The phenomenon of the continuing action of the red, orange, or yellow rays, on the simple compound of iodide of silver, was discovered by M. Ed. Becquerel ; and soon after M. Gaudin found, that not only those rays continue the action by which mercury is deposited, but that they develope without mercury an image having the same appearance as that produced by mercurial vapour.
M. Gaudin, not having observed the fact of the white precipitate, which is the result of the decomposition by the action of light, could uot explain the cause of the image brought out under the influence of the yellow ray.
M. Claudet states that the iodide of silver without bromine is about 100 times more sensitive than the bromo-iodide to the action of the rays which produce the decomposition of the compound forming the white precipitate of silver, while it is 100 times less sensitive for the effect which gives the affinity for mercury; It may be that, in the case of the iodide of silver alone, the decomposition being more rapid, and the affinity for mercury slower than when bromine is added to the compound, the red, orange, or yellow rays having to act upon an incipient decomposition, have the power, by their own photogenic influence, of continuing the decomposition when it has begun. This may explain the development of the image under red, orange, or yellow glasses, according to M. Gaudin's discovery. But in the case of the bromo-iodide of silver, the red, orange, or yellow rays have to exert their action on the affinity for mercury, begun a long time before the decomposition of the compound; and they have the property of destroying that affinity.
So that it would appear that all the rays of light have the property 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 rays continue 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. This would explain the various phenomena of the formation of the two different deposits, and also the anomaly of the continuation of the action by the red, orange, or yellow rays, according to M. Ed. Becquerel's discoveries on the iodide of silver ; and of the destruction 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 refrangible rays, of imparting the affinity for mercurial vapour on both the iodide and bromo-iodide of silver ; and they destroy that affinity when it has been produced on the bromo-iodide of 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.
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 half 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 fully represented, and in nearly as short a time, as when no blue glass had been interposed.
Thus we might construct a room lighted only through an inclo-sure of pale yellow glass, in which light would be very dazzling to the eye, and in this room no photographic operation could be performed ;* or in a room inclosed by deep blue glass, which would appear very dark, and in which the photographic operation would be nearly as rapid as it would be in open air.
Thus we may conceive certain states of the atmosphere under which there will be an abundance of illuminating rays, and very few actinic rays ; and some others, under which the reverse will take place. Considering how difficult it is to judge by the eye alone of the chemical state of light, we can understand why the photographer is constantly deceived in the effect he tries to produce, having no means to ascertain beforehand, with any degree of certainty, the intensity of light. For these reasons M. Claudet turned his attention to contrive an apparatus by which he could test at the same time the sensitiveness of the daguerreotype plate and the intensity of light. This instrument he called a Photographometer.
" By this instrument," says the inventor, " I have been able to discover at what degree of intensity of light the effect called solarization is produced : on well-prepared plates of bromo-iodide it does not begin under an intensity 512 times greater than that which determines the first effect of mercury; and also at what degree the decomposition producing the white precipitate without mercury manifests itself, both on iodide and on bromo-iodide of silver. On the first, it is 100 times quicker than on the brorno-iodide: and on the last it is produced by an intensity 3000 times greater than that which developes the first affinity for mercury.