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November 19, 1880.] THE PHOTOGRAPHIC NEWS. 557 had been bleached by the action of the spectrum, and the density curve followed the curve of absorption of the dye. Photographers have been long aware of the fact that some sorts of organic matter on a glass plate will cause the reduction of sil ver on those portions of the plate on which it exists. The bleached dyes partake of the nature of this organic matter, and we are forced to conclude that a state of oxidation favours this disposition. From exposure of the dyes in different media to the action of the spectrum, it seems that the bleaching is due to a state of oxidation of the dye. If a film containing silver bro mide, and dyed, were exposed in hydrogen to the action of the spectrum, the density curve was that due alone to the action of silver bromide, and the dye did not affect it, excepting so far as it acted as a screen to prevent the full intensity of the light falling on the bromide. The dyes which are most active are those which are known as yielding fugitive colours ; a permanent colour produces no effect beyond the above screening the light from the silver bromide. It should be remarked that when a film containing finely-sus pended matter is dyed, the change effected on the dye is much more rapid than when a continuous film is acted upon, for the dye surrounds these very small particles, and thus a large surface of it is exposed to the air, and renders oxidation comparatively easy. As a result of these experiments I can confidently state that in no case did the addition of the dye cause any chemical effect to be produced by the rays below A of the solar spectrum, nor has Vogel claimed that they do. I am aware that in the Pro ceedings of the Royal Society, Major Waterhouse has stated that by staining the film with turmeric he obtained evidence of the existence of lines in the solar spectrum a little less refran gible than A, but it must be observed that the lines so shown are, except in one instance, “ reversed that is, absorption lines appear as opaque on the transparent body of the spectrum in stead as of a normal character, viz. transparent on an opaque body. This reversal is a matter to which I have referred in the Proceedings of the Royal Society, and is dependent on a differ ent action entirely to that which I am now considering. Preparation of a film sensitive to the infra-red region.—My earliest endeavours were directed to extending this action of organic matter, so that sensitiveness of tbe compound might be obtained in the ultra-red regions. By weighting the molecules of the silver bromide with gum resins, the spectrum was impressed considerably below A, and the absorption lines were unre versed. Measures of the heating effect of the solar spectrum on lampblack, as shown by the thermo-pile and Sir J. Herschel’s well-known researches, showed that the lower limit of the pris matic spectrum had not been reached ; it therefore seemed ad- visable to search in a different direction for a more sensitive com pound. The salts of silver still seemed the most feasible to work with, and more especially the bromide, and efforts were made to obtain this compound in a different molecular condition to that generally found. I need not detail the different methods of pre paration of this compound in collodion that were carried out. In some cases I obtained it in a state which when viewed in a film by transmitted light appeared of a sky-blue colour, inclining to a green tint, visibly absorbing the red. In this condition it was sensitive to the whole spectrum, visible and invisible. There was much uncertainty attaching to the preparation ; about one batch of the salt suspended in collodion out of five or six fulfill ing the requisite conditions. I am now, however, in a position to prepare it without any risk of failure, my experiments of the last nine months having showed the conditions absolutely neces sary for success. The following is the mode of preparation. A normal collodion is first made according to the formula below :— Pyroxylins (any ordinary kind) ... 16 grains Ether (725 s.p.) 4 ounces Alcohol (-820) 2 This is mixed some days before it is required for use, and any undissolved particles are allowed to settle, and the top portion is decanted off. 320 grains of pure zinc bromide are dissolved in 2 oz. to 1 oz. of alcohol ('820) together with 1 drachm of nitric acid. This is added to 3 ozs. of the above normal collodion, which is subsequently filtered. 500 grains of silver nitrate are next dissolved in the smallest quantity of hot distilled water, and 1 oz. of boiling alcohol '820 added. This solution is gradu ally poured into the bromized collodion, stirring briskly whilst the addition is being made. Silver bromide is now suspended in a fine state of division in the collodion, and if a drop of the fluid be examined by transmitted light it will be found to be of an orange colour. Besides the suspended silver bromide, the collodion contains zinc nitrate, a little silver nitrate, and nitric acid, and these have to be eliminated. The collodion emulsion is turned out into a glass flask, and the solvents carefully distilled over with the aid of a water bath, stopping the operation when the whole solids deposit at the bottom of the flask. Any liquid remaining is care fully drained off, and the flask filled with distilled water. After remaining a quarter-of-an-hour, the contents of the flask are poured into a well-washed linen bag, and the solids squeezed as dry as possible. The bag with the solids is again immersed in water, all lumps being crushed previously, and after half -an-hour the squeezing is repeated. This operation is continued till the wash water contains no trace of acid when tested by litmus paper. The squeezed solids are then immersed in alcohol '820 for half-an-hour to eliminate almost every trace of water, when, after wringing out as much of the alcohol as possible, the con- 3 tents of the bag are transferred to a bottle, and 2 ozs. of ether ! (720) and 2 ozs. of alcohol (-805) are added. This dissolves the [ pyroxyline, and leaves an emulsion of silver bromide, which, when viewed in a film, is essentially blue by transmitted light. All these operations must be conducted in very weak red light —such a light, for instance, as is thrown by a candle shaded by ruby glass,* at a distance of twenty feet. It is most important that the final washing should be conducted almost in darkness. It is also essential to eliminate all traces of nitric acid, as it re tards the action of light on the bromide, and may destroy it if present in any appreciable quantities. To prepare the plate with this silver bromide emulsion, all that is necessary is to pour it over a clean glass plate, as in ordinary photographic processes, and to allow it to dry in a dark cupboard. For development after exposure I recommend what is known as the ferrous oxalate developer. This is prepared by dissolving ferrous oxalate in a saturated solution of neutral potassium oxa late, adding the iron salt till no more is taken up. To make up the developing solution, equal parts of this solution of ferrous oxalate, and of a solution of potassium bromide, 20 grains to the ounce, are employed. This mixture is placed in a clean develop ing glass just before development is to take place. The film is first softened by flowing over it a mixture of equal parts of alco hol and water, and is then well washed. The developer is now poured over the plate, taking care to keep the fingers from touch ing any part of the film. The image will appear gradually, and should have fair density when all action is exhausted. The intensity can be materially increased by using the ordinary intensifying solutions of pyrogallic acid, citric acid, and silver nitrate. The unreduced silver bromide is removed by a saturated solution of sodium thiosulphite in water, from all traces of which the film should be thoroughly washed before being allowed to dry. The operation of development should take place in a very sub dued red light, that recommended for the preparation of the emulsion being the safest. It is, however, somewhat remarkable that when the developing action has once been set up, a greater quantity of light may be admitted to fall on the plate than before the action commences. The bromide of potassium probably pre vents any further action by the light, which may account for it. It should be noted that the image may be developed by the ordinary alkaline method, though not so satisfactorily, a slight veil being usually apparent. I may here state that by diminishing the amount of nitric acid to one-fourth the amount given in the preparation of the emulsion it is posssble in very cold weather to obtain plates which are sensitive to very low radiations, such as the radiations pro ceeding from boiling mercury, or even boiling water. In summer-time this emulsion, as would naturally be expected, produces what are known as “ foggy pictures ; ” but it can be rendered of use by flooding with hydrochloric acid. In the preparation of such an emulsion the water bath must be kept at a temperature but little above that of the boiling point of ether. For details of the apparatus employed, and other interesting particulars of Captain Abney’s paper, we must refer the reader to the “ Philosophical Transactions.” • If a green light of the refrangibility of about half way between E and D could bo obtained, it would be better than the faint red light transmitted by ruby glass, since the bromide is less sensitive to it than to the latter.