14 de Março
ABSTRACTS OF THE PAPERS,
PRINTED IN THE PHILOSOPHICAL TRANSACTIONS
ROYAL SOCIETY OF LONDON
Pag. 131, 132, 133
PROCEEDINGS OF THE ROYAL SOCIETY
A paper was also read, entitled, “ Note on the Art of Photography, or the application of the Chemical Rays of Light to the purposes of Pictorial Representation.” By Sir John F. W. Herschel, Bart., K.H., V.P.R.S., &c.
The author states that his attention was first called to the subject of M. Daguerre’s concealed photographic processes, by a note from Captain Beaufort, dated the 22nd of January last, at which time he was ignorant that it had been considered by Mr. Talbot, or by any one in this country. As an enigma to be solved, a variety of processes at once presented themselves, of which the most promising are the following; 1st, the so-called de-oxidizing power of the chemical rays in their action on recently precipitated chloride of silver; 2ndly, the instant and copious precipitation of a mixture of a solution of muriate of platina and lime-water by solar light, forming an insoluble compound, which might afterwards be blackened by a variety of agents; 3rdly, the reduction of gold in contact with deoxidizing agents; and, 4thly, the decomposition of an argentine compound soluble in water, exposed to light in an atmosphere of peroxide of chlorine, either pure or dilated.
Confining his attention, in the present notice, to the employment of chloride of silver, the author inquires into the methods by which the blackened traces can be preserved, which may be effected, he observes, by the application of any liquid capable of dissolving and washing off the unchanged chloride, but of leaving the reduced, or oxide of silver, untouched. These conditions are best fulfilled by the liquid hyposulphites. Pure water will fix the photograph, by washing out the nitrate of silver, but the tint of the picture resulting is brick-red; but the black colour may be restored by washing it it over with a weak solution of hyposulphite of ammonia.
The author found that paper impregnated with the chloride of silver was only slightly susceptible to the influence of light: but an accidental observation led him to the discovery of other salts of silver, in which the acid being more volatile, adheres to the base by a weak affinity, and wich impart much greater sensibility to the paper on which they are applied: such as the carbonate, the nitrate, and the acetate. The nitrate requires to be perfectly neutral; for theleast excess of acid lowers in a remarkable degree its susceptibility.
In the application of photographic processes to the copying of engravings or drawings, many precautions, and minute attention to a number of apparently trivial, but really important circumstances, are required to ensure success. In the first transfers, both light and shadow, as well as right and left, are the reverse of the original: and to operate a second transfer, or by a double inversion to reproduce the original effect, is a matter of infinitely greater difficulty; and in which the author has only recently ascertained the cause of former failures, and the remedy to be applied.
It was during the prosecution of these experiments that the author was led to notice some remarkable facts relating to the action of the chemical rays. He ascertained that, contrary to the prevailing opinion, the chemical action of light is by no means proportional to the quantity of violet rays transmitted, or even to the general tendency of the tint to the violet end of the spectrum: and his experiments lead to the conclusion that, in the same manner as media. Have been ascertained to have relations sui generis to the calorific rays, not regulated by their relations to the rays of illumination and of colour, they have also specific relations to the chemical spectrum, ditferent from those they bear to the other kinds of spectra. For the successful prosecution of this curious investigation, the first step must consist in the minute examination of the chemical actions of all the parts of a pure spectrum, not formed by material prisms, and he points out, for that purpose, one formed in Fraunhofer’s method, by the interference of the rays of liglit themselves in passing through gratings, and fixed by the heliostat.
He notices a curious phenomenon respecting the action of light on nitrated paper; namely, its great increase of intensity, under a certain kind of glass strongly pressed in contact with it; an effect which cannot be explained either by the reflection of light, or the presence of moisture; but which may possibly be dependent on the evolution of heat.
Twenty-three specimens of photographs, made by Sir John Herschel, accompany this paper: one, a sketch of his telescope at Slough, fixed from its image in a lens: and the rest copies of engravings and drawings, some revers, or first transfers; and others second transfers or re-reversed pictures.