Page:Proceedings of the Royal Society of London Vol 4.djvu/233

Rh stating the result of his trials with the iodide, chloride, and bromide of silver, he suggests that trials should be made with the fluoride, from which, if it be found to be decomposed by light, the corrosion of the glass, and consequently an etching, might possibly be ob- tained, by the liberation of fluorine.

As it is known that light reduces the salts of gold and of platinum, as well as those of silver, the author was induced to make many ex- periments on the chlorides of these metals, in reference to the ob- jects of photography ; the details of which experiments are given. A remarkable property of hydriodic salts, applied, under certain cir- cumstances, to exalt the deoxidating action of light, and even to call into evidence that action, when it did not before exist, or else was masked, is then described.

4. The chemical analysis of the solar spectrum forms the subject of the next section of his paper. It has long been known that rays of diff'erent colours and refrangibilities exert very diff^erent degrees of energy in eff'ecting chemical changes; and that those occupying the violet end of the spectrum possess the greatest deoxidating powers. But the author finds that these chemical energies are distributed throughout the whole of the spectrum ; that they are not a mere function of the refrangibility, but stand in relation to physical quali- ties of another kind, both of the ray and of the analysing medium ; and that this relation is by no means the same as the one which de- termines the absorptive action of the medium on the colorific rays. His experiments also show that there is a third set of relations con- cerned in this action, and most materially influencing both the amount and the character of the chemical action on each point of the spec- trum ; namely, those depending on the physical qualities of the sub- stance on which the rays are received, and whose changes indicate and measure their action.

The author endeavoured to detect the existence of inactive spaces in the chemical spectrum, analogous to the dark lines in the lumi- nous one ; but without any marked success. The attempt, however, revealed several curious facts. The maximum of action on the most ordinary description of photographic paper, namely, that prepared with common salt, was found to be, not beyond the violet, but about the confines of the blue and green, near the situation of the ray F in Fraunhofer's scale ; and the visible termination of the violet rays nearly bisected the photographic image impressed on the paper : in the visible violet rays there occurred a sort of minimum of action, about one-third of the distance from Fraunhofer's ray H, towards G : the whole of the red, up to about Fraunhofer's line C appears to be inactive ; and lastly, the orange-red rays communicate to the paper a brick-red tint passing into green and dark blue. Hence are de- duced, first, the absolute necessity of perfect achromaticity in the object-glass of a photographic camera ; and secondly, the possibility of the future production of naturally coloured photographs,

5. The extension of the visible prismatic spectrum beyond the space ordinarily assigned to it, is stated as one of the results of these researches ; the author having discovered that beyond the extreme