T H E ELECTROCHEAIISTRY O F LIGHT. VI1 BY W I L D E R D. B A S C R O F T
The Theory of Solarization.
Part I
In the preceding three papers, I have brought out the extraordinary differences of opinion in regard to the facts of solarization and the theories concerning them. As a basis for discussion, I give cz list of certain conclusions drawn by various people in regard to the subject of photographic reversals. It must be recognized that some of the conclusions are mutually exclusive and that some of them are probably inaccurate. I have been very much pleased, howe.i-er, to find that the differences and inaccuracies are more superficial than real, and that a slightly different wording of the conclusions i d 1 bring order out of what is apparently a hopeless chaos. So far as I can see, everybody has been more or less right and the trouble has been due largely to the failure to consider the field as a whole. In the following list of conclusions, names are occasionally appended in parentheses. n'hen this happens, it means that the men in question vouch for the facts or have held the opinions cited; but not necessarily that they hare discovered the facts or originated the opinions. (I) ,;1 reversal of the photographic image may be obtained with daguerreotype plates, with collodion plates, with gelatine plates, and with pure silver bromide. The reversal is, therefore, due primarily to a change in the silver salt and i t is independent of the nature of the material, such as gelatine or collodion, in which the silver salt may be embedded. The nature of the gelatine or collodion, however, has a marked effect on the readiness with which reversal takes place. (2) It is much easier to obtain a reversal with a gelatine plate than with a collodion plate, and it is quite difficult t o obtain a reversal with pure silver bromide. Reversals
450
W i l d e r D . BancroJt
occur more readily with a coarse-grained gelatine emulsion than with a fine-grained one. ( 3 ) -4 reversal of the photographic image may be obtained : by a prolonged exposure; by the antagonistic action of light of different colors (Herschel, Draper) ; by an exposure to white light followed by bathing in a solutiotl of an oxidizing agent and exposure to the spectrum (-qbney); by the use of stained plates (n'aterhouse); by the action of white light during development (Sabatier, Nipher) ; by very slow development of an under-exposed plate (Guebhard) ; by addition of thiocarbamide, etc., to the developer (Waterhouse) ; by very short and very intense exposures with a subsequent short exposure to diffused light (Clayden, ITood) ; by cathode rays followed by sunlight1 (Goldstein) ; by positive light of two intensities' (Goldstein), (4) The stronger the developer and the longer one develops, the more easy it is to obtain a reversed image. Very slow development of an over-exposed plate will give a normal image. The latent reversed image is destroyed more rapidly than the latent normal image by bromine, nitric acid, ammonium persulphate, chromic acid, etc. (5) n'ith pure silver bromide collodion and chemical development, the reversal begins a t approximately the same time as the visible blackening of the silver bromide; but with silver bromide collodion containing silver nitrate, visible blackening takes place long before a latent reversed image is formed iEder). (6) The visible blackening of the plate which causes the second reversal is probably due to the formation of allotropic or metallic silver. ( 7 ) The reversal of the image is facilitated by oxidizing agents and prevented by reducing agents iAbney). (8) Oxidizing agents and reducing agents have nothing to do with the reversal of the image. Halogen absorbers * Goldstein studied the blackening of silver iodide and did not a t t e m p t to obtain images. I t would be possible t o obtain reversals and consequently the cases are included here.
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prevent the reversal of the image if present during the exposure (Liippo-Cramer). (9) n’hen a plate is exposed to light during development, the partially developed image acts as a screen, protecting the silver bromide below it (Seely). TTrhen a plate is exposed to light during develop(IO) ment, it makes no difference whether the light falls on the plate from above or below. There is, therefore, no screening effect due to the partially developed image (Sipher, Trix-elli). In most cases there is a simple quantitative re(11) lation between the original exposure and the amount of light necessary during development if the plate is to be brought into the ‘zero state’ (Sipher). (12) The latent reversed image can be developed physically. If a plate is fixed first and then developed physicallj-, we ma\- get a normal image or a reversed one, depending on the conditions of the experiment. (13) In the developed plate, the reversed image is chieflv in the lower layers. (14) According to Eder, there are three images on an undeveloped over-exposed plate : a latent normal image, a latent reversed image, and a normal silver image. ,lccording to Homolka, there are two latent normal images. ( I j ) TT-hen a platinum electrode, coated with silver bromide, is dipped in a bromide solution and exposed to a moderate light, it becomes the cathode. n’hen a stronger light is used, the platinum electrode will sometimes tend to become’the anode. iI 6 ) -1film of silver iodide swells and clouds when exposed to light. No measurable change in chemical composition takes place, and the rate of clouding under the influence of light seems to be increased by the catalytic action of oxygen, iodine, etc. (Scholl). (17) The substance forming the latent reversed image is a silver oxybromide of the general type X,oBr,O, (-1bney Eder).
JViZdey I?.Bancrojt
45 2
(IS) The oxidation hypothesis of ,Ibney is absurd (Luppo-Cramer). (19) There is no such thing as a latent reversed image. The phenomena are due to the regeneration of silver bromide. (20) The latent reversed image is a reduction product of silver bromide (Luppo-Cramer). (21) The developable image consists of silver bromide and an cr silver sub-bromide which is itself not reduced by the developer; but which acts as a catalytic agent, causing the developer to reduce the silver bromide. This a silver subbromide can be reduced by light to a 2 silver sub-bromide which is also not reduced by the developer but which has no catalytic properties. Reversal occurs when the a silver sub-bromide has been changed in the high lights very largely to 2 silver sub-bromide and this change has not taken place to any great extent in the shadows (Trivelli). ( 22 ) The n’aterhouse process with thiocarbamide in the developer gives pseudo-positives (Nipher). Since the latent normal image is unquestionably a sub. stance of variable composition’ containing less bromine than silver bromide, the first question to be decided is whether the latent reversed image consists of silver, bromine and oxygen, or of silver and bromine only. In other words, we have to decide for or against ,Ibney’s theory or any possible modification of it. It must be admitted that most of the objections to ,Ibney’s theory are unsound and that a very strong case can be made out for it. Luppo-Cramer contends that Bbney is wrong in his statement that reducing agents prevent the reversal, because silver nitrate and sodium nitrite are not reducing agents and yet they are effective in preventing the reversal. Liippo-Cramer says that these two substances act as they do because they are halogen absorbers. This is precisely the same quibble’ which was made >Tears ago in regard to chemical sensitizers. Since bromine is an oxidizing agent, any substance which destroys it is to that l
1T.eisz: Zeit. phys. Chem., 54, 334 (1906). Luppo-Cramer: Photographische Prohleme, 140 ( 1 9 0 ~ ) .
extent and for those conditions a reducing agent. It is entirelj- a question of definition as is shown by the fact that Aibneyhimself states that potassium nitrite‘ is very effectiT-e in preventing the reversal of the image. -1s in the case of the chemical sensitizers, the whole difficdty disappears if we sa)- that depolarizers prel-ent the reversal of the photographic image when present in the filni in sufficient quantities during the exposure. If we accept Xbney’s theory, a good many phenomena become intelligible a t once. Oxidizing agents facilitate the reversal of the image and reducing agents check it. In the case of long exposure the bromine reacts with moisture in the film, forming hypobromous acid which oxidizes the latent normal image. Since acids increase the oxidizing power of an oxidizing agent, we see that acids must increase the tendency t o reverse, a point which troubled *Ibney a good deal. Since the concentration and the oxidizing power of the hypobromous acid depend, under these circumstances, on the concentration of bromine, we shall consequently expect a greater tendency t o reversal in a coarse-grained emulsion than in a fine-grained one where the bromine may be lost by diffusion. The difficulty in getting reversed images with pure silver bromide’ might be due to the lack of moisture. This seems more probable when we reflect that Luggin3 got marked reversal under conditions which involved high bromine concentrations and therefore relatively high concentrations of hypobromous acid. This is very satisfactory as far as it goes, and we began our experiments with the firm conviction that some niodification of Xbney’s theory would probably account for all the facts. The first shock came when it was found that a solution of pure hypobromous acid did not enable us to get satisfactory reversals even though the concentration was varied over fairly \Tide limits. If the hypobromous acid Cf. Bancroft: Jour, Phys. Chern., 13, 20 (1909) TYeisz: Zeit. phys. Chem., 54, 351 (1906). Zeit. phys. Chem., 23, 592 (189;).
were sufficiently dilute, it appeared to hare very little effect one way or the other. V-hen niore concentrated, it enabled us to get a much better normal image than we should otherwise have obtained. In fact, it is an admirable agent for removing fog. Further investigations showed that both of Xbney’s generalizations are inaccurate in the form in which they are usually put. It is not true that oxidizing agents facilitate the reversal; but it is true that one gets a better, reversed plate after development if oxidizing agents are present in the film during exposure. It is not true that depolarizers check the reversal; but it is true that they produce results which mask the res-ersal. This discovery destroyed the foundations of the oxybromide theory and we are therefore forced to conclude that the latent normal image consists primarily of silver and bromine only. Of the subhalide theories so-called, Trivelli’s is the latest and the most complete. I cannot accept any theory which postulates two silver subhalides neither of nhich can be reduced by the developer while one of them has catalytic properties and the other has not. X further argument against Trivelli’s theory is that he comes to grief oT-er -4bney’s experiments. theory which accounts equally well for facts and for alleged facts is not a safe guide. I shall, therefore, try to show that all the observed phenomena can be accounted for on the assumption that the latent image, whether normal or reversed, consists essentially of silver and halogen. n‘hile it is necessary to assign certain properties to some of the intermediate stages, it is neither necessary nor desirable to assume the existence of allotropic modifications. ll-hen studying solarization, it has been customary to make different exposures and to develop for a given length of time. The resulting densities are then plotted as ordinates ~
ofcourse
in so f a r as the substance in the gelatine plate is colloldal,
i t nil1 not lie absolutely pure
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against the degree of exposure as abscissas. Quantitative data for such curves are to be found in the papers of Hurter and Driffield, Sterry, hlees and Sheppard, and others. -1 qualitative diagram by Trivelli has been reproduced in the preceding paper.' In Fig. I , I give an imaginary sketch of what one would get if one were to point the camera a t the sun. The degrees of exposure of that portion of the plate on which the image of the sun is focussed will always be greater than that of the rest of the plate and consequently will always be represented by a point further to the right on the curve O-IBS. If the degree of exposure of the sky is represented by the point H and that of the sun by the point I,, we see that the sun will develop black against a relatively light background. If the degree of exposure of the skj- is repre-
Degree o f exposure
Fig.
I
sented by the point I, and that of the sun b>- the point l f , the developed plate will show the sun and the sky of the same degree of blackness and there will be no visible picture. If the txTo points are 11 and N, we shall get the sun coming out light against a relatively darker sky. In other words, m-e shall have a positive image instead of the negatil-e which we g o t from H and I,. If the degree of exposure of the sky be represented bj- R and that of the sun by S, we shall get a negative, the second negative as it is called. It has been pretty well established that this last image is due to the reduction of silver bromide to metallic silver b>- light. It is the image which we ordinarily get when we make prints with Jour, Phys. Chem., 13, ' 9 3 (1909)
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albumenized paper. So far as the facts go, it seems probable that this metallic silver image is intensified when the plate is developed. This is a point which is apparently not of any especial iniportance so far as the theory of solarization is concerned. The diagram shown in Fig. I is merely a yualitatiT-e graphical representation of observed facts. There is nothing in it to show the unknown degree of decomposition of the silver halide and yet that is what we need if we are going to account for the phenomena. I make two assumptions in regard to the behavior of the silver halides : (I) The action of light on a silver halide corresponds to that of a direct current or of a fairly weak reducing agent. It causes the silver halide to pass through all the possible stages. (2) -1fairly strong reducing agent (a developer) does not cause the silver halide to pass through 311 the possible stages and does reduce certain sih-er-halogen solid solutions or subhalides faster than others which contain less halogen. S o one can take exception to the first assumption, vhich is m.ere1y a formal statement that the theorj- of Grotthuss applies to the casc. I t would be perfectly legitimate to object to the second assumption. It is avon-edlj- an aswmption cd hoc, and reuuires justification. Some day I hope to furnish experimental proof of this assumption; but that is not going to be an easy matter a: best and may take a very long time. For the present, thercAfore, the usefulness of the assumption will have to be its justification. It is well, however, to note that the assumption is not an impossible one. lye know that phenylhjrdroxylamine is ordinarilJ- reduced to aniline without passing through the possible intermediate stages of azobenzene or hydrazobenzene. IT-e also know that phenl-lhydroxplamine is reduced to aniline more rapidly than hydrazobenzene. I agree entirely with Trivelli that a sharp distinction should be drawn between the ease or rate of development of a plate, and the sensitil-eness of the plate to light. lye
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45 7
know that the rate of development of an unexposed plate is practically zero when an ordinary developer is used. When the silver bromide has lost a little bromine through exposure to light, the plate can be developed and therefore has a definite rate of development. On the other hand, the rate of development of metallic silver is of course zero. The rate of development for a given plate and a given developer, therefore, passes through a least one maximum as the composition of the silver salt changes from silver bromide to sill-er. The senqitiveness of the unexposed plate is high. IThether it is as high as that of a plate which has received an infinitesimal exposure is a point on which there is some difference of opinion; but which does not concern us at present. The important thing is that the rate of development may be low when the sensitiveness to light is high. This shows that the two sets of phenomena do not even run parallel. In Fig. 2 are given exaggerated representations of the behas-ior of a plate with two different strengths of the same developer. The drawing is not to scale. The ordinates are rates of development while the variation of any grain of the emulsion between silver bromide and silver is represented along the abscissas. If the possible phases are a series of solid solutions from -4gBr to -Ig2Br, and then the appearance of metallic silver, there will be a break in the curve a t the composition ,4g,Br and metallic silver will be present beyond this point to an ever-increasing amount. It is not necessary, however, to assume an\- such state of things. So long as there is not a continuous series of solid solutions from silver bromide to silver, metallic silver will have to appear a t some point before we reach the extreme right of the diagram and a break in the curve will occur wherever that point may- be. If further investigation s h o w the existence of phases other than a solid solution and metallic silver, there will be breaks n-herever the new phases appear. If v e postulate that the degree of reduction of the silver bromide is some direct function of the light intensity into the time of exposure, the abscissas also represent
W i l d e r I).Rancroft
458
degrees of exposure in unknown units. This is indicated by the words “degree of exposure” and by the symbols o and X. I n so far as the blackening of the plate is proportional to the rate of development, we could measure densities along the ordinates as in Fig. I . The curve for density and degree of exposure ceases to correspond to that for rate of development and composition at the point B in Fig. I , where metallic silver first appears. As in Fig. I , for any given exposure, the high lights will be represented by points on the curves in Fig. 2 lying to the right of the corresponding points for the shadows. The curve OI%B represents the action when a normal developer is used. At -1,the maximum of the curve, we get the greatest blackening for a given time of development. If a plate is exposed for a short time, so that the decomposition in the high lights is represented by the point ,1 or by some point x
0
D e g r e e of exposure
+
/‘
Composition
A9
Ag Br
Fig.
2
t o the left of that, the decomposition in the shadows will be represented bj- a point still nearer to 0. The diagram shows that, in this case, the high lights will develop faster than the shadows, giving us a negative. If we expose for so long a time that the decomposition in the shadows corresponds to X or to some point to the right of it, the decomposition in the high lights will be represented by some point further down the curve towards B. The shadows will develop
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faster than the lights and we shall have a positive, in other words, a reversal of the image. For intermediate exposures in which the shadows are represented by a point to the left of -1and the high lights by a point to the right of A, we shall get a partial reversal, the high lights reversing the first and the shadows reversing the last. If the difference between the high lights and the shadows is not very great or if the curve O.IB is very flat at the top, an exposure corresponding to *A for the half lights will give a plate which is pretty uniformly fogged. -111 these phenomena are matters of record and they have also been checked in my laboratory. -111 the experiments which will be referred to in this paper were made with Cramer’s Lantern Slide Plates. Plates were exposed in a printing frame behind a given lantern slide for different lengths of time. The intensity of the light, the distance of the plate from the light and the nature and composition of the developer were kept constant. n‘ith an exposure of two seconds a good negative was obtained on development. U3th an exposure of five minutes, a good positive was obtained on development. JYith an exposure of two and one-half minutes, the plate developed very flat and only partially reversed, the deep shadows being the last to change, One reason for supposing that the curve OXB may be very flat and relatively broad a t the top is that amateurs often over-expose their plates badly and yet it is very seldom that they get beyond the stage of fog and into the stage of partial or complete reversal. Since the form of the curve OA4Bin Fig. 2 depends on the nature of the developer, the position of X may vary. If we should add something to the developer which would make the curve OA4 more nearly vertical and which would thus move A over very close to the axis of ordinates, we should have a reversed image with a much shorter exposure than is necessarj- when using the ordinary developer. This is perhaps what happens with Iyaterhouse’s thiocarbamide developer; but me have not yet tested his method experimen-
T Vildev D . B a m i o j t
460
tally, so I cannot speak definitely with regard to this. Of course, the really important thing is to explain why thiocarbamide should displace the maximum to the left. I do not care to offer any hypothesis in regard to this now; but I feel certain that a knowledge of how thiocarbamide acts will help us in finding out why it acts as it does. If we start with a dilute developer or with a developer to which we have added a good deal of bromide, the rate of development will be less and we shall, therefore, get a curve corresponding to OCD in Fig. 2 . Since we can obtain a fair negative with such a developer from a plate which would fog badly with a normal developer, we know that we must still be on the ascending portion of OC when we have already reached the top of the curve OAR. I n other words, the point C lies to the right of the point A. I do not know whether the curve OCD lies entirely inside of OAB or whether it crosses the latter somewhere in the right-hand portion of the diagram. Since the matter is of no importance so far as this paper is concerned, I have adopted provisionally the simpler of the two hypotheses, that OCD lies entirely inside of OAB. The relative positions of A and C have been deduced from the experimental facts in regard to the development of negatives after ordinary exposures. No assumption has been made in regard to the development of positives. From the diagram, we see, however, that exposures corresponding to'points on the abscissas between ,4 and C will develop as positives if we use the normal developer and as negatives if we use a weak or a restrained developer. In other words, the reversal of the image is checked by the use of a weak developer or of a restrained developer. It was this fact which caused Prechtl to believe that polarization is a phenomenon of development only. There is, however, still another conclusion to be drawn from the two curves in the diagram. For exposures corresponding to points to the right of C we shall get positives with either developer, pro~ _ _ _ _ _- - _ _
Cf Jour. Phys Chem , 13, 76 (1909)
The Electioclzeniistry
0)
Light
461
vided we do not expose for such a time that we get the second reversal. This has been confirmed experimentally. 113th a certain exposure, the plates developed as positives with the normal developer and as negatix-es with a diluted developer. 1Yith a still longer exposure, positives were obtained with both developers. \Ye next took up the question of the cfiect of a depolarizer in the film during the exposure. If a plate be dipped into a solution of a depolarizer and then given a long exposure, no reversal is obtained and the conclusion is therefore drawn that depolarizers prevent the reversal. lye shall modify this conclusion radically if we take into account Eder’s work on the latent image. Eder’ believes that an over-exposed film usually contains three images in varying relative amounts : a latent normal image, a latent reversed image and a normal silwr image. If n-e expose a plate in contact with a depolarizing solution, we increase the rate of reduction and consequently the intensity of the silver image very much. After development, we may have a reversed image on the plate but we shall not see it if the normal silver image formed during the exposure is the more intense. The reversal will have taken place but it nil1 have been masked. 1Vhen describing somc experiments with silver bromide collodion t o which a silver nitrate solution had been added, Eder’ says: “The traces of silver nitrate which cling to the silver bromide even after long washing with water, cause a very quick and dense blackening of the plate, so that the reversal is quite masked by it. A short treatment of such a plate with nitric acid (sp. gr. 1 . 2 0 ) , after the exposure and before the development, changes the plate to such an extent that reversal can readily be detected when the exposure is one thousand times the induction limit.” This normal silver image, which can be destroyed by nitric acid, is the one which constitutes the second reversal and which gives rise to the portion BRS in Fig. I. -An\-
Zeit niss. Photographie, 3, jjY (190j). Cf Jour Phys Chem , 13, 60 (1909)
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Erz'Zdey D . Bunciojt
state of things which intensifies this normal silver image will tend to prevent the first reversal from showing. The reason why it is difficult t o obtain reversals with silver chloride dry plates' is that light reduces silver chloride readily to rrietaltic silver. Consequently the normal silver image before development is apt to be so intense as to mask any reversal. So\\ that we k n m - why the depolarizer acts as it does, it is clear that the way to obtain a reversal is to shorten the exposure and not to lengthen it. Experiments were accordingly made with plates which had been dipped into a solution of potassium nitrite. IT-hen a relatively short exposure was made, the plates developed as positives ; v ith longer exposures the plates developed as negatives. Intermediate exposures gave partial reversals. Since we are dealing with a second reversal, the conditions are exactlj- opposite t o those previously considered and the high lights are the first to change from positive to negative while the shadows remain black up to the last minute. Luppo-Cramer did not succeed in getting a reversal when he had sodium nitrite in the film; but he worked under absolutely unknown conditions. He says2 himself that he dipped dry plates for two minutes in a two percent solution of sodium nitrite and then dried them. There is nothing to show how he dried them and consequently we do not know at all what the concentration of the sodium nitrite really was or whether it was the same in any two plates. A11 we can be sure of is that the solution in the film contained more than two percent of sodium nitrite. It is probable, as suggested by Trivelli, that the experiments of Waterhouse with stained plates are really experiments with films containing weak depolarizers. Reversals would, therefore, take place with shorter exposures than in the case of plates which had not been stained. TTe have not yet had time to take up this point experimentally and so Liippo-Cramer: Photographische Probleme, I j3 (190;). Z C f . Jour. Phys. Chem., 13, 183 (1909).
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I cannot be certain that other factors may not have to be considered. If the presence of a depolarizer masks the reversal but does not prevent it, there is no reason to suppose that the presence of oxidizing agents in the film will really increase the tendency to reverse. A plate was bathed in a dilute permanganate solution and then exposed along with another plate froni the same box which had not been so treated. The conditions of exposure and of d e d o p m e n t were as nearly identical as possible for the two plates. It was possible to find a time of exposure such that the permanganate plate gave a negative on development and the other plate a positive. This particular oxidizing agent, therefore, apparently checks reversal instead of helping it. I do not care to lay any stress on this point because, of course, there is some absorption of light by the permanganate and the effective exposure was therefore somewhat less than with the other plate. Since there would be a slight oxidation of the developer, the permanganate plate was possibly developed with a somewhat weaker developer than the other, which would decrease the tendency to reverse. These are trifles however. The important fact is that treatment with an oxidizing agent does not cut down the tirne of exposure necessary to produce a reversal when we expose to the same source of light and develop iyith the same initial develope;. On the other hand, with a good liberal over-exposure, the permanganate plate gives us a much clearer reversed image than does the plate which has not been treated. This is really what Xbney observed and what he meant when he said that oxidizing agents facilitate reversal. The explanation is a very simple one. n'hen we use no oxidizing agent, the finished plate contains a more intense reversed image and a less intense normal silver image. IYhat we see is the difference between the two. Since the presence of depolarizers in the film causes a more rapid reduction to silver, the presence of an oxidizing agent will retard this same reduction. Our finished plate will then contain less of the normal silver image and will
464
IVilder D . Bamrojt
therefore give us a clearer reversed image. The masking of the reversal is decreased by the presence in the film, during exposure, of a substance which checks the reduction to silver. by light. If we expose a plate for such a length of time that we are still on the curve OX in Fig. 2 , we get a negative on development. If we make a second exposure to white light of uniform intensity before development , we increase the decomposition over the \Thole plate and bring the plate into a state corresponding to some portion of the curve AIB, On developing, we then get a reversed image. This accounts for what Trivelli calls the Sabatier polarization. It is, of course, possible to make the exposures in the reverse order. nrhether the resulting positive will be as good of its kind as a good negative depends on a number of factors, which I prefer to discuss at some other time when I take up the whole question of developing positives from instantaneous exposures. Tri~-ellilstates that a second exposure to light is identical in effect with an exposure during development. It seems to me very probable that there are some quantitative differences. far as the qualitative side of the phenomenon is concerned, the matter is quite simple. The light pushes the plate along while the development is going on. If the effect of the light is relatively intense, the image develops as a positive. If the effect of the developer is the more important, we get a negative. In one experiment three plates were exposed for fifteen seconds each, behind the same lantern slide at the same distance from the same source of light. The same developer was used in each case; but the first plate was developed at a distance of one meter from a Welsbach burner and came out a negative. The second plate was developed a t a distance of fifteen centimeters from the same Welsbach and came out a positive. The third plate was developed a t a distance of fifty centimeters from the Welsbach and came out partly positive and partly negative. Zeit. wiss. Photographie, 6 , 2 3 8 (1908).
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There was one thing about the last plate which n a s distinctlj- interesting though it has nothing to do with the theory of solarization. lye were copying a lantern slide made from a photograph of a distinguished French physicist dressed in his academic robes. The gown was black silk and there were large white buttons down the front. For obvious reasons it was impossible to see the shadows which the buttons cast upon the black gown. On the plate in question, merything had reversed except the gown and consequently TVC had the white buttons casting a black shadow on the apparently white gown. lTe have now to consider the antagonistic effect of different lights, a phenomenon which Trivelli has called the Herschel effect. In the case of the silver halides, the matter is complicated by the formation of photo-halides having different colors and consequently absorbing light in different degrees. These are mere matters of detail and the general theory is quite simple. If we work with a sufficient quantity of pure silver halide in a closed space, there will be an equilibrium pressure of the halogen corresponding to the chemical potential of each wave-length of light or to the ciiemical potential of any other form of energy, cathode rays for instance, which tends to set free the halogen. If we pass from light which causes one equilibrium pressure to light which causes another equilibrium pressure, we shall have a change in the dissociation and consequently an antagonistic effect which may or may not lead to a reversal, depending on the conditions of the experiment. This appears very nicely in some experiments that Goldstein' made with a silver bromide so prepared that it was not sensitive t o light. This salt is blackened by cathode rays and brought back to the yellow state by positive light of a certain intensity. Since positive light of a certain intensity blackens the salt, it was possible first to blacken and then to bleach it by changing the intensity of the positive light. Cathode rays set free Cf. Jour. Phys. Chem., 13, 48 (1909)
M,‘ildey D . Bancrojt
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enough iodine from silver iodide to blacken the salt, positive light enough to stain it orange, while daylight restores the original color. Alkali salts can be colored by cathode rays or by ultra-violet light and are blackened by daylight. Goldstein also showed that, in a closed tube lighted from above, it is impossible to blacken both the upper and the under sides of the silver bromide simultaneously. n‘hat is a t the moment the surface is always blackened while the under side loses its black color. “n‘ith a continuous illumination in the open air, diffusion and air currents prevent the utilization of the free bromine and there is, therefore, an increasing blackening and decomposition without regeneration.’ ’ This question of diffusion is probablq- of extreme iniportance in several cases. If the grain of the silver bromide gelatine plate is relativelj- coarse, some of the bromine set free by light will gather in the inner portion of the grain and will tend to prevent the reduction to silver. n‘ith such plates, we should expect to get a reversed image without difficulty and this is exactlv what happens. TI-ith a finegrained emulsion there is a greater tendencj- for the bromine to pass off and an increased tendencv for the reduction to silver to take place. Luppo-Cramer’ states that it is impossible to obtain reversal phenomena with a silver bromide gelatine having an extraordinarily fine grain. I think it is much more probable that this is a case of masked reversal and I am inclined to offer the same explanation in regard to the difficulty of obtaining reversals with pure silver bromide containing no binder. In the Clayden effect, a very short exposure to an intense light followed by brief exposure to diffused light causes a reversal while no reversal is obtained if the original exposure is longer than about one fifty-thousandth of a second. The fact of the reversal presents 110 difficulty a t all. JTe know that the more intense the primary exposure, the less intense -
~~
~~~~
-
Photographische Problenie, 14; (190;)
The Electl-ocliemistl-y of Light
467
does the secondary exposure have to be in order to cause a reversal. The t r o u b l e s ~ ~ nfact x is that no res-ersal takes place if the original exposure is' prolonged beyond a s-ery brief interval. lye have not yet had time to repeat these experiments and consequently I cannot be certain as to the explanation. I suspect, however. that we are dealing with a case of masked rex-ersal. -1bney worked v i t h a light of moderate intensitj- arid added a depolarizer to the film. Clayden worked with a light of great intensity and added no depolarize; to the f i h . In the experiment prel-iously referred to, Eder showed that a reversal did take place in the case of silver bromide collodion to which silver nitrate had been added, and I hope to be able to show the same thing some daj- for the ClaJ-den effect. Trivelli' has called attention to a case in which an Eastman film was developed thrce years after exposure and came out a positive instead of a negative. I n the last few months my attention has been called by Prof. Jloler of Cornell, by Prof. Friedburg of the College of the City of Kew York, and by others to cases in which films had developed as positives instead of negatives. In every case an interval of several months or more had elapsed between the exposure and the development. The explanation offered by Trivelli is undoubtedlj- the right one, that chemical decompositions in the film had produced the same effect as a longer exposure to light. I n the film shown me by Professor Illoler, apparently the whole picture had reversed except where the sunlight fell on the white collar of a man. That half of the collar was black. Xt first sight this seems abnormal because we usually have the high lights reversing before the shadows. It all becomes intelligible if we assume that the chemical reduction due to, or accompanying, the decomposition of the film has carried the plate along nearly to the point where we get the second reversal and the appearance of the normal silver image. TT'hen discussing the effect of a reducing agent Cf.Jour. Phys. Chem.,
13, 309
(1909)
in the film, I h a w pointed out that the change from positive to negative takes place first in the high lights. I therefore conclude that the blackness of the man’s collar was due to a second reversal and not to the first reversal having failed to take place. ;1 necessary corollary is that by treatment with certain chcmicals one ought to be able to bring a plate from the point where i t will develop as a negative to a point where it will develop as a positive or even to a point \\.here we get E! second negative. This apparently was done by Carey Leal twentj- years ago, using sodium hypophosphite as the preliminary reducing agent. The theorj- outlined in this paper is a T-ery simple one involi-ing practically no assumptions. I t is based on the theory of Grotthuss and appears to be a logical deduction from the experimental fact that silver bromide which has been slightly decomposed b>- light is reduced faster by a developer than either silver bromide which has not been exposed at all or silver bromide which has been completely decomposed bj- light. The theory seems able to account for all the phenomena of reversal and it reconciles the divergent views of such men as *Ibney, Eder, Liippo-Cramer, Englisch, and Precht. In the next paper I expect to discuss the methods of developing a positive after an instantaneous exposure. There are also some points in regard to the experiments of Nipher, Luggin and others which can perfectly well be left till later. Cornel1 Cunwsztj’ Am Jour Sci ( 3 ) , 33, 186 ( r b b i l
