The least refrangible part of the solar spectrum possesses also, under certain circumstances, a bleaching property, by which the metallic salts are restored to their original whiteness after being blackened by exposure to common daylight, or to the most refrangible rays of the solar spectrum.
Paper prepared with iodide of silver, when washed over with ferrocyanite of potash, blackens rapidly when exposed to the solar spectrum. It begins in the violet rays and extends over all the space occupied by the dark chemical rays, and over the whole visible spectrum down to the extreme red rays. This image is coloured, the red rays giving a reddish tint and the blue a blueish. In a short time a bleaching process begins under the red rays, and extends upwards to the green, but the space occupied by the extreme red is maintained perfectly dark. Mr. Hunt found that a similar bleaching power is exerted by the red rays on paper prepared with protocyanide of potassium and gold with a wash of nitrate of silver.
The application of a moderately strong hydriodate of potash to darkened photographic paper renders it peculiarly susceptible of being whitened by further exposure to light. If paper prepared with bromide of silver be washed with ferrocyanate of potash while under the influence of the solar spectrum, it is immediately darkened throughout the part exposed to the visible rays down to the end of the red, some slight interference being perceptible about the region of the orange and yellow. After this a bleaching action begins over the part occupied by the red rays, which extends to the green. By longer exposure an oval spot begins again to darken about the centre of the bleached space; but, if the paper receive another wash of the hydriodate of potash, the bleaching action extends up from the green, over the region occupied by the most refrangible rays and considerably beyond them, thus inducing a negative action in the most refrangible part of the spectrum.
In certain circumstances the red rays, instead of restoring darkened photographic paper to its original whiteness, produce a deep red colour. When Sir John Herschel received the spectrum on paper somewhat discoloured by exposure to direct sunshine, instead of whiteness, a red border was formed extending from the space occupied by the orange, and nearly covering that on which the red fell. When, instead of exposing the paper in the first instance to direct sunshine, it was blackened by the violet rays of a prismatic spectrum, or by a sunbeam that had undergone the absorptive action of a solution of ammonia-sulphate of copper, the red rays of the condensed spectrum produced on it, not whiteness, but a full and fiery red, which occupied the whole space on which any of the visible red rays had fallen; and this red remained unchanged, however long the paper remained exposed to the least refrangible rays.