PHOTOGRAPHIC NOTES.
Mr. Warnerke's New Discovery.--Very happily for our art, we are at the present moment entering upon a stage of improvement which shows that photography is advancing with vast strides toward a position that has the possibility of a marvelous future. In England, especially, great advances are being made. The recent experiments of our accomplished colleague, Mr. Warnerke, on gelatine rendered insoluble by light, after it has been sensitized by silver bromide and developed by pyrogallic acid, have revealed to us a number of new facts whose valuable results it is impossible at present to foretell. It seems, however, certain that we shall thus be able to accomplish very nearly the same effects as those obtained by bichromatized gelatine, but with the additional advantage of a much greater rapidity in all the operations. In my own experiments with the new process of phototypie, I hit upon the plan of plunging the carbon image, from which all soluble gelatine had been removed, into a bath of pyrogallic acid, in order to still further render impermeable the substance forming the printing surface. I also conceived the idea of afterward saturating this carbon image with a solution of nitrate of silver, and of subsequently treating it with pyrogallic acid, in order to still further render impermeable the substance forming the printing surface. But the process described by Mr. Warnerke is quite different; by means of it we shall be able to fix the image taken in the camera, in the same way as we develop carbon pictures, and afterward to employ them in any manner that may be desirable. Thus the positive process of carbon printing would be modified in such a manner that the mixtures containing the permanent pigment should be sensitized with silver bromide in place of potassium bichromate. In this way impressions could be very rapidly taken of positive proofs, and enlargements made, which might be developed in hot water, just as in the ordinary carbon process, and at least we should have permanent images. Mr. Warnerke's highly interesting experiments will no doubt open the way to many valuable applications, and will realize a marked progress in the art of photography.
Method for Converting Negatives Directly into Positives.--Captain Bing, who is employed in the topographic studios of the Ministry of War, has devised a process for the direct conversion of negatives into positives. The idea is not a new one; but several experimenters, and notably the late Thomas Sutton, have pointed out the means of effecting this conversion; it has never, however, so far as I know, been introduced into actual practice, as is now the case. The process which I am about to describe is now worked in the studios of the Topographic Service. The negative image is developed in the ordinary way, but the development is carried much further than if it were to be used as an ordinary negative. After developing and thoroughly washing, the negative is placed on a black cloth with the collodion side downward, and exposed to diffuse light for a time, which varies from a few seconds to two or three minutes, according to the intensity of the plate. Afterward the conversion is effected by moistening the plate afresh, and then plunging it into a bath which is thus composed:
Water 700 cub. cents.
Potassium bichromate 30 grams.
Pure nitric acid 300 cub. cents.
In a few minutes this solution will dissolve all the reduced silver forming the negative; the negative image is therefore entirely destroyed; but it has served to impress on the sensitive film beneath it a positive image, which is still in a latent condition. It must, therefore, be developed, and to do this, the film is treated with a solution of--
Water 1,000 grams
Pyrogallic acid 25 "
Citric acid 20 "
Alcohol of 36° 50 cub. cents.
The process is carried on exactly as if developing an ordinary negative; but the action of the developer is stopped at the precise moment when the positive has acquired intensity sufficient for the purpose for which it is to be used. Fixing, varnishing, etc., are then carried on the usual way. The great advantage of this process consists in the fact of its rendering positives of much greater delicacy than those that are taken by contact; and, on the other hand, by means of it we are able to avoid two distinct operations, when for certain kinds of work we require positive plates where a negative would be of no service. M. V. Rau, the assistant who has carried out this process under the direction of Captain Bing, has described it in a work which has just been published by M. Gauthier-Villars.
Experiments of Captain Bing on the Sensitiveness of Coal Oil.--The same Captain of Engineers has undertaken a series of very interesting experiments on the sensitiveness to light of one or two substances to which bitumen probably owes its sensitiveness, but which, contrary to what takes place with bitumen, are capable of rendering very beautiful half tones, both on polished zinc and on albumenized paper. These sensitive substances are extracted by dissolving marine glue or coal-tar in benzine. By exposure to light, both marine-glue and coal-tar turn of a sepia color, and, in a printing-frame, they render a visible image, which is not the case with bitumen; their solvents are in the order of their energy; chloroform, ether, benzine, turpentine, petroleum spirit, and alcohol. Of these solvents, benzine is the best adapted for reducing the substances to a fluid state, so as to enable them to flow over the zinc. The images obtained, which are permanent, and which are very much like those of the Daguerreotype, are fixed by means of the turpentine and petroleum spirit. They are washed with water, and then carefully dried. It is possible to obtain prints with half-tones in fatty ink by means of plates of zinc coated with marine-glue. Some attempts in this direction were shown to me, which promised very well in this respect. We are, therefore, in the right road, not only for economically producing permanent prints on paper, but also for making zinc plates in which the phototype film of bichromatized gelatine is replaced by a solution of marine-glue and benzine. The substance known in commerce under the name of pitch or coal-tar will produce the same results.
Bitumen Plates.--A new method of making bitumen plates by contact has also been introduced into the topographical studios. The plan, or the original drawing, is placed against a glass plate, coated with a mixture of bitumen and of marine-glue dissolved in benzine. The marine-glue gives the bitumen greater pliancy, and prevents it from scaling off when rubbed, particularly when the plate is retouched with a dry point. These bitumen plates are so thoroughly opaque to the penetration of the actinic rays, that the printing-frame may be left for any time in full sunlight without any fear of fog being produced on the zinc plate from which the prints are to be taken.
Method for Topographic Engraving by Commandant de la Noë.--Before leaving the interesting studios of which I have been speaking, I ought to mention a very ingenious application which has been made of a process called topogravure, invented by Commandant de la Noë, who is the director of this important department. A plate of polished zinc is coated with bitumen in the usual way, and then exposed directly to the light under an original drawing, or even under a printed plan. So soon as the light has sufficiently acted, which may be seen by means of photometric bands equally transparent at the plate, all the bitumen not acted upon is dissolved. As it is a positive which has acted as matrix, the uncovered zinc indicates the design, and the ground remains coated with insoluble bitumen. The plate is then etched with a weak solution of nitric acid in water, and the lines of the design are thus slightly engraved; the surface is then re-coated with another layer of bitumen, which fills up all the hollows, and is then rubbed down with charcoal. All the surface is thus cleaned off, and the only bitumen which remains is that in the lines, which, though not deep, are sufficiently so to protect the substance from the rubbing of the charcoal. When this is done we have an engraved plate which can be printed from, like a lithographic stone; it is gummed and wetted in the usual way, and it gives prints of much greater delicacy and purity than those taken directly from the bitumen. The ink is retained by the slight projection of the surface beyond the line, so that it cannot spread, and a kind of copper plate engraving is taken by lithographic printing. Besides, in arriving at this result, there is the advantage of being able to use directly the original plans and drawings, without being obliged to have recourse to a plate taken in the camera; the latter is indispensable for printing in the usual way on bitumen where the impression on the sensitive film is obtained by means of a negative. It will be seen that this process is exceedingly ingenious, and not only is its application very easy, but all its details are essentially practical.
Succinate of Iron Developer.--I have received a letter from M. Borlinetto, in which he states that he has been induced by the analogy which exists between oxalic and succinic acids to try whether succinate of iron can be substituted for oxalate of iron as a developer. To prove this he prepared some proto-succinate of iron from the succinate of potassium and proto-sulphate of iron, following the method given by Dr. Eder for the preparation of his ferrous oxalate developer. He carried out the development in the same way as is done by the oxalate, and he found that the succinate of iron is even more energetic than the oxalate. The plate develops regularly with much delicacy, and gives a peculiar tone. It is necessary to take some fresh solution at every operation, on account of the proto-succinate of iron being rapidly converted into per-succinate by contact with the air.
Method of Making Friable Hydro-Cellulose.--At the meeting of the Photographic Society of France, M. Girard showed his method of preparing cellulose in a state of powder, specially adapted for the production of pyroxyline for making collodion. Carded cotton-wool is placed in water, acidulated with 3 per cent. of sulphuric or nitric acid, and is left there from five to fifteen seconds; it is then taken out and laid on a linen cloth, which is then wrung so as to extract most of the liquid. In this condition there still remains from 30 to 40 per cent. of acidulated water; the cotton is divided into parcels and allowed to dry in the open air until it feels dry to the touch, though in this condition it still contains 20 per cent. of water. It is next inclosed in a covered jar, which is heated to a temperature of 65° C.; the desiccation therefore takes place in the closed space, and the conversion of the material is completed in about two or three hours. In this way a very perfect hydro-cellulose is obtained, and in the best form for producing excellent pyroxyline.--Corresp. Photo Mews.