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The photographic news
- Bandzählung
- 7.1863
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- 1863
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 236, March 13, 1863
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The photographic news
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Band 7.1863
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- Register Index 619
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Band 7.1863
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March 13, 1863.] THE PHOTOGRAPHIC NEWS. 125 EXPEDITIOUS METHOD FOR THE COMPARISON OF PHOTOGRAPHIC SUBSTANCES. BY M. MC. A. GAUDIN. In order to be able to make the numerous trials I have undertaken in that branch of photography which relates to the employment of collodion for the formation of images in the camera-obscura, I contrived a process at the same time comparative and expeditious, of which I have spoken before, and now proceed to describe in full, being persuaded that it will be found useful to photographers on many occasions. At the present day the main thing is sensitiveness, and that has been the steady aim of all my efforts for more than twenty years. Notwithstanding the multiplicity of my attempts, of which no one can form any idea, I have dis covered very little ; but this has not discouraged me, and in any case this process will be very useful in practice, if it does not become fruitful in inventions in the hands of every body. Success in photography depends particularly upon the mutual accord existing between the collodion, the silver bath, and the developing agent, which is fully realized only in taking proofs ; but to appreciate the sensitiveness the question becomes complicated with that of the intensity of the light, and the focal distance of the objective and its diaphragm, the time of exposure, the nature of the object, and the intensity of the image obtained: so that, the opera tion concluded, we find ourselves greatly embarrassed to decide the principal question started, namely, the sensi bility. While, -with the process I now proceed to describe, we can, at all times, by night as well as by day, prove the effective sensitiveness of its products on the relative sensi tiveness of two collodions, with great promptitude, and without any apparatus. We employ plates of glass eight inches long and two inches wide, which, by their elongated form, permit of our operating upon a small scale without the contact of the fingers or frames, which is often the cause of the proofs becoming fogged. After pouring the collodion upon the extremity of the plate, so as to cover a space of two inches square, we immerse this extremity, collodion downwards, into the dish containing the silver solution, the other ex tremity resting upon the edge of the dish, which must be covered with a piece of card-board. After sensitizing, we place upon the back of the plate a piece of moistened yellow paper, cut square or triangular, which adheres, and we then present this paper to the flame of a candle, at the distance of four inches, counting seconds from 0, 1, 2, 3 to 9, re moving the paper between 9 and 10. By this means we have, side by side, two exposures; the one of ten seconds, the other of one only, which, after the application of the reducing agent, will mark, if the sensitiveness is at the maximum, the one, an intense black, the other, sensibly of a half tint; but if the sensitiveness is weak, the exposure of ten seconds only will be marked. We at once understand that this manner of operating gives results always comparable at any season, and at any hour of the day and night. If the developer is susceptible of fogging, it will show itself after the washing with hypo sulphite of soda, the space left masked in the last place by the screen of yellow paper will not appear perfectly black upon a black ground, provided, however, that it be held three feet from the candle during development. To compare the sensitiveness of two collodions is also very easy : we pour on the extremity of the glass plate a film of each collodion, thus forming two contiguous squares, which are sensitized together; and after placing at the back a slip of yellow paper across the line of separation of the two col lodions, and impressing these collodions by the light of a candle as before described, upon the application of the developing agent, we may clearly ascertain which is the most sensitive of the two collodions. . W ith 150 grains of collodion we can make twenty trials in an hour. It is by following this method that I have recently made numerous experiments upon the introduction of iodide of silver, protochloride of tin, and the iodides of iron, tin, and antimony into the collodion. Iodide of Silver in Collodion.—The introduction of iodide of silver into collodion appeal's to me a good way of increas ing the porosity of collodion and augmenting its sensitive ness. This idea is not a new one, as the first collodions prepared contained iodide of silver, but theoretic notions have caused this accessory to fall into disuse. Still, it is quite certain, that when iodide of silver exists in collodion at the same time with iodide of potassium, we need not fear that the iodide of silver will be impressed by light, since, on the contrary, if it were impressed in advance, in the highest degree, the contact with iodide of potassium would restore it to its original state. During my long researches upon the photoglne for collodion, I have been led to believe that most of the iodides act upon iodide of silver, like the iodide of potassium, but in a much less degree. This is why I have introduced some iodide of silver into a collodion iodized with iodide of cadmium, working well, and I have left the bottle exposed to sunshine for half an hour. When I afterwards coated my slip of glass impressed by the flame of a candle, I obtained as pure an image as ordinary. I have, therefore, con cluded that iodideof silver, with excess of iodide of cadmium, is insensible to light, and that we may carry the addition of iodide of silver to collodion very far. Only the great difficulty is in obtaining it in the collodion in the nascent state, and not in the state of suspension: in the nascent state, the collo dion is opaline, just like a sensitized film, and the iodide is never precipitated, while, if in a state of suspension, it very quickly subsides to the bottom of the bottle if left to repose. This difference in the behaviour of iodide of silver declares itself at once under very different circumstances : for example,—to prepare collodion with opaline iodide of silver, it is better first to introduce into simple collodion a drop or two of a concentrated solution of nitrate of silver, then to add the alkaline, or metallic iodide, in the usual pro portion, and shake the bottle until the whole is dissolved. By adding nitrate of silver to collodion already iodized, the collodion almost always turns, and the iodide very quickly separates upon standing, with the exception of a very minimum portion, which is incapable of producing the least useful effect. Protochloride of Tin in Collodion.—The protochloride of tin, introduced in a very minute quantity into collodion, iodized or not, produces an extraordinary effect—fully com parable to that of gallic acid : after sensitizing, the film has assumed a light-brown hue, which is not modified by the action of light; so that a plate thus prepared and exposed yields no image, but under the action of the reducing agents its whole surface assumes a uniform red hue of extraordinary intensity. The addition of free iodine to this collodion ultimately gives sensitive plates, so that protochloride of tin produces the opposite effect to free iodine; and in the present case it may serve for this latter. A collo dion become very red by old age will almost imme diately recover its sensitiveness by the addition of a few drops of a collodion prepared in the ordinary proportions with protochloride of tin. The reason of this phenomenon is very simple : the protochloride of tin, in presence of nitrate of silver, forms chloride of silver and nitrate of per-oxyde of tin, which immediately gives rise to the formation of deut- oxyde of tin, which causes the reduction of the silver. Proto chloride of tin is already known for reducing the salts of gold, and its reaction is still more marked with nitrate of silver: it takes place instantly, which prevents its employ ment for developing photographic images. Preservation of the Ferroso-Acetate.—The preservation of the ferroso-acetate being a condition of primary importance, for its employment as a developer in photography, I have tested it under this relation. I left a pint of a weak solution of it in a wide-necked bottle, uncorked, exposed to the atmosphere for three weeks, using a portion every day, without observing any diminution in its power of reduction,
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