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The photographic news
- Bandzählung
- 27.1883
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- 1883
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- Englisch
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1298, July 20, 1883
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The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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452 THE PHOTOGRAPHIC NEWS. [July 20, 1883. to : one is this—you will notice the light blue shadow creeping up the distant snow mountain, this shadow is absolutely lost in the negative, there is no trace whatever of it; the other feature is, that in the coloured print, the sky near the horizon is made lighter by the introduction of a pale yellow colour, but the photograph, as one would expect, has rendered this darker than the upper part of the sky, instead of lighter, thus absolutely reversing the effect the artist intended. Still, with all these discrepancies, the photograph has a certain harmony in its way. There is gradation, and light and shade, although the gradation and light and shade are not those of the original; but considering the violent colour and glaring effect of the lithograph, if we were compelled to make a choice between the t wo, most probably we should all desire the photographic copy rather than the original. The problem to be solved appears to me to be this. Supposing an artistic engraver had to reproduce this picture in monochrome, how would he do it, and what value would he attach to the various colours ? Taking this idea of reproduction as a standard, can there be any method devised by means of which an artist would have as much control over a photographic negative as an engraver has over his plate ? My first step in the consideration of this problem was to produce a very tbiu negative, giving all the detail in the drawing that it was possible to get. A print from this negative is before you. You will observe that there are no high-lights, no deep shadows : the whole thing is flat and in half-tone. This negative I took as a basis to work upon, and the result of this negative worked up is also before you. I do not pretend that the print from this negative is such a repre sentation as a skilled engraver would give us, but it will serve to demonstrate to you the power of alteration consequent upon the method of working I have pursued. The negative has been retouched by a clever artist, but one who has had no experience in this kind of work, and it is very possible that the effects produced are neither good nor true ; for to produce fine artistic effects from pictures by this method will require both talent and practised skill. My object is to demonstrate to you what could be done. By having the nega tive in half-tone it has been possible to give vigour to the shadows by covering the negative with paper and cutting out those portions which were wanting in depth ; to bring up the reds and yellows by working with chalk and pencil; to create the blue shadow creeping up the mountain by working upon the high-light of the enow, and leaving the other portion of the mountain in its original half-tint to represent the shadow ; to give luminosity to the sky near the horizon by stumping with chalk and powder colour, allowing the upper portion to remain as photographed. Thus I trust to have demonstrated to you this evening that, given a thin, well-exposed negative, such as can so well be pro duced with a gelatine-bromide plate, it is possible to work largely upon such a negative, to put in lights, and give depth and force to shadows, to lower the tone of colours that come out too prominently, and to raise to their proper importance colours to which the photographic sensitive surfaces refuse to render justice. THE EFFECT OF SHEARING STRESS ON SENSITIVE SALTS.* BY CAPTAIN ABNEY, E.E., F.R.s.t The subject which I have to bring before the Society to-night is one which has arisen from the observation that when plates are packed with masks, as by Mr. England’s plan, it sometimes happens that marks are found at the corners of the nlates where the gummed paper forming the corners of these masks comes in contact with them. The same phenomenon is also observable in plates packed by the ordinary zigzag plan. My attention was especially called to the fact, as I have observed, that plates from the top are not so liable to masks as plates at the bottom of (say) a packet of twenty-four. The question I asked myself wa%, then, “ What is the cause of these markings ? ” In order to investigate the matter, I commenced by taking a piece of glass rod, rounded so as to be quite blunt at the end, and writing lightly on a gelatine plate. On applying the alkaline deve loper, the part written on showed as a black line ; here there was a something on which to start an enquiry. Next it was * The author’s original title was, “The Effect of Pressure on Sensitive Salts.’’ He has thought it bettor to modify the title to the above. + Read before the Photographic Society of Great Britain. found that ferrous-oxalate development acted equally well. It struck me that this might be due to the action of the gelatine, which had been scraped, forming a nucleus on which develop ment could take place. The question was, how to settle that point. A plate was written on, and then half of it soaked in water till thoroughly swelled, and then developed ; the action was the same as before. Next half a similar one was immersed in bichromate solution, and where the plate was immersed no action was seen on applying the developer. Thus, then, the same oxidizing solution which will destroy an image produced by light will also destroy one produced by shearing stress. The question then again arose, as to whether it might not be the material of the glass rod which caused this effect; so plates were written on by gold, silver, ivory, talc, and wood, and still the same effects were in every case obtainable. A black-lead pencil seemed to be an exception, but this was only due to the repel- lant action of the black-lead by the developer. When the graphite was sponged away by water, and development proceeded with, the pencil-marks showed all the signs of development. If a plate is moistened, you do not get the effect, because the silver is not crushed. To my own mind, these experiments settled the question as to gelatine plates; but was the same obtainable on sensitive salts or other media, such as collodion ? A collodion-emulsion was taken, and treated the same way with the different media; and it was found that, where the film was not torn by the writing process, this same action took place, but more feebly, owing to the small shearing force that could be applied. To try further, a piece of gelatine paper was taken, and coated with collodion-emulsion, and written upon, with the result that the writing in each case developed. A piece of bromo iodized paper, prepared as I have described in the Journal, gave results, but very feeble, owing, in my belief, to the fact that the particles of sensitive salts could escape from beneath the writing implement. My next experiment was with a piece of mosquito netting placed on a gelatine film between two pieces of thick glass. These were squeezed together in a vice, and the film developed. Where the pressure had been brought to bear on the film, there the network developed as black lines. A collodion-bath plate was treated with an albumen solution, and dried, and the writing implements applied to the film, and in this case the result was almost imperceptible, for a reason which will presently be appa rent. A piece of cream-laid paper was placed over a gelatine plate, and writing executed on the film through the paper. With a gentle pressure the words did not develop, while when the pressure was strong they showed themselves perfectly, but the lines rather thicker than usual. In this case there was no abrasion of the film, as far as 1 could see. Let me lay the facts so far obtained before you. 1st. All rounded points applied direct to the film gave images on development, if the pressure was very slight, and the deve loping power was destroyed by bichromate. 2nd. The same points through a medium, such as thin paper, gave no result unless the pressure used was strong, when the lines seemed increased in breadth. 3rd. Pressure in a vice of a raised surface gave an image of that surface. Now the foregoing are different results to those which have been previously obtained by Warnerke and myself. Pressure from the back of a film produces insensitiveness of the film. In old experiments where friction had to be resorted to to bring green bromide to the state of orange bromide, in some cases the film fogged where friction had been applied, which is in accord ance with what is now brought forward, whilst in others the sensitiveness to light was evidently diminished. The former was so much the rule, that I have in many cases resorted to un- fogging the image by passing it through dilute hydrochloric before conducting experiments. I set this down, however, at the time, to the eflect of organic matter from the hand with which I usually rubbed. Can the effect be due to phosphores cence ? I think not, for I could not perceive any light when rubbing, and, if it were present, it must be very feeble—in fact, too feeble to produce a strong image such as I obtained. Again it struck me that it might be due to the breaking-down of the very sensitive form of bromide in the gelatine plates; orange bromide, however, gave the same result. Again, when I first tried the experiments, my thoughts naturally turned to Mosers experiments; but on testing the matter by using recently ignited mica as a writing material, this explanation was void. Other experiments, performed with the same view, conoborated this one experiment, and we must look for some other explanation.
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