These remarks apply to Photographs taken by sunlight. Mr. Crookes states that in working with artificial light, such as gas or camphine, the case is different. Actinic rays of high refrangibility are comparatively wanting in gas-light, the great bulk of the Photographic rays beings found to lie within the limits of the visible spectrum, and consequently acting more energetically upon Bromide than on Iodide of Silver.
Explanation of the mode in which Coloured Objects impress the Sensitive Film.—The fact of which we have been speaking, viz. that the natural colours are not always correctly represented in photography, is often urged in depreciation of the art,—"when lights, are represented by shadows," it is said, "how can a truthful picture be expected?" The insensitiveness of Iodide of Silver to the colours occupying the lower portion of the spectrum would indeed present an insuperable difficulty if the tints of Nature were pure and homogeneous: such however is not the case. Even the most sombre colours are accompanied by scattered rays of white light in quantity amply sufficient to affect the sensitive film.
This is especially seen when the coloured body possesses a good reflecting surface; and hence some varieties of foliage, as for instance the Ivy, with its smooth and polished leaf, are more easily photographed than others. So again with regard to drapery in the department of portraiture—it is necessary to attend not only to the colour, but also to the material of which it is composed. Silks and satins are favourable, as reflecting much light, whilst velvets and coarse stuffs of all kinds, if at all dark, produce very little effect upon the sensitive film.
SECTION V.
On Binocular Vision and the Stereoscope.
An object is said to be "stereoscopic" (στρεοϛ solid, and σκοπεω, I see) when it stands out in relief, and gives to the eye the impression of solidity.
This subject was first explained by Professor Wheatstone in a memoir on binocular vision, published in the 'Philosophical Transactions' for 1838; in which he shows that solid bodies project different perspective figures upon each retina, and that the illusion of solidity may be artificially produced by means of the "Stereoscope."
The phenomena of binocular vision may be simply sketched as follows:—If a cube, or a small box of an oblong form, be placed at a short distance in front of the observer, and viewed attentively with the right and left eye separately and in succession, it will be found that the figure perceived in the two cases is different; that each eye sees more of one side of the box, and less of the other; and that in neither instance is the effect exactly the same as that given by the two eyes employed conjointly.
A silver pencil-case, or a pen-holder, may be used to illustrate the same fact. It should be held at about six or eight inches distant from the root of the nose, and quite at right angles to the face, so that the length of the pencil is concealed by the point. Then, whilst it remains fixed in this position, the left and right eye are to be alternately closed: in each case a portion of the opposite side of the pencil will be rendered visible.