Page:Proceedings of the Royal Society of London Vol 4.djvu/156

Rh Confining his attention, in the present notice, to the emplojrment 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 w^ater will fix the photograph, by washing out the nitrate of silver, but the tint of the picture result- ing is brick-red ; but the black colour may be restored by washing it over with a weak solution of hyposulphite of ammonia.

The author found that paper impregnated with the chloride of silver vv'as 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 which impart much greater sensibility to the paper on which they are appHed : such as the carbonate, the nitrate, and the acetate. The nitrate requires to be perfectly neutral ; for the least 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 tranfers, 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 repro- duce 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 au- thor was led to notice some remarkable facts relating to the action of the chemical rays. He ascertained that, contrary to the prevail- ing 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 ex- periments 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, different 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 aU the parts of a pure spectrum, not formed by material prisms, and he points out, for that purpose, one formed in Fraunhofer's me- thod, by the interference of the rays of light 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 Her-