The picture being now finished, is preserved from dust by placing it in a frame, and covering it with glass. In every successful operation, the picture is almost as perfect in its details as that of the camera obscura itself; but, as the light of the sun is only white, there can be, of course, none of the varied tints of nature. The shades are supplied by the black polish of the metallic surface which, when it reflects a luminous object, the white vapour of the mercury appears in shade, and thus gives us either a positive or a negative picture, according to the light in which it is viewed. Various improvements have gradually been made in the processes of the Daguerreotype and the Talbotype, which our limited space forbids us to describe. Mr. Beard has added colour to his Daguerreotype portraits, which is uniform and so transparent as not to affect the likeness in any degree, while the life-like effect is greatly heightened. M. Claudet has found that, when the sun is rendered red by the vapours of the atmosphere, it not only produces no effect upon the Daguerreotype plate, but that it destroys the effect previously produced by the white light. If the image of the red sun be taken in the camera obscura, it produces upon the Daguerreotype plate a black image. By covering a Daguerreotype plate previously affected by light with a red, orange, or yellow glass, the radiation through these coloured media has also the property of destroying the action produced by white light. The most interesting part of M. Claudet’s statement refers to the fact that, after the destroying action of the red, orange, and yellow radiations, the plate is restored to its former sensitiveness; so that, after having been affected by white light, and restored by the destructive action of the red, orange, and yellow radiations, it is possible to produce a photographic effect, as upon a plate just prepared with iodine and bromine. This alternate acting and destroying action may be repeated _ad infinitum_, without altering the final state of the plate. This curious fact proves, evidently, that, in the Daguerreotype process, light does not alter the chemical compound on the plate, and that the affinity for mercury is the result of some new property imparted by the action of the rays of light. M. Claudet’s experiments prove, also, that the red and yellow rays are endowed with a photographic action of their own, which, as well as that of the blue and violet rays, gives an affinity for mercurial vapour. The photographic action of the red ray is destroyed by the yellow, that of the yellow by the red; the red and yellow destroy the photographic action of the blue, and the blue destroys the action of the others. The photographic, or the destroying action of any particular ray cannot be continued by any other. It appears, therefore, that each radiation changes the state of the plate, and each change produces the sensitiveness to mercurial vapour when it does not exist, and destroys