent directions of objects. One is, that they are perceived in the direction of lines pass- ing from their pictures on tlie retina, through the centre of the eye; the other, that their apparent directions coincide with their visual rays*. But both of these theories are inconsist- ent with the phenomena of single vision with two eyes. For according to neither of them can an object, placed at the concourse of the optic axes, be seen single, unless we have a most ac- curate knowledge of its distance ; nor will either admit two objects to be seen as one, which are situated in the optic axes, whether on this side, or beyond where they meet, unless the united object be referred by sight to their very point of intersection ; both of which conclusions are contradicted by experience. It is evident, therefore, that some other theory of visible di- rection is required, which shall not be liable to * Mr. D’Alembert has said (Opuscules Mathematiques, Tom. I. p. 265) that all optical writers before him had re- garded it as an axiom, that every visual point is seen in the direction of its visual ray. But the assertion is not well founded. For Kepler long ago taught (Paralipomena in Vitellionem, p. 173)> that objects are perceived in lines pass- ing from their pictures upon the retina, through the centre of the eye j in which he was followed by Dcchales and Doctor Porterfield ; to the latter of whom Dr. Reid improperly at- tributes the discovery of the same supposed law. /