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The photographic news
- Bandzählung
- 35.1891
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- 1891
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1690, January 23, 1891
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The photographic news
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JANUARY 23, 1891.] THE PHOTOGRAPHIC NEWS. 69 mously greater activity of the blue and violet rays—i.e., those of least visual intensity—in decomposing the compounds which they employ. It will be convenient here to point out another contrast of this method with those more commonly employed. The prints obtained with it are positives, the light and shadow in the object being exactly produced in the coloured picture. Natural objects, therefore, of convenient form, such as leaves, may be photographed directly ; reproductions from camera pictures require glass positives, or positive paper prints made transparent in the usual way with vaseline. In view, perhaps, of the chemistry of the method, it would I perhaps be more correctly termed a negative method, since the action of the light is destructive, and when it acts, the construc tion or development of the colour is rendered impossible. In the ordinary methods, on the other hand, the light is an agent, as it were, in the synthesis of colour—when there is most action the deepest tones are developed—and the photographic action would, in this case, perhaps, be more correctly termed positive. I The point, however, is not a very important one. There now remains to be noticed a second photographic I process, based upon the peculiar properties of the diazo-deriva- ■ tives of the coal-tar bases ; a method which, to use the ordin ary term, is a negative one, i.e., the light plays a constructive part in the development of a coloured picture. When the diazo compounds are treated with an alkaline bisulphite, they are converted into the diazo sulphonates. These compounds are sensitive to light, the action of which is to set free the diazo group from its combination, but they do not react with phenols and amines, as do the diazo compounds. The mixture of a diazo sulphonate with the latter is unattended by any colour reaction, but, on exposure to light, the diazo group being set free in presence of a phenol, the development of an azo-colour takes place pari passu. This reaction is the basis of the process invented and patented by A. Feer. The photographic surface is a mixture of a diazo sulphonate with the alkali compound of a phenol applied to any suitable material. On exposure to light under a transparency, deve lopment of colour takes place in proportion to the quantity of light transmitted, giving, therefore, a reversed reproduc tion, or negative picture. When printed, the unattacked mixture is dissolved away by copious washing, leaving the picture, already developed in the azo-colour, permanently fixed upon the fabric or material. This process is not rapid enough to admit of being con veniently demonstrated by means of the artificial light at our disposal. We must, therefore, content ourselves with this brief description, referring those who wish for further information to the specification of the original German patent, No. 53,455/ga. In concluding our brief sketch of this new departure in the application of the coal-tar colours, we need not, perhaps, apologise for its brevity. We learn that our friend, Professor Meldola, is to give a course of Cantor Lectures on “ Photo graphic Methods ; ” and his survey of the field will be wide and complete. We have not attempted to give a general account of photographic methods partly on this account, and partly, also, because these diazotype processes are entirely typical of all printing processes, and, as subjects of lecture or educational demonstration, have the great advantage of in volving the very simplest reaction possible; whereas, the changes which take place in silver compounds are very com plicated, and as yet but imperfectly understood. The primuline process is simplicity itself. It can be prac tised with the minimum of apparatus, and requires no technical training. The results are striking and pleasing, as we hope we have been able to show. At the close of the reading of the foregoing paper a few questions were asked. Mr. T. R. Dal’meyer asked if the authors knew the spectral qualities of the colours obtained by the process with a gela tine vehicle, and whether the qualities were equal to those obtainable by means of manufactured glass. Mr. Leon Warnerke wished to know whether the authors had, so far, been able to get rid of the yellow ground to the prints. Mr. Cross, in reply to these and other questions, said that he would leave the question of the spectral values of the colours obtained to Captain Abney. When primuline is used the yellow ground cannot at present be avoided, but the whole subject is new ; with other dyes of the class the authors have obtained a nearly colourless ground. The applications of the primuline process are general, and the results obtained will resist the action of hot soap and water ; they are also exceed ingly fast to light. Captain Abney, the Chairman, stated that Mr. Bevan had been at his laboratory experimenting upon the absorption spectrum of the sensitive compound, and could give all the particulars. The absorption extends over nearly the whole spectrum, but fades off between the yellow and the red. He (the speaker) had worked the process to a limited degree, and had become quite fascinated by it ; he thought that it would become a perpetual source of amusement in the decora tion of houses. Everyone knew how ladies bought pictures to stick them to glass, spending much time over such work ; but by means of the process just demonstrated they could decorate their furniture and their homes, and go to nature for their subjects. The materials were simple, and he thought that he was right in saying that they were harmless ; they also can be contained in a few beakers ; the only other adjunct required was a common photographic printing frame ; obtaining natural objects from the garden to photograph was then all that was necessary. He had to thank the authors for their interesting process. Not every day is it that we get a thoroughly new photographic process, and this was thoroughly novel ; at least, he had never heard of anything of the kind until about six months ago. Unfortunately, the sensitiveness of the com pound is not great enough for the camera, but there is no knowing what may happen in such matters. London and Provincial Photographic Association.— Subject for January 29th, “ Photo-Micrography.” Lord RAYLEIGH will lecture to-night at the Royal Institu tion on “ Some Applications of Photography,” and in the library some objects of photographic interest will be exhibited, including some “pigeon-post films.” Process for Detecting Wood-Paper.—M. Wurster.—The following reagents serve to distinguish wood-paper from pure cellulose:— Reagent. W’ood-paper. Cellulose. Orcine Dark red Nothing Resorcine ... ... Deep green... ... Violet Pyrogullic acid ... Blue-green „ Phenol Yellow-green ... ,, Phlorglucine ... Blue-Violet ... Nothing The presence of wood in paper can be detected, and the quantity even estimated, by means of dimethyl paraphenylen- diamine.—Journal de Pharmaciei,et de Ohimie. Recovery of Uranium Residues. — For the recovery of uranium residues, Lambe (according to the Zeit Angcioandtc Chcmic) recommends the following modification of the Reichardt process :— “The residues from titrations are col lected together, and the clear liquid is decanted off from time to time. When a sufficient quantity has been collected, the pasty mass is either heated in the pot by introducing steam, or in an iron pan over an open fire. Soda crystals are then added until the precipitate appears chiefly dissolved. It is let cool, mixed with a sufficiency of ammonia, and the phosphoric acid is precipitated with magnesia mixture. After standing for twelve hours the supernatant liquid is syphoned off, and the precipitate is washed with ammoniacal water. The alka lide liquids are neutralised either with hydrochloric or sul phuric acid, heated to expel carbonic acid, and the uranium is precipitated at a boiling heat with ammonia as uranium oxide ammonium. The precipitate is washed by decantation. A small quantity of some ammonium salt must be added to the last washing waters, as the precipitate otherwise does not settle well. For obtaining uranium nitrate, the precipate is dis solved in excess of nitric acid and concentrated by evapora tion.”
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