The hand: its mechanism and vital endowments as evincing design / By Charles Bell.

  • Bell, Charles, Sir, 1774-1842.
Date:
1837
Catalogue details

Licence: Public Domain Mark

Credit: The hand: its mechanism and vital endowments as evincing design / By Charles Bell. Source: Wellcome Collection.

    380/400 (page 352)
    paper will seem yellow. These phenomena are to be explained by considering, that the nerve is exhausted by the continuance of the impres- sion, and becomes more apt to receive the sen- sation of an opposite colour. All the colours of the prism come into the eye from a surface that is white : accordingly, when we remove the coloured wafer, (take that which was red) from the white paper, all these combined colours of the prism enter: but if the nerve has been ex- hausted by the red colour of the wafer, it will be insensible to the red rays, when reflected from the paper; and the effect of the rays of an op- posite kind will be increased : consequently, the spot will be no longer white, nor red, but of a green colour. Let us next observe how this exhaustion of the sensibility of the nerve produces an effect in en- graving ; where there is no colour, and only light and shade. Is it possible that a high tower, in a cloudless sky, can be less illuminated at the top, than at the bottom ? Yet if we turn to a book of engravings, where an old steeple, or tower, is represented standing up against the clear sky, we shall find that all the higher part is dark ; and the effect is picturesque and pleas- ing. Now this is perfectly correct; for although the highest part of the tower be in the brightest illumination, it is not seen as if it were—it never appears so to the eye. The reason is, that on