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The photographic news
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- 27.1883
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- Bandzählung
- No. 1277, February 23, 1883
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THE PHOTOGRAPHIC NEWS. Vol. XXVII. No. 1277.—February 23, 1883. CONTENTS. PAGE Latitude of Exposure 113 Thorough Washing and Effectual Fixation of Gelatine Nega tives 114 By-the-Bye.—Photography and Map-Making 115 Notes on Photography. By E. Howard Farmer 116 Alkaline Hoomatoxylin as a Developer. By J. Vincent Elsden 117 French Correspondence. By Leon Vidal 118 Marion’s Ferro-Prussiate Process. By Edgar Goold 118 Pellet’s Patent Direct Copying Process. By Edgar Goold 119 Notes 119 Patent Intelligence PAGE Zinc Bromide in Emulsion, and Excess of Bromide. By Captain Abney, R.E., F.R.S 122 Photo-Lithography and Photo-Zincography. By Major J. Waterhouse, B.S.C 122 On the Mounting of Photographs, and Injuries caused Thereby 124 Pho to-Micrography. By G. J. Johnson 125 Correspondence 126 Proceedings of Societies 126 Talk in the Studio 127 To Correspondents 128 I The Every-Day Formulary 128 LATITUDE OF EXPOSURE. The subject which forms our title is by no means a new one, and in fact is one about which so much has been written, that we need almost to apologise for introducing it once more. The fact that Mr. Woods read a paper on it at a recent meeting of the Photographic Society of Great Britain must be our excuse. Mr. Woods pointed out that, given a knowledge of the exposure, whether it is normal, a little below normal, or a little or much above it, there is possibility of com pensating in the development, and of producing in all cases a similar result. The agent which he mentioned as a restrainer was bro mide of ammonium or potassium, and he indicated that different plates show varying properties in regard to the latitude of exposure which is possible with them. The only distinction made by Mr. Woods was between bromo gelatine and bromo-iodide gelatine plates. We have lately been experimenting with a view of discovering what are the factors which give in one plate greater latitude of exposure than is to be found in another. As regards under-exposure the matter is simple. Lt is usual to consider the normal exposure for any plate to be the shortest which is compatible with good results ; that is to say, the developer recommended to be used with any given plate is generally the strongest which such plates will stand without endangering its good qualities, and such developer is considered to be normal for that par ticular plate. A somewhat stronger developer may cer tainly be used without actual fog or other defect making its appearance, and thus a small amount of under-exposure may be compensated for, but the amount is truly small. It is in the direction of over-exposure that compensa tion is likely to be of use; so that, if we know that a negative is either normally exposed, or over-exposed, we may still get a good result from it. The amount to which different plates are amenable to compensating treatment varies enormously ; or, more strictly speaking, the treat ment necessary to bring about compensation varies enormously with different plates. First, as regards the restrainer to be used. It is common to suppose that only soluble bromides act as restrainers, or that, if other subtances do the same, they act in a quite different manner; and it is usual to cite ammonium and potassium bromide as those most efficient. As a matter of fact, so far as we are yet able to judge, any neutral salt soluble in water, and which is not in itself incompatible with one or other of the developing agents, acts as a restrainer ; and there does notappear to be avast difference in the efficiency of different salts. We have tried bromides, iodides, chlorides, nitrates, and citrates, and of potassium and ammonium, and find that all act as restrainers. We, however, confine our remarks to the result of bromide of ammonium, as our experience of this salt has been the most extensive, and as it is the one commonly used. The next question is, as to what is the practical func tion of a restrainer ? Does it prevent detail from appear ing at all, or does it simply make the time which it is necessary for it to make its appearance longer than it would otherwise be ? In fact, is bromide a restrainer, or merely a retarder? So far as we are able to judge, it acts the part of a retarder merely, up to a certain point, and, after that, acts as a restrainer ; but the point where it ceases to be the one, and begins to be the other, varies vastly for different plates. This is a matter not altogether easy to understand at first, and requires further explanation. The following ex periment will teach much. Let a set of plates be exposed, each for a similar time under a sensitometer tablet, the exposure being such that with normal development the figure 20 or thereabout will appear. Let each of these plates be developed with different developers, containing each (say) one grain pyrogallic, three minims ammonia, but with the quantity of bromide varying from nothing in the first, to three grains in the second. The plates are not to be fixed, but to be examined immediately after develop ment and washing. It will be found that the first plate— that is, the one which was developed without bromide, and probably several more, show, looked at from the back, a positive impression of the first few squares, the parts which are black in front being white behind : that is to say, there is reversal of image. According to the nature of the plate, it may be found that only the one developed entirely without bromide shows this reversal, or that all up to (say) the plate developed with two grains show it. Now let us look on the face of the plates. We will find that up to about the plate where reversal of the image ceases, there is no decrease in the number which shows with in crease in the bromide, but sometimes, on the contrary, an increase. On the other hand, it will be found that after the plate which is the last to show reversal, there is a decrease of detail with an increase of bromide. From this we deduce the fact that bromide acts as a re tarder—merely causing development to be slower—up to the point where a tendency to reversal with a moderate exposure ceases; that in quantities greater than sufficient to prevent such reversal it acts as a restrainer ; and further, we deduce, as a carollary, that the plate which will require most pushing, either by decrease of bromide or increase of ammonia, to produce reversal, is the one in which, by the use of a certain given quantity of bromide, the greatest amount of compensation may be brought about. It does not follow, however, that, if properly treated, a plate showing early reversal of the image may not also be compensated for.
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