Volltext Seite (XML)
660 THE PHOTOGRAPHIC NEWS. |September 18, 1891. I was quite surprised, at our last two conventions, by some of the criticisms offered on the pictures upon the screen; the criticisms were on the pictures having more than one point of light. It is a fact that a bust portrait should only have but one light, but in half length of figures it is not contrary to art to have more than one point of light. And so we could go on to enumerate instances where art and photography, or art and art in photography, come to a perfect agreement. Our photographers who make a study of genre photo graphy must study invention, which is claimed to be the great soul of painting. The subject he desires to illustrate must be created within the mind, and with his hands he must arrange every detail, from the most important to the minutest, thereby proving that art in photography and art go hand-in-hand. The operator has no choice in subject, but must do his best with each subject presented. His choice must be in the point of view, in the light and shade, and the general use of background and accessories. One essential in every really good photograph is a point of vision—one that will please the eye and not tire it after carefully observing it in all its phases. In case of a front view the eyes form the point of view, with all the rest subservient and gradually fading out of view. By observing a few of these rules we obtain what the artist calls “drawing,” which we must acknowledge a great many photographers lack, and when we touch the point it brings us to one of the essentials, and that is the background. There is more thought and study required in that accessory than in any other. Writers in our photographic journals have been advocating reform in backgrounds, and advising to keep them back—not bring them too near the subject. In case of a bust picture, I think, if your ground is the right kind, you can bring it well up, and thereby secure better effects than by keeping the ground five or ten feet away. If your ground is too plain, take a little common chalk and a tuft of cotton, and in a few minutes you can have the relief you aim at by a few well-directed strokes. Art seems to be made up of small bits, and if we are able to carry them in our mind, and then put them together correctly, we shall be able to behold art in photography. PHOTOGRAPHIC CHEMISTRY.* BY PROFESSOR R. NELDOLA, F.R.S. Continuing the study of those properties of silver salts which are of photographic importance, the next point to be dealt with is the vexed question of the existence of sub-salts. Here, in the present state of knowledge, it is most advisable to avoid dogmatic statements. The utmost that can be done is to summarise the evidence, and to let the student see there from that, from a scientific point of view, the existence of such sub-salts has not been conclusively demonstrated. I will only repeat now what I said some years ago, namely, that so far as analogy is to be trusted as a guide, it would seem improbable that the sub-haloid salts of silver should be highly coloured compounds, because the analogous salts of copper, mercury, and thallium are not highly coloured. Now, all the attempts which have been made to produce sub-haloid salts of silver by partial reduction or by other methods give rise to coloured products, which have been held by some investigators to consist of the sub-haloids, and by others (Carey Lea) to consist of molecular compounds of the sub-haloids with the haloids proper. It may further be suggested that these coloured compounds might con sist of oxyhaloids, mixed or combined (molecularly) with the haloids, that in some cases they might consist of metallic silver or its oxide in molecular combination with the haloid, and that * Continued from page 639. in other cases they might consist of the foregoing compounds or mixtures, or of the true haloids coloured by the retention of a small quantity of some metallic oxide as an impurity. The study of these coloured products is of importance to the photographic chemist, whether they are definite chemical com pounds, or whether they are molecular compounds, or what ever subsequent research may prove them to be. They are of importance to us here, among other reasons, because there may be some relationship between these compounds and the pro ducts formed by the photo-chemical decomposition of the silver haloids. I have thought it desirable, therefore, to summarise, in a collected form, the various methods by which these com pounds have been produced :— 1. Rose-coloured silver chloride, obtained by reducing a hot solution of silver citrate with hydrogen, exhausting the dark product (before complete reduction) with citric acid, and then treating it with hydrochloric acid. Obtained also by reducing the dry nitrate in hydrogen at 100° C., extracting the product with water, and treating the residue with hydrochloric and nitric acids. (Brit. Assoc. Rep., 1859, p. 105.) 2. Chocolate-coloured chloride ; obtained by adding a solu tion of silver arsenite in nitric acid to a strong boiling solution of caustic soda, when “ an extremely black powder” is pro duced. This, on treatment with hydrochloric acid, becomes grey, and the washed product, on boiling with dilute nitric acid, loses silver and leaves the chocolate-coloured chloride. (Brit. Assoc. Rep., 1859, p. 106.) 3. Coloured products obtained by acting upon silver with solutions of ferric or cupric chloride have long been known. 4. Coloured products obtained by Carey Lea, and described as “photochloride,” “photobromide,” and "photoiodide" (“ photosalts ”). (a) Purple or black chloride, obtained by the action of alkaline hypochlorites on finely-divided (reduced) silver. (J) Red chloride, prepared by adding ferrous sulphate to an ammoniacal solution of silver chloride, and then acidifying with dilute sulphuric acid. The precipitate is washed, boiled with dilute nitric acid, washed, and finally boiled with dilute hydrochloric acid. (c) Red or copper-coloured chloride, prepared by heating silver oxide or carbonate to a point short of complete reduc tion, and then treating the residue with hydrochloric acid. (d) By precipitating silver oxide in the presence of the lower oxides of iron, manganese, &c., and treating the product with hydrochloric acid. (e) Dark purple chloride, obtained by treating finely divided (reduced) silver with a solution of ferric chloride. (Contains 76'07 per cent, of silver.) (f) Red chloride, similarly prepared by the action of cupric chloride. (g) Red chloride, prepared by pouring dilute solution of silver nitrate on to cuprous chloride, and boiling the black pre cipitate thus obtained with dilute nitric acid. (h) Brownish purple chloride, prepared by pouring an ammoniacal solution of silver nitrate into a strong solution of ferrous chloride, and treating the dark precipitate with dilute sulphuric acid. Becomes lighter with nitric acid. (Similar to b.) (i) Purple chloride, prepared by reducing the citrate in a current of hydrogen at 100“ C., and treating the product with hydrochloric and nitric acids successively. (Similar to No. 1.) (j) Red and purple shades of chloride, obtained by reducing (partially) a silver salt with alkali and an organic reducing, agent, such as milk sugar, dextrine, &c., and then treating with hydrochloric and nitric acids successively. (k) Red, brown, or lavender chloride, produced by treating the white chloride with a boiling solution of sodium hypo phosphite. The dark, chocolate-coloured product is washed, and boiled with dilute nitric acid. By somewhat similar methods, coloured forms of the bro mide and iodide have been obtained. From these special studies of the silver compounds we may now pass to another phase of the subject, viz., the combina tion of silver and its salts with organic compounds. At this stage the technology—i.e., the sources and methods of manu facture of the more important organic compounds used by the
