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May 29, 1885.] THE PHOTOGRAPHIC NEWS. 339 with very long exposure a little beyond the bright blue. If one compares eoside of silver with pure bromide of silver as regards the action of the spectral yellow, the eosine com pound turns out to be three times more sensitive than the indigo-sensitive modification of silver bromide. 2. Eoside of silver in conjunction with bromide of silver gives considerable sensitiveness to the yellowish-green rays; the sensitiveness of the two conjointly for these rays being at least sixty times that of a simple bromide plate. 3. Eoside of silver with a small addition of iodide gives a sensitiveness for the green about four times as great as the sensitiveness for the blue. Although one might suppose that eoside of silver itself would be the best medium for photographing coloured objects, it is not sufficiently sensitive for the purpose, hence it is better to use bromide and iodide as recom mended in my receipt for colour-collodion published last year. This circumstance leads one to the enquiry, whether it is not better rather to reduce the blue sensitiveness by re ducing the amount of the substance which is specially sensitive to the blue (bromide of silver); and to corres pondingly increase the amount of the substance specially sensitive to the green (eoside of silver). This question I have endeavoured to answer experimentally by omitting the iodide in my formula, and increasing the eoside about tenfold; in this case the lowering of the sensitiveness to the blue, and the exaltation of the sensitiveness to the yellow, were so great that use of the yellow glass was found to be no longer necessary in order to secure very marked results. The experiments were now continued, and the proportion of eoside was increased to twenty-fold the original, with the result that a plate was obtained which worked satisfactorily without the yellow glass, and was more acted upon by chrome yellow than by ultramarine blue. Still the result was hardly equal to the azaline plate used with yellow glass ; but the convenience of being able to work without a yellow glass is very considerable, as the yellow screen prolongs the exposure very consider ably. For the reproduction of oil paintings containing deep blue, the eosine collodion process, without the yellow glass, should suffice; especially if but little importance is attached to the red tints. The image which is obtained on the collodion plate highly charged with eosine is thinner than when a plate is used containing a small proportion only of eosine, but this thin image can be easily intensified with silver and pyrogallic acid. ABOUT SULPHITE OF SODA AS A DEVELOPER AND FIXING AGENT. BY CAPTAIN ABNEY, R.E., F.R.S. Sulphite of soda is so commonly used by photographers as an ingredient in the alkaline developer that it is a matter of some surprise that we have not heard more regarding its capabilities in other directions. Sulphite of soda has several curious properties, one of which was announced some little time ago by Mr. Watmough Webster, I believe —viz., its supposed power of taking the place of the alkali in the alkaline developer. He found if the developer be mixed as for development, and if the alkali be absent, but sulphite of soda be present for the purpose of keeping the pyrogallic acid colourless, that after a long interval an image may be developed on a gelatino-bromide plate. It appeared to me, however, that this developing action might be due to a little free alkali which is often present in the sulphite, and to test this some experiments were made. To the sulphite solution was added a little hydrochloric acid, just sufficient to make it redden blue litmus paper, and a collodio-bromide emulsion plate was, after exposure, im mersed in a saturated solution of the acidified salt, to which a few grains of plain pyrogallic acid were added. After a lapse of about three minutes, a perfectly developed pic ture was obtained, and quite free from fog; but it had a curious metallic look on the surface, and was a little browner in colour than that usually obtained by other means. This proved, then, that the sulphite per .sc can be substi tuted for an alkali in the ordinary alkaline developer. Another plate was exposed and immersed in a mixture of sodium sulphite and sodium bisulphite (this latter is an acid salt), to which pyrogallic acid had been added, with the result that an excellent picture was obtained, but rather slower in coming out, and of a still browner colour. It will be seen that this developer worked without any restrainer whatever. It struck me, there fore, that it should be useful for collodio-chloride. Plates were prepared and exposed wet, and, in the first instance, a saturated solution of the sulphite was used; but, for reasons which will be apparent further on, the strength was diminished to half saturation. With long exposure to light the image developed up rapidly, and had a rather ruddy tone, but nothing very remarkable. By giving a short exposure, however, the image was long in coming up ; but it developed eventually with great inten sity, and assumed a beautiful ruby-purple colour by trans mitted light, but by reflected light showed a green metallic lustre, which could not be distinguished from the colour of a very bad example of green fog. This was only on the high-lights, however, and not to be traced on the trans parent portions of the picture. We have here a further proof, if any be required, that green fog in gelatine plates may be caused by the reduction of an haloid salt to the metallic state, as Mr. Woods and myself have shown in different manners. The ruby-purple colour by transmitted light and the reflected light are extremely significant. Silver, we know, can be caused to precipitate as ruby crystals. When metallic silver is immersed in a neutral solu tion of peroxide of hydrogen, argentous hydrate (2HAg,0) is formed, and dissolves. When the solution is evaporated it leaves a crystalline residue, which, on treatment with water, gives up argentic hydrate, and deposits metallic silver in red microscopic crystals. The general resulting action of development is well understood ; but it by no means follows that each minute reaction has been studied or followed, and it seems quite possible that the colour of the silver may be due in this case (as in other cases of ruddy images) to the formation at some stage of argentous hydrate, and a subsequent deposition of ruby crystals of silver. Now as to the action of the developer on gelatine plates. Gelatine bromide plates do develop with the acidified sodium sulphite, but they are long in coming out, but they give pleasing images: with the strictly neutral solu tion the image develops more rapidly. With gelatino- chloride plates we have an anomaly. When exposed and placed in the saturated solution the plate fogged, but fogged in a peculiar way. Gelatine-bromide plates, as is well known, tarnish round the edges more readily than do bromide plates. It was when this tarnish existed that the fog appeared. The fog was very peculiar in appearance, as it gave a very bright metallic appearance, and spread over the whole of the surface of the plate, and when the plate was dry could be polished like a mirror. Now for the cause of this. When a collodio-chloride plate was developed it was found that the liquid, on rock ing, was streaked with a reddish, soluble substance, and that after the plate was developed the liquid was of a rosy tinge, and moreover, it was found that after long immer sion the plate was fixed. Now I knew well that silver chloride did dissolve in sulphite of soda, and it was evi dent to me that the rosy tint was really due to the dis solved chloride becoming reduced by the pyro and sulphite. Nay, more, it was evident that as this reduction was gradual, intensity was given to the image by the precipi tation of those particles on the silver already reduced in