A Manual Of Photography

Mi 148 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHE. „à of decomposing the iodide of silver in a longer or shorter time as they have that of producing the affinity for mercury on the bromo-iodide of silver: with the difference, that on the former compound the separate actions of the several! rave. con- tinue each other, and that on the second compound these separate actions destroy each other. We can understand that in the first case, all the rays are capable of operating the same decomposition ; and that in the second, the affinity for mercury when imparted by one ray is destroyed by another. U>>sw°u^ explain the various phenomena of the formation of the two different deposits, and also the anomaly of the continuation ot the action by the red, orange or yellow rays, according to M Ed. Becquerel's discoveries on the iodide of silver ; and of the de- struction of that action by the same rays according to M. Claudet’s observations on the bromo-iodide of silver. The red, orange, and yellow rays, when acting on an unaffected surface, are considerably less capable than the most refran- Sble rays of imparting the affinity for inercuria vapour on both the iodide and bromo-iodide of silver ; and they destroy that affinity when it has been produced on the silver by the photogenic rays. It follows from this fact, that when the red, orange, or yellow rays are more abundant in the light than the most refrangible rays, the photogenic effect is retarded in proportion to the excess of these antagonistic rays. This happens when there exist in the atmosphere some vapours which absorb the most refrangible rays. In these circumstances the light appears rather yellow ; but it is very difficult to judge by the eye of the exact colour of the light, and of the proportion of photogenic rays existing in the atmosphere at any given moment. , ,. r The vapours of the atmosphere which render the light yellow, act as any other medium intercepting the blue rays, and those which have the same degree of refrangibility. If we cover an engraving one-half with light yellow glass, and place it before a camera obscura, in order to represent the whole on a daguerreotype plate we shall find that during the time which has been necessary to obtain the image of the halt not covered, not the slightest effect has been produced on the half covered with the yellow glass. Now, if we cover one-half with deep blue glass, and the other with the same light yellow glass, the engraving will be seen very distinctly through the yellow glass, and not at all through the blue. In representing the whole, as before on the daguerreotype plate, the half which was clearly seen by the eye has produced no effect; and in the other, which could not be seen, is as fui y