A Manual Of Photography

146 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. the white precipitate alluded to, it operates another effect, which is the wonderful property of attracting the vapour of mer- cury. This vapour is condensed in the form of a white powder, having also, when examined by the microscope, the appearance of reflecting crystals.,,.... It is probable that light exercises a two-fold action on the iodide of silver whether it is combined or not with chlorine or bromine. By one, the iodide is decomposed, and the silver set free is pre- cipitated on the surface in the form of a white powder or small crystals ; by the other, which begins long before the former, the parts affected by light have been endowed with an aümity for mercurial vapour. , ..... By means of his photographometer, this investigator has been able to ascertain that the pure light of the sun performs m about two or three seconds the decomposition of the bromo- iodide of silver, which is manifested by the white precipitate; while the same intensity of light determines the affimty tor wer- curial vapour in the short space of about part of a second. So that the affinity for mercury is imparted by an intensity of light 3000 times less than that which produces the decomposi- tion manifested by the white precipitate. For this reason it is difficult to suppose that the two actions are the same. We must admit that they are different. Long before it can effect the decomposition of the surface, light im- parts to the sensitive coating the affinity for mercurial vapour ; and this appears to be the principle of the formation of the image in the daguerreotype process. In a paper communicated to the Loyal Society on the 17th of June, 1847, M. Claudet stated that the red, orange, and yellow rays were destroying the action of white light, and that the surface was recovering its former sensitiveness or unaffected state after having been submitted to the action of these rays. It was inferred from that curious fact that light could not have decomposed the surface ; for if it had, it would be difficult to understand how the red, orange, or yellow rays could combine again, one with another, elements so volatile as bromine and iodine, after they had been once separated from the silver. These experiments have much in common with those of M. Edmond Becquerel, who has been led to a division of the spec- trum into exciting rays and continuating rays. But he had not yet been able to ascertain that, when light has decomposed the bromo-iodide of silver, the red, orange, or yellow rays canno restore the surface to its former state. The action of ligli, which can be destroyed by the red, orange, or yellow rays, does not determine the decomposition, which would require an inten-