A Manual Of Photography

Hg HISTOET OF PHOTOGRAPHY. pictures obliterated. They were then exposed to free air, the one in the dark, the other in sunshine. Both recovered, but the former much more slowly than the latter. The restoration of the picture exposed to the sun was completed in twenty-four hours, that in the dark not till after a lapse of two or three days. “ A slip of the stained paper was wetted with liquid sulphurous acid, and laid on blotting-paper similarly wetted. Being then crossed with a strip of black paper, it was laid between glass plates and (evaporation of the acid being thus prevented) was exposed to full sunshine. After some time, the red colour (in spite of the presence of the acid) was considerably restored in the portion exposed, while the whole of the portion coA ered by the black paper remained (of course) perfectly vhite. “ Slips of paper, stained as above, were placed under a receiver, beside a small capsule of liquid sulphurous acid. When com- pletely discoloured, they were subjected (on various occasions, and after various lengths of exposure to the acid fumes, from half an hour to many days) to the action of the spectrum; and it was found, as indeed I had expected, that the restoration of colour was operated by rays complementary to those which destroy it in the natural state of the paper ; the violet ray s doing chiefly active, the blue almost equally so, the green lit e, and tie yellow, orange, and most refrangible red, not at all. In one ex- periment a pretty-well defined red solar image was developed by the least refrangible red rays also, being precisely those for which in the unprepared paper the discolouring action is abruptly cut off. But this spot I never succeeded in re- producing ; and it ought also to be mentioned, that, according to differences in the preparation not obvious, the degree of sensibility, generally, of the bleached paper to the restorative action of light, differed greatly ; in some cases a perceptible reddening being produced in ten seconds, and a considerable streak in^two minutes, while in others a very long time was re- quired to produce any effect. The dormancy of this colouring principle under the influence of sulphurous acid, is well shown by dropping a little weak sulphuric acid on the paper bleached by that gas, which immediately restores the red colour in all its vigour. In like manner alkalies restore the colour, convert- ing it at the same time into green. - - Papaver orientale.— The chemical habitudes of the sulphurous acid render it highly probable that its action, in inducing a dormant state of the colorific principle, consists in a partial deoxidizement, unaccompanied, however, with disorganization of its molecules. And this view is corroborated by the similar action of alcohol already spoken of ; similar, that is, in kind,