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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 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. 1286, April 27, 1883
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Zeitschrift
The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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260 THE PHOTOGRAPHIC NEWS. LAPRIL 27, 1883. been required to obtain a silver print, after which the print—now of a light brown colour—was soaked in water until all traces of unaltered bichromate were removed. The wet print was now partially dried by means of blot ting-paper, and then exposed to the air until dry, after which it was laid between sheets of ordinary white paper, and well ironed with an ordinary flat iron, heated to about 150° Centigrade; the object of this proceeding being to harden the altered starch, and to enable it to hold the fatty ink firmly. The sheet was next moistened, laid on a sheet of damp blotting-paper, and inked by a velvet roller charged with rather thin lithographic transfer ink. This ink adhered to the exposed portions, which refused to take up water, as a kind of granular deposit, leaving the thorough damp por tions of the paper clear and white. The stippled ink picture thus obtained was then laid on a cleaned zinc plate, and etched into relief just in the manner which we have already detailed in our last volume. We pre pared two blocks of the same subject, that which we now place before our readers, and one on about double scale. Either block could be printed from in any ordinary newspaper ; and there would be no difficulty in making them up to three feet square, and in six or seven hours after the receipt of the negative. We cannot print the larger example in the Photographic News, but our readers will understand that the same grain would show to much greater advantage in the case of a large picture. We shall shortly publish further particulars as to the Asser process, and other methods by which stipple photo- typic blocks may be made. St. Nicholas’ Church, Chiswick, the subject of ouretching, is now no more; the whole of the original structure, except ing the tower, having been demolished to make place for a new building, which is to be erected at the expense of Mr. Henry Smith, a resident in the neighbourhood. The tower is nearly five hundred years old. COMPARING THE SENSITIVENESS OK DIFFERENT PLATES. That there should be a difficulty in naming a number which shall precisely denominate how much more sensitive gelatine plates are than wet collodion plates is not un natural, seeing that the sensitiveness of each varies much ; but it would at first appear somewhat strange that there should be difficulty in stating, with any degree of certainty, how much one gelatine plate is more or less sensitive than another. Yet such is the fact. Give to each of half- a-dozen photographers samples of plates coated with two different emulsions, and ask them to tell what is the differ ence between the sensitiveness of the one and the other, it will be found that opinions differing vastly will pro bably be given. Quite likely several will say one sample is the more sensitive, whilst the rest will give the palm to the other. Even if they are all agreed as to which is the more rapid plate, it is far from likely that they will agree as to what figure represents the ratio of sensitiveness between the one and the other. This difference of opinion is so far from uncommon—in fact, is a thing of such everyday occurrence—that it is worth while enquiring somewhat into the cause of it. The reason which will first suggest itself to the minds of many is, that different plates show the maximum of sensitiveness to rays of different refrangibility; in fact, that it is because the tests of sensitiveness are made in different colours of light, more favourable in certain cases to one plate than to another, that the difference of results arises. That this has something to do with the matter we have no doubt, but we believe it has less than is sup posed. True, the ratio between the sensitiveness of one plate and another maybe somewhat different in a brilliant white light and in that of a day if yellow fog; but the difference will be altogether too small to account for the very great difference of opinion which exists. We believe we state the true reason when we say that, except in a few cases, there is no common denominator whereby to compare plates; that, in fact, to try to tell how much more sensitive one plate is than another, fre quently is as difficult—or rather, we should say, as impossi ble—as to say how much more bright a blue light is than a red one ; or, to throw the matter into a strong light by taking an extreme case, to say how much stronger the sound of a certain bell is than the light of a certain lamp. We shall try to explain our meaning clearly by taking practical examples. There are in general use two ways of comparing the sensitiveness of plates. The one is by the use of the sen sitometer, and is the most common in this country. The other, which is used by Dr. Eder, and would appear to be in the most general use on the Continent, is by giving test exposures in the camera till a negative is obtained on each of the samples of plates to be tested, as nearly as possible similar to what is got on the others. The sensitiveness is then said to be in the inverse ratio to the exposure found necessary. We shall first consider the sensitometer method, and take an example. Let us suppose that we use a gas light to expose by, and that we wish to compare the sensitiveness of a wet plate and of a gelatine plate of moderate sensitiveness. We expose each for the same length of time, and on deve lopment we get the figure 10 with the wet plate, the figure 20 with the dry. Now what conclusion do we draw? If we follow the regular sensitometer method our conclusion is, that the dry plate is sixteen times more sensitive than the wet one, and, consequently, that an exposure of six teen in the case of the wet plate will give exactly the same result as an exposure of one in the case of the dry. Let us try the result of exposing the wet plate under the sensi tometer tablet sixteen times as long a’ both were exposed before, and see if we get a result similar to what we got before on the dry plate. We shall probably find that we get the figure 20, and so far the results are similar ; but let us now intensify our wet plate to such an extent that the first figures are of about the same density as are the first figures of the dry plate, and compare the general appear ance. We shall find that although the same last figure is visible in each case, the general appearance of the one is totally different from that of the other. In the case of the wet plate there is a rapidly rising scale of density in the last figures; in the case of the dry plate there is a long series of figures in which there is a gradual increase of density between one and the other. Now let us expose a wet and a dry plate in the camera, giving exposure of sixteen to one. We shall find that we get negatives totally different, and that the wet plate nega tive would be pronounced far more fully exposed than the others. By giving a somewhat shorter exposure to the wet plate (say one-eighth of what we gave the dry) we get nega tives somewhat more similar ; but let us vary the exposure as we may, we cannot get precisely similar results on the wet and on the dry plate. In fact, the curves of sensitive ness are different in the different plates, and there is no common denominator. Probably we shall arrive most nearly at a just comparison if we expose both till we get what is somewhat indefinitely described as a “ well-exposed negative.” This difference in the curve of sensitiveness, which is so marked in comparing a wet and a dry plate, also exists in the case of different dry plates, although to a somewhat less degree. It is particularly noticeable when we attempt to compare plates of very different degrees of sensitiveness. Good commercial dry plates give on the standard sensi tometer figures varying from 15 to 20. Very often a plate of very fair sensitiveness will be found not to give more
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