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Nature Photography in Scientific Research 356 NATURE AUGUST 26, 1939, VoL. 144 PHOTOGRAPHY IN SCIENTIFIC RESEARCH A conference was held in l\Ianchester during The migration of atoms through solid bodies by July 3-4, under the auspices of the Man­ a process in which electrons and ions move separ­ chester and District Branch of the Institute of ately, is a fairly general phenomenon. It occurs, Physics, and under the chairmanship of Dr. F. C. for example, in the atmospheric oxidation of metals, Toy, and was devoted solely to photography, with as is sho'm by the dependence of the rate of that the principal object of giving an opportunity for dis­ process upon the oxygen pressure3• A similar cussion of the many photographic problems met mechanism is responsible for the thermal dis­ '\ith in scientific research, where photography integration of certain azide compounds at 100° c.•. plays so large a part. A large amount of time was To understand the formation of the photo­ necessarily given to reviews on the various aspects graphic latent image itself it is necessary to assume of photography and its applications, but in the the existence of an electron trap before exposure. first session fundamental problems of latent image This probably consists of impurities like silver formation were discussed and some original work sulphide, the so-called sensitivity specks•. A very was reported ; this will occupy most of the space small silver speck is considered to be not as stable available for the present report. as a larger one ; electrons and ions may 'evaporate' Prof. N. F. Mott opened the session by a from, and thus break l\p, a small speck•. This "Description of the Photographic Processes in process was regarded as responsible for the Terms of Atomic Physics", Dr. J. L. M. Brentano, 'reciprocity failure' at low intensities ; at constant Dr. ,V. F. Berg and· others contributing to the exposure (intensity X time) the density depends discussion. Dr. S. 0. Rawling gave a paper on on the intensity level, because at low intensities "Some Theories of the Action of Developers". a speck may partly evaporate, and thus be too Much of the matter dealt with bas already been small to make the grain developable, whereas at reported in NATURE' and elsewhere. The latent higher intensity the speck reaches a stable size image which can so far be revealed only by the more quickly without losing matter by evapora­ process of development was assumed to be a small tion. speck of silver. Modern atomic theory enables a Some of the experiments reported by Dr. more detailed model of the flocking together of Brcntano appeared to require at least some quali-· the isolated silver atoms formed by light to be fication of this picture. These experiments were given'. The silver halides arc photo-conductors, made on diluted emulsions, thus avoiding the that is, mobile electrons arc formed if light is complications due to the multi-layer nature of absorbed. The silver halides are also electrolytic commercial materials. The shape of the foot of conductors : some silver ions (in 'interstitial' experimental characteristic curves required the positions) will move through the crystal if an assumption that a grain must absorb 10 or more electric field is applied. Thus, if there is a place quanta to be made developable. This is of impor­ in the crystal where electrons can be collected tance for the consideration of low intensity re­ and held 'trapped', a field '\ill be set up and the ciprocity failure and suggested experiments in ions will move up and neutralize the electrons, which a certain exposure was given in a few thus forming metallic silver. A piece of silver in instalments, '\ith varying intervals of time in contact with silver halide constitutes such an between. Below and near the threshold, ex­ electron trap. In this way, once a small speck of posures were additive, even when separated by silver is formed it will continue to grow if light is intervals up to 300 hours. Thus, a· markedly stable absorbed by the crystal. During that process the result is obtained already at the very first stages halogen atoms must escape, otherwise a regression of an exposure. At higher densities, an inter­ would occur. It is important that in the model ruption of an exposure causes a regression, which suggested the halogen will escape through the is greatest where the characteristic curve is crystal surface and will not be liberated at the steepest. For small exposures an interruption after interface of silver and silver halide. The depth }, ! or i of the exposure has been given causes the of the electron traps can be determined optically same amount of regression. Several breaks even by the Herschel effect : red light will release when close together cause a larger regression than electrons from the traps and thus bleach the latent a single break. These findings might be accounted image. Thermal energy would not be enough to for in two ways. One is to assume that the last release electrons, and thus a silver speck in contact electron or ion joining a growing speck is for some "ith silver halide is stable. time in a less consolidated state. The other is to © 1939 Nature Publishing Group No. 3643, AuGUST 26, 1939 NATURE 357 modify the assumption of the small specks break­ slowed down or arrested. The experiments lead ing up and getting lost by a hypothesis introducing to the following views•. At low temperatures free a certain delay in the early stages of latent image electrons arc formed and then trapped, partly in formation. the sensitivity specks, partly in other less stable Two groups of experiments were made with the trapping centres in the grain. The depth of the intention of shm,ing whether such a delay could traps could be investigated by releasing the be due to a process of temporary trapping of trapped electrons by red and infra-red light. Ionic electrons outside the latent image. Experiments movements occur on warming up only and depend in which exposures to light of different wave­ solely on the total number of electrons trapped, length, balanced to produce the same density for which in turn depends only on the total number the same time of exposure, were added, showed a of quanta absorbed. Thus, no reciprocity failure small but distinct difference in density if the order should occur at these temperatures, as has actually of the exposures was reversed _; this was inter­ been shown10•11 • These experiments, while com­ preted as demonstrating a small amount of patible with the Gurncy-Mott theory, cannot be An X-ray examination of the state regarded as proving it, since several additional of crystalline perfection of a number of emulsions hypotheses were required for their explanation. showed that in general the more sensitive emulsion Tho value of the theory as a working hypothesis grains arc subdivided into smaller crystallites as that had given new stimulus to research into indicated by the broadening of the X-ray lines. photographic problems was stressed. Another possible interpretation is the existence of Mr. E. R. Davies's paper on "Some Important a bigger strain in the more sensitive grains. As a Properties of Photographic l\Iaterials" in which particular instance it was mentioned that sensitizing the basis for many applications of photographic by mercury vapour produced such a broadening. methods in scientific research were given, and the This may be cause or part-effect of the increase advantages and dangers of these methods pointed 12 in sensitivity. out, was given in his absence by Dr. Bcrg • Reciprocity failure has been shown to be, at :Mr. C. W. Bradley, Dr. J. G. Wilson, Dr. C. A. least to some extent, a development effect, the Adams, Dr. C. A. Bruck and Prof. W. H. Lang rate of development depending on the intensity then gave examples of the application of photo­ of the exposures. This effect has not been explained graphic methods to specific problems. The three satisfactorily, requiring a deeper insight into the principal advantages of the photographic method mechanism of development. lie in the high amplification factor of about 10 10 : The process of development may be explained a few quanta striking a grain make the whole by a mechanism similar to that of latent image grain developable ; in the possibility of integrating formation, the developer being regarded as a weak light over long periods-in astronomy the medium that will hand over electrons to the latent photographic method enables stars to be detected image. Thus, a field ''ill be set up, and interstitial that are ten times weaker than the weakest detect­ silver ions will move through the silver halide. For able by the eye ; and finally, in the possibility this process to work, the developer must come into of recording radiation outside the visual range. intimate contact with the silver speck. The Some of these advantages have to be paid for; difference in adsorption between various agents, the granular structure leads to loss of resolution which might othen\ise be suitable as developers, due to the graininess and the scattering of light may be regarded as responsible for the differences causing 'image spread'. This, however, may some­ in their behaviour. Another point of view considers times be useful: stellar magnitudes can be deter­ the reduction of the silver to occur in the solution, mined by the spread of the pin-point images. which soon becomes supersaturated and deposits l\Iany difficulties arise from penetration and ex­ sih·er on to the latent image. Since the solubility haustion effects during development, making of sih·cr halides in ordinary developers is low, this photographic photometry a difficult problem.
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