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SEPTEMBER 10, 1880.] THE PHOTOGRAPHIC NEWS. 435 Of backgrounds there were very few in the studio, but a snow landscape deserved attention. The trunk of a tree with matted snow on one side, as the result of a drifting storm, was a prominent feature, but the white landscape was not overdone ; the flooring was of canvas, with a few roughly-marked foot prints, and cotton-wool was employed to add to the effect. A grass ground was also very clever, made of green silk threads—one of Atkinson’s, we were told —and with this some good recumbent models had been photographed : gentlemen lying at their ease on the sward, reading and smoking. “We have all sorts of possible and impossible accessories,” said our host, “ pointing to a large stock of furniture, “ but we like to employ them as little as possible now-a-days.” The silver bath has been for some months past dispensed with at the Worthing studio, and only gelatine is em ployed ; and this, strange to say, without any alteration to the dark room. This has an orange stained window and two tammy curtains. But the relief in being able to do without collodion was beyond description. The dark room is on the roof, and therefore exposed to the action of the sun, the consequence being that in summer the ether and alcohol fumes were well-nigh insupportable. “ Only those who work in a dark room like this,” said our host, “ can appreciate the value of gelatine plates. The health question alone would be enough to bring gelatine into favour.” Messrs. Russell and Sons print a proportion of their work in carbon by the so-called chromotype method, “ but it requires a very good negative to stand it,” we were assured. Beside the printing room is an open flat roof that serves as an out-door waiting room for visitors. We thought of M. Libert's “terrasse d’agrement,” where models in waiting smoke their cigarettes ; if he could only secure a prospect like this, the bright expanse of sea, the green waves and white cliffs extending as far as Beechey Head, our Parisian confrere would be happy indeed. Gelatine plates have given little difficulty in the ex perience of Messrs. Russell. Never to proceed with a batch if the first or second film turns out questionable, is the invariable rule followed, and in this way much trouble and worry is saved. Gelatine plates, when good, are simple and easy to work, and when they give trouble the fault, nine times out of ten, lies in imperfect preparation. Therefore, rather than lose precious time in attempting to secure an inferior result with a second-class plate, the questionable batch is put on one side and returned. When the developer is once fairly at work on the film, one or both of the tammy blinds arc drawn up, and the manipula- tion proceeds behind a single thickness of orange glass. From this it will be seen that Messrs. Russell and Sons do not prepare their own plates. They give two reasons for this, and it must be admitted that they are good ones. In the first place, they are quite satisfied with the films that they purchase ; and in the second place, the time of their establishment is already fully occupied in camera and printing work. So long as it is possible to obtain trust worthy films at a reasonable rate, say Messrs. Russell, we shall prefer to leave the difficult work of preparing gelatine emulsion to others. They are very proud of their mounting material at the Russell Studio ; as at many other establish ments, the mounting of pictures caused a good deal of trouble and anxiety, but these have not been known since the employment of a material proposed in one of our Year- Books. Since that time the Messrs. Russell have never employed any other material, and seeing it has given so much satisfaction, we take this opportunity of repeating its composition:— Best Bermuda arrowroot ... 3} ounces Water 28* ,, Sheet gelatine or glue 160 grains Methylated spirit ... ... 2 ounces Pure carbolic acid 12 drops Mix the arrowroot with 6 ounces of water into a paste, then add 22 ounces of water and the gelatine. Boil and stir for three or four minutes, then let it partly cool. Finally add carbolic acid and spirit; keep stirring till properly mixed. The next “At Home” will be “Messrs. Brown, Barnes, and Bell at Liverpool.” ON THE REVERSAL OF THE DEVELOPED PHOTO GRAPHIC IMAGE. BY CAPTAIN ABNEY, R.E., F.R.S.* In the Proceedings of the Royal Societyt I explained the theory of the reversal of the photographic image on development, showing that it was due to oxidation when ordinarily met with. On looking back, however, to the Philosophical Transactions for 1840, I find Sir J. Herschel gave a description of a similar phe nomenon which perhaps might admit of a different explanation. This subject I examined some time ago ; and since the whole question seems to have revived in interest, it has seemed an op portune moment to put on record my more recent investigations. I need scarcely say that the fact as to the reversal of the image is anything but new, as will be seen from Herschel’s, Hunt’s, and Draper’s well known works. The explanation, however, as far as I know, has been confined to my researches already men tioned ; and these were summarized very briefly. Sir John Herschel, in his memoir just referred to, experi mented on a prepared paper. In article 48 of his communica tion he says, “ A paper endowed with a pretty high degree of sensibility may also be prepared with the following triple solu tion, viz.:—1st, acetate of lead ; 2nd, hydriodate of potash ; 3rd, nitrate of silver .... If paper so prepared and darkened in the sun be washed over with a fresh dose of hydriodate, the exposure to sunshine being sustained, it whitens with great rapidity ; and were it practicable (which I have not found it) to ensure precisely the same ingredient-proportions, and the same degree of blackening in the sun to start from, I should not hesi tate to propose this as an excellent process for a positive photo graphic paper.” This process in Sir J. Herschel’s hand was not then uniformly successful; and it must be noted that here we are dealing with a visible image, and not a developable one. But it will be found that the same argument applies to both, since the visible image and the developable image are of precisely the same nature, vary ing only in the matter of degree. It has been some time' known, and more recently has been brought forward by Dr. Angus Smith, + that a slightly acidified solution of potassium iodide liberates iodine in the presence of light, though it remains unaltered in the dark for considerably long periods. If, then, paper impregnated with a silver-salt be blackened by light, and be then treated with potassium iodide, we have the exact explanation of Sir John Herschel’s experi ment, presuming the paper be slightly acid. When iodide of silver is exposed to light in the presence of a neutral solution of silver nitrate, we have this acidity produced, the iodine liberated from the silver iodide in its conversion to subiodide combining with the silver nitrate and liberating nitric acid, probably with the formation of an iodate. This is true more especially when the paper is not absolutely desiccated, for when desiccation is perfect the iodine might not combine. Hence may arise the uncertainty to which Sir J. Herschel refers. If paper so prepared be kept damp by any means, the reaction will invariably take place, and iodine from the potassium iodide will be liberated and combine with the semi-metallic silver on the paper to produce silver-iodide ; in other words, the blackened surface will bleach. The experiment may be tried in a variety of ways. The sim plest, perhaps, is to salt ordinary unglazed paper with a 10 per cent, solution of common salt, and when dry to float it on a solu tion of silver nitrate of about the same strength, and then to dry and expose it to the daylight to blacken. When the black ening is produced, if the paper be slightly washed and then be treated with a 5 per cent, solution of potassium iodide (slightly acidified with nitric acid) in the dark, and while still damp be • PhilonophioalMng^’ne. t Vol. xxvii. pp 291 and 451. t Since writing this paper, the sudior has noticed a paper in the Philo sophical Magazine for August 1880, by Dr. Lends, in wh’ch this solution has been investigated.