Chemical science. On the Coloured Photographic Image of the Solar Spectrum.

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MDCCCXLIX (1849)By M. Edmond Becquerel.

The author, in the course of his researches upon the chemical action of Light, was led to this remarkable fact, that the Solar Spectrum could form its image with colours corresponding to its own, upon a plate of silver properly prepared. For this purpose, the plate may be attacked by free chlorine, with the precautions indicated in the note presented to the Academy: the sensitive coating which is formed upon the surface of the plate is red in the prismatic red, yellow in the yellow, green in the green, blue in the blue, and violet in the violet. The reddish tint turns to purple in the extreme red, and extends even beyond Fraunhofer's line A; as to the violet, it continues far beyond A, gradually becoming more feeble. When the action of the spectrum is permitted to last a long time, the tints become dark, and the image finally takes the metallic lustre: the colours have then disappeared.

According to the preparation of the plate and the thickness of the sensitive coating, any of the tints of the spectrum may be made to predominate: thus, a surface well prepared, and previously in diffused light coloured purple under a deep red glass, gives a beautiful coloured photographic image of the spectrum, in which the orange, yellow, the green, and the blue, are marked with the greatest clearness. The substance formed upon the surface of the silver is not the white chloride, but probably a subchloride, since it is not strongly coloured beyond the visible violet, as the chemically precipitated chloride is, and the maximum of action is found in the yellow, where the maximum of luminous intensity is, or moves towards the red, according to the preparation to the plate. To get a tolerably rapid action, it is necessary to use a strongly concentrated spectrum. These effects explain the red colour of the chloride of silver, and of the sensitive paper formed with that compound, in the red rays, which has been already observed by MM. Seebeck and Herschel.

The author has succeeded in preparing, by means of free chlorine, and also by using bichloride of copper, a sensitive coating of the chloride of silver, so impressed that now only certain parts of the spectrum are represented with their colours; but besides, white light makes a white impression.

The compound formed upon the surface of the silver by the action of chlorine, is the only one hitherto found which shows the properties here mentioned. Up to the present time, it appears necessary to keep the coloured prismatic images in the dark, and the author has not found the means of fixing it under the influence of light. If the fixation could be accomplished, and if the sensitiveness of the material was greater, we could not only draw, but also paint by light; nevertheless, the results mentioned show that the solution of the problem is possible.