835 ight. rays had acted, a green band where the green had acted, whilst in the blue and indigo it took an intense blue, and over the violet there was a ruddy brown. In reference to these coloured images on paper it must not be forgotten that pure salts of silver are not being dealt with as a rule. An organic salt of silver is usually mixed with chloride of silver paper, this salt being due to the sizing of the paper, which towards the red end of the spectrum is usually more sensitive than the chloride. If a piece of ordinary chloride of silver paper is exposed to the spectrum till an impression is made, it will usually be found that the blue colour of the darkened chloride is mixed with that due to the coloration of the darkened organic com pound of silver in the violet region, whereas in the blue and green this organic compound is alone affected, and is of a different colour from that of the darkened mixed chloride and organic compound. This naturally gives an impression that the different rays yield different tints, whereas this result is simply owing to the different range of sensitiveness of the bodies. In the case of the silver chlorinized plate and of true collodio-chloride, in which no organic salt has been dissolved, we have a true coloration by the spectrum. At present there is no means of permanently fixing the coloured images which have been obtained, the effect of light being to destroy them. If protected from oxygen they last longer than if they have free access to it, as is the case when the surface is exposed to the air. That photography in colours may one day be accomplished is still possible, though the bright tints of nature can never be hoped for, since, as a rule, they are produced by sunshine, whereas on the plate they have to be viewed by diffused light, leducing Action of Light on Silver Salts. The action of light on ction ot sensitive bodies has occupied the attention of many experi mentalists from a very early period of photography. In 1777 Scheele, according to Hunt (Researches in Light), made the following experiments : " I precipitated a solution of silver by sal-ammoniac ; then I edulcorated it and dried the precipitate and exposed it to the beams of the sun for two weeks ; after which I stirred the powder, and repeated the same several times. Hereupon I poured some caustic spirit of sal-ammoniac (strong ammonia) on this, in all appearance, black powder, and set it by for digestion. This men struum dissolved a quantity of luna cornua (horn silver), though some black powder remained undissolved. The powder having been washed was, for the greater part, dissolved by a pure acid of nitre (nitric acid), which, by the operation, acquired volatility. This solution I precipitated again by means of sal-ammoniac into horn silver. Hence it follows that the blackness which the luna cornua acquires from the sun s light, and likewise the solution of silver poured on chalk, is silver by reduction. ... I mixed so much of distilled water with well - edulcorated horn silver as would just cover this powder. The half of this mixture I poured into a white crystal phial, exposed it to the beams of the sun, and shook it several times each day ; the other half I set in a dark place. After having exposed the one mixture during the space of two weeks, I filtrated the water standing over the horn silver, grown already black ; I let some of this water fall by drops in a solution of silver, which was immediately precipitated into horn silver. " This, as far as we know, is the first intimation of the re ducing action of light. From this it is evident that Scheele had found that the silver chloride was decomposed by the action of light liberating some form of chlorine. Others have repeated these experiments and found that chlorine is really liberated from the chloride ; but it is necessary that some body should be present which would absorb the chlorine, or, at all events, that the chlorine should be free to escape. A tube of dried silver chloride, sealed up in vacua, will not discolour in the light, but keeps its ordinary white colour. A pretty experiment is to seal up in vacua, at one end of a bent tube, perfectly dry chloride, and at the other a drop of mercury. The mercury vapour vola tilizes to a certain extent and fills the tube. When exposed to light chlorine is liberated from the chloride, and calomel forms on the sides of the tube. In this case the chloride darkens. Again, dried chloride sealed up in dry hydrogen discolours, owing to the combination of the chlorine with the hydrogen. Poitevin and H. W. Vogel first enunciated the law that for the reduction by light of the haloid salts of silver halogen absorbents were necessary, and it was by following out this law that the present rapidity in obtain ing camera images has been rendered possible. To put it briefly, then, the action of light is a reducing action, which is aided by or entirely due to the fact that other bodies are present which will absorb the halogens. There is another action which seems to occur almost simultane ously when exposure takes place in the absence of an active halogen absorbent, as is the case when the exposure is given in the air, that is, an oxidizing action occurs. The molecules of the altered haloid salts take up oxygen and form oxides. An example f this has already been shown in the section on " photographs in natural colours." If a sensitive salt be exposed to light and then treated with an oxidizing substance, such as bichromate of potash, per manganate of potash, hydroxyl, ozone, an image is not developed, but remains unaltered, showing that a change has been effected in the compound. If such an oxidized salt be treated very cautiously with nascent hydrogen the oxygen is withdrawn, and the image is again capable of development. 1 Spectrum Effects on Silver Compounds. The next in- Spec- quiry is as to the effect of the spectrum on the different trum silver compounds. We have already described Seebeck s . (1810) experiments on the chloride of silver with the com . spectrum whereby he obtained coloured photographs, but pounds. Scheele in 1777 allowed a spectrum to fall on the same material, and found that it blackened much more readily in the violet rays than in any other. Senebier s experi ments have been already quoted at the beginning of this article. We merely mention these two for their historical interest, and pass on to the study of the action of the spectrum on different compounds by Sir J. Herschel which is to be found -in the Philosophical Transactions for 1840. He there describes many interesting experiments, which became the foundations of nearly all subsequent researches of the same kind. The effects of the spectrum have been studied by various experimenters since that time, amongst whom we may mention Becquerel, Draper, Poitevin, H. W. Yogel, Schumann, and Abney. Fig. 1 (see pp. 836-38), which appeared in the Proceedings of the Royal Society for 1882, shows the most recent researches by the last-named experimenter as regards the action of the spectrum on the three principal haloid salts of silver. We may mention that in two instances exception has been taken to these results (1) by H. W. Vogel, who recognizes a difference of behaviour in the spectrum in chloride and bromide of silver when precipitated in alcoholic and aqueous solutions, and (2) by Schumann to the effect of the spectrum on the double iodide and bromide, and iodide and chloride. The latter experimenter finds that when the two salts are mixed after precipitation the results are correct, but that if the precipitations of the two salts take place together the most refrangible maximum of sensitiveness disappears. The dia gram (see fig. 1), however, will give a very approximate approach to the truth. Nos. 33 and 34 show the effect of the spectrum on a peculiar modification of silver bromide made by Abney, in which the silver bromide is seen to be sensitive to the infra-red rays. This modification is, and will be, largely used in investigating this part of the spectrum. 1 See Abney, "Destruction of the Photographic Image," in Phil. Mag., vol. v., 1878 ; also Proc. Roy. Soc., vol. xxvii., 1878.
