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The photographic news
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- 24.1880
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- 1880
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- Bandzählung
- No. 1159, November 19, 1880
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The photographic news
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Band 24.1880
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- Register Index 631
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Band 24.1880
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THE PHOTOGRAPHIC NEWS. [November 19, 1880. remains with the potassium oxalate in the form of a double salt. On the air being admitted there is found, in addi tion to the substances above enumerated, basic ferric oxa late, which separates as a flocculent red brown mass ; this last must be considered as a secondary product caused by the oxidation of the potassic-ferric oxalate which has been produced by the action of light. The addition of oxalic acid prevents the separation of this basic salt, owing to the formation of the normal green soluble salt. The sodic and ammonia ferric oxalates behave in a simi lar way, except that more of the insoluble ferrous oxalate is separated by the action of light ; this is due to sodium and ammonium possessing a slighter dissolving power on the latter salt. The ferric citrates and tartrates are also reduced by light. Ferric citrate and tartrate are reduced by light to the cor responding ferrous salts at first without any gas being given off ; afterwards carbonic acid is developed. Ferrous tartrate is precipitated from a solution of ferric tartrate as a green crystalline powder. Ammonia ferric citrate was used by Herschel* so far back as 1842 for the production of photographs, and the action of the solar spectrum on this salt was investigated both by him and by Draper.t It appears to be more especially sensitive to the blue and violet rays, but this photo-chemical action extends as far as the line F. {To be continued.') NOTE OF AN EXPERIMENT ON INTENSIFYING A PHOTOGRAPH IN GELATINE. BY T. W. IART, F.CS. The plate was exposed in Newfoundland, and brought to me with others to print from. This particular one was so very weak that the paper coloured through almost as rapidly as if under ground glass ; a second print was tried in deeper shade with no better result. As it was full of detail, I was desirous to try an experiment with it, proceeding as follows. The plate being unvarnished, it was put direct into cold water, just sufficient to cover the whole plate. After a few minutes I poured into the dish half a drachm of a saturated solution of mercuric chloride in methylated alcohol, wherein it remained about fifteen minutes, turning throughout to a slate grey colour. It was now thoroughly washed, and put into a solution of hydrogen-sulphide. At this instant the idea occurred to try if it could be intensified locally (knowing how slowly solutions penetrate gelatine). I therefore removed it from the dish, and applied fresh solution after the manner of the ordinary pyrogallic and silver in the wet collodion process, and with a most satisfactory result. There being no crystallisable salts used after washing away the small quantity of mercuric chloride, very little washing was now required, so it was just held under the water tap for a few seconds, stood to drain and dry, then heated and varnished. It may not be universally known to photographers that there are two solutions of hydrogen sulphide prepared, one in glycerine, the other in water. I used the latter, as that is in constant use for many other purposes in my manufactory ; but the glycerine solution is more suitable for those who have not the materials and apparatus in daily use for its preparation. PHOTOGRAPHIC METHOD OF MAPPING THE LEAST REFRANGIBLE END OF THE SOLAR SPECTRUM. It may be within the memory of our readers that Captain Abney, R.E., F. R.S, was selected by the Royal Society to deliver । his year the Bakerian Lecture t an honour highly courted among the Fellows of the Society. Captain Abney chose the above sub ject for his text, and with his paper presented a map of the solar • “On the Action of the Solar Spectrum.’’ Philosophical Transactions 1842. See also Photographische Archiv, 1864, p. 467. + Philosophical Mayazinc^ 1845, vol. xxvii, p. 435. spectrum between wave lengths 7,600 and 10,750, based upon measurements from a series of photographs. Referring first to the action of dyes on silver bromide, Captain bney says :— In December, 1873,* Dr. H. C. Vogel, of Berlin, aanounced that by means of dyed collodion films, which contained silver bromide, he had been enabled to photograph with the yellow and green rays of the solai spectrum, which had hitherto been supposed to be possessed of but little chemical effect. About the same time I had set myself the task of mapping the ultra-red region of the spectrum, and I was naturally led to examine the method advocated by Dr. H. C. Vogel. If a spectrum be thrown on an ordinarily prepared photographic plate, containing only silver bromide, it is well known that the length of the spectrum impressed varies considerably from that obtained by a plate containing silver iodide or silver bromo-iodide. The commencement of the photographic action in the first case is somewhere near the line B, or slightly below, and in the last two, near E in the green ; in all the three the action ex tends to the limit of the solar spectrum in the ultra-violet. The relative chemical effects produced by the different rays show themselves by a varying thickness, or what is usually called den sity, of the metallic silver reduced or precipitated by the action of developing solutions. For the above-named silver compounds the maximum effect is somewhere about the line G ; and if we represent the density of the metallic silver at any point by ordi nates,it will be found that the area of the curve formed by joining the ordinates fairly represents the relative sensitiveness of the compound to the action of white light. With silver bromide the curve falls gradually towards B, and similarly towards the ultra-violet. With silver iodide and bromo iodide the curve falls suddenly near G, and the chemical effect from the bottom of this descent to E is but very feeble ; whilst for the more refrangible end above G, it follows pretty nearly the curve of the bromide. When collodion films containing silver bromide are dyed, by flowing over them an alcoholic or aqueous solution of certain dyes, and exposed to the spectrum, the resulting curves are modified in a marked manner. They almost exactly correspond to curves compounded of the absorption curve of the dye used, and of the absorption curve of the unreduced silver bromide. For exam pl , if we examine a simple bromide film with the spectroscope, it will be seen that an absorption takes place along the 'whole visible spectrum, corresponding with the density curve of this compound. An examination of the spectrum of eosine, supposing we are going to employ that dye to stain the film, gives an absorption band in the green, together with a less marked region of absorption in the blue and violet. If these two absorp tion curves be combined, we shall find that the density curve obtained by exposing a stained film to the action of the spectrum we get a reproduction of this compound curve. Vogel explained this remarkable action by a theory which seemed to be contrary to our present idea of molecular physics ; and in my experiments conducted in this direction 1 had the good fortune to arrive at a more acceptable solution, involving no new laws of the action of radiation on matter. Some of the dyes employed (and most, if not all,'of these belonged to the aniline series) could form com pounds with silver, and when brought in contact with silver nitrate, the action of light on the compounds was perfecty in- telligible. Vogel, however, pointed out that if a bromide film were perfectly freed from all silver nitrate, by immersion in potassium bromide, then washed, next treated with tannin, and dyed, a similar density curve was obtained. I need scarcely recount the numerous experiments which I undertook; one of the final ones will be sufficient to show how a reasonable ex planation can be offered. A few granules of the dye were taken and dissolved in norma Collodion, a glass plate was coated in the ordinary manner, and the film dried and exposed to the spectrum. It was found that in those regions were the spectrum was absorbed, a bleaching of th dyed film was evident. Thus, with eosine, a bleaching took place in the green rays, corresponding with its absorption band, and to a lesser degree in the yellow and blue rays. If, after such an exposure, the dyed collodion were coated in the dark-room with collodion containing silver bromide in suspension, and an alkaline developing solution applied, it was found that the silver bromide was reduced to metallic silver on those parts of the plate which ♦ PHOTOGRIPHIC NHws, Decembr 1874.
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