DECEMBER 11 I 1937 NATURE 997 Worcestershire seem to be com pounded of Welsh deposits. These points are brought out by the and Eastern elements. accompanying table. Over much of that part of the Warwickshire• Staffordshire Plateau which is drained by the Height at l:pstream Limit Compo- upper waters of the Tame, Cole and Blythe, the Severn Avon• Mouth of sition mantle of drift is comparatively intact, and Severnt Severn Avon frequently forms the valley floors ; but on the Woolridge - ? about 2001 Tewkesbury - 8, ?W,M** O.D. a fewP Bushley Stratford Severn-Avon side of the watershed of England it No.5 110/75 O.D. Tewkesbury do. Green Bewdley Stone- becomes very ragged, projecting outwards as Kidder- No. 4 65 /35 O.D. do. (goes up leigh near promontories or forming outliers on the highest minster Stour) Kenii· hills. This is so far a rule that one is forced to worth view the capping as remnants of a more or less Main Xos. 2 35?/15? O.D. Coalport Church do. and S & ?3 (goes up T>awford continuous sheet which once stretched far into the Worfe) I near vales of Severn and A von. Here it has in most Rugby places been completely destroyed. Evidence of its Worcester ? No.1 ?- 2ii O.D. Shrewsbury ? do ..

Mechanism of the Photographic Process A GENERAL discussion on "The Mode of and theory of development. Prof. N. F. Mott Action of the Photographic Plate" in described a theory of light action and latent image Section A of the British Association meeting formation which was developed on a wave• at Nottingham, was opened by three speakers, mechanical basis and which provides an explana• who treated the problem of photography from tion for many hitherto puzzling fa cts and a useful three different points of view. Mr. E. R. Davies working hypothesis. drew a picture •of the action of light on photo• The sensitive layer of any piece of photograpl:J.ic graphic materials mainly from physical evidence. material consists of a thin film of gelatin in which Dr. S. 0. Rawling gave an account of the action is embedded an enormous number of minute

© 1937 Nature Publishing Group 998 NATURE DECEMBER 11, 1937 crystals of a silver halide. Silver bromide is most destroying the latent image. Similar results are commonly used and is precipitated in the course obtained on a partly developed piece of photo• of manufacture in a gelatin sol. Mter various graphic material, which, of course, contains bigger operations which greatly affect the sensitivity to particles of silver. At another critical potential light, the so-called emulsion is spread out on a no change occurs; above it the density is dimin• support and dried down to form a layer about ished, below it further development occurs. This 1/1000 in. in thickness. critical potential is higher than that of the The light-sensitive unit in the photographic layer latent image. The conclusion from these experi• is the individual silver halide crystal. Its sen• ments is that the latent image silver speck must sitivity manifests itself in two ways. Sufficient be above a certain critical size--which cannot, exposure to light causes the crystal to darken however, be worked out from these data-but visibly-print-out effect. Exposures smaller by below the size at which it would act as solid a factor of 1/10• produce the 'latent image', which silver. makes the crystals developable. These thermodynamical considerations do not The print-out effect consists of the direct pro• tell us anything about the actual mechanism of duction of silver by the action of light, with development. Two essentially different processes liberation of the halogen. If this can be taken up have been suggested. One is that the silver halide by the surrounding medium the process will pro• is dissolved, is reduced to silver in the solution, ceed to completion. Two points were emphasized. which soon becomes supersaturated with reduced The quantum efficiency is of the .order of one silver silver, and the supersaturation is relieved by de• atom formed per absorbed quantum of light. Light position of silver upon the latent image. From is absorbed all over the crystal, but the silver the behaviour of artificially produced silver coagulates and forms specks. particles it has been estimated that an aggregate The latent image provides the ever-new problem of four or five atoms of silver may be necessary in photography which cannot yet be regarded as for this action to occur. The other suggestion solved. No satisfactory proof of its nature has is that the developer, while not being adsorbed so far been offered, although the print-out effect by the surface of silver bromide, which is nega• strongly suggests that it consists of specks of tively charged, is strongly adsorbed by a speck metallic silver. A few to a few hundred light of latent image, and from this foothold is quanta must be absorbed to make a grain develop• able to reduce the rest of the crystal, either through able. It is difficult to say what effect so few silver the body or at the interface between crystal and atoms could have unless they coagulate to form developer. Experiments have shown that some of a speck. Since there is so far no sufficiently sensitive the commonly used developing agents, which physical method by which the latent image can yield negatively charged reducing ions, are indeed be detected, we have to use the results of develop• only feebly adsorbed to negatively charged silver ·ment for its interpretation. A thorough under• bromide but are strongly adsorbed to colloidal standing of the process of development is therefore silver, and that by this very adsorption their essential. reducing activity is enhanced. These facts lend The latent image acts as a trigger or a catalyst, strong support to the second hypothesis. which enables the developer to reduce the silver It is, however, probable that both mechanisms halide crystal. Practically all the energy required can operate, sometimes together. With some is supplied chemically, and the latent image itself developers, yielding positively charged ions, is lost in the process of development. Developers the deposition mechanism probably operates are often regarded as solutions possessing certain almost exclusively, and we then have the slow, redox potentials. (In the physicist's language, so-called physical development. With other the term redox potential may be explained by developing solutions the adsorption mechanism, saying that the potential measures the ease with with direct reduction at the interface between silver which electrons are given off by the solution, a and silver bromide, may predominate. This seems low potential meaning a stronger reducer, giving to be the more likely action with ordinary com• off electrons more easily.) If the potential is too mercial development. low, silver halide crystals will be reduced in• Another mechanism is based on the fact that the discriminately, whether they carry a latent negative charge on colloidal silver particles is image or not : if the potential is too high, they discharged by light, when the particles become will not be developed at all. By preparing a series developable. A similar process may occur in the of solutions of decreasing potential it can be bigger silver halide crystals. shown that at a certain critical potential no Development always takes place in two seem• development takes place ; below that a solution ingly definite steps : an induction period, in which will act as a developer; abon it as an oxidizer, no visible changes occur, and the development

© 1937 Nature Publishing Group DECEMBER 11, 1937 NATURE 999 proper, which proceeds very rapidly. Unless special photographic materials, where the latent image is precautions are taken, a crystal is either found to bleached out by consequent exposure to red light. be completely developed or not at all. The process Furthermore, the spectral sensitivity of the of development seems to be an autocatalytic Herschel effect coincides well with the absorption reaction, but the significance of this for the band observed in single crystals, suggesting mechanism of development is not clear. strongly that the particles responsible in both cases We are thus led to the assumption that the must be the same. The particles in single crystals latent image consists of a silver speck of critical are usually regarded as colloidal silver particles, size. the absorption band being due to colloidal scatter• The formation of this silver speck was then ing. This, however, leads to impossible dimensions considered. Light is absorbed all over the crystal for the silver particles, which cannot, from photo• and forms isolated silver atoms, which then graphic evidence, be bigger than a few hundred coagulate into a speck. The distinction was silver atoms. The absorption band observed in drawn between a primary process, the formation single crystals cannot, therefore, be due to colloidal of silver atoms, and a secondary process, the scattering, but must be caused by a more specific coagulation. absorption process arising from the intimate con• The primary process in latent image formation tact between the silver speck and the silver halide is strongly influenced by changing the absorption crystal. of the individual silver halide crystal. This can be done by dyeing the emulsion with 'sensitizing X • X • X dyes'. The natural absorption of silver bromide • in the ultra-violet and the blue, and with it its spectral sensitivity, can be extended right into the X X red and infra-red. Silver bromide containing a • • • X few per cent of silver iodide is much more sensitive than pure silver bromide ; this probably also X X X influences the primary process. The absorption • • • spectrum has mostly been found to coincide with the spectral sensitivity of an emulsion. Sensitizing X X X dyes are always strongly adsorbed to silver halide: • • • an intimate contact between dye and crystal seems essential. X X X The secondary processes strongly influence the • • • sensitivity, that is, the number of quanta required Fig. I. to be absorbed to make the crystal developable. IONIC CONDUCTIVITY IN THE SILVER HALIDE LATTICE. The sensitivity of crystals increases with their size : coarse grain materials are more sensitive. The The mechanism of latent image formation was explanation is not solely that a bigger crystal will then considered. The crystals are made up from absorb more light and thus form a bigger speck silver and halide ions. In order to produce a more easily, because it has been found that crystals silver atom we have to remove an electron from ofthe same size differ widely in their sensitivity, a bromine to a positively charged silver ion. This which is also influenced by the gelatin and foreign act is thought to constitute the primary process substances generally. The active medium is found in photography. In dye sensitization the electron to be mainly silver sulphide. There has been much may come from the dye molecule instead. The speculation as to the way it acts. Silver sulphide fact that silver halides are photo-conductors probably forms specks on the surface of the silver shows that the electron is not very strongly halide crystals and acts as a condensation nucleus. attached to the silver ion, and may move about Much evidence, mainly of chemical nature, con• in the lattice. Calculations show that the mobility firms this hypothesis. of such an electron is almost as high as in a Here the present writer would point out metal. that, while the latent image in photographic A speck of silver in contact with a silver halide materials cannot be detected by physical crystal was considered. There is a certain prob• methods, there exists a seemingly strict analogy ability· for some electrons to escape from the silver in big single crystals of silver halide. On illumina• into the crystal, and an electron vapour pressure tion with blue light they become coloured and is set up which depends on the temperature. The develop an absorption band in the red. On illumina• speck thus becomes positively charged. The tion with red light this absorption band is bleached sensitivity specks of silver sulphide may behave out. This corresponds to the Herschel effect on in a similar way. Illumination with light increases

© 1937 Nature Publishing Group 1000 NATURE DECEMBER 11, 1937 the electron concentration in the crystal. If this certain density after development, against the concentration is higher than the equilibrium con• intensity, that is, the rate at which the quanta centration of the speck, electrons condense on it. are received. Two facts are most striking ; that The speck becomes negatively charged and tends there is an optimum intensity where a minimum to attract the positive silver ions. Silver halides of exposure is necessary, and that with lowering are also ionic conductors ; the conductivity is of the temperature the material becomes more entirely due to the silver ions. A mechanism has sensitive at low intensities-a fact which should been suggested for this ionic conductivity which be most valuable for spectrography. Since a critical is illustrated by Fig. l. At any temperature there electron vapour pressure has to be reached before the are some silver ions in the wrong position in the latent image can be formed, at sufficiently low centre of the cube of four others, leaving an empty intensities no latent image may be formed at all; space in the lattice behind them. These silver since there is a certain chance for recombination of ions have a certain mobility, and so have the holes the electrons with the bromine atoms the critical whence they came. The ions will be attracted by vapour pressure may never be reached. On raising the charged silver or silver sulphide speck and will the intensity the critical pressure will be reached for increase its size, pushing it out of the grain. We the more sensitive grains in the material, but not for others ; on raising it still further the optimum CMULSION 0 A QAOIATION

1'7 OCNSITV • 1.5 will be At high intensities there is no 1'4 difficulty in reaching a

1'1 sufficiently high vapour pressure, but since the o·s ionic processes take time, :;' it is not unreasonable to s 0'5 ," •o assume that the vapour "' 0'2 pressure has to be kept up ;::: 1·9 as long as possible for the ..."'0 optimum effect. For too 1·6 short exposures the ions 1·3 have no time to move up to the speck, and recom• I·o bination will take place more readily. At low tem• 2·7 peratures the critical va• 2·4 pour pressure is more eas- 5·4 4-:0 2·4 fo o·z o·s ilyreached, and the biggest Lr.g I (ergs{cm.'/sec) Fig. 2. effect is obtained at low in. VARIATION OF THE RECIPROCITY CHARACTERISTIC WITH TEMPERATURE. tensity when the pressure is kept up for a long time. are now left with a neutral bromine atom and a It seems that we have arrived at a fairly simple spare hole whence the silver ion came. There is picture of the mode of action of photographic reason to assume that the primary process mostly materials, but it should be pointed out that the takes place on the surface of the crystal, so that the picture is by no means complete and will not be bromine atom escapes into the gelatin. Only a regarded as correct by all workers in this field. slight reshuffie of the lattice is necessary to get There is space for mention only of a few more rid of the hole, when the crystal will be smaller facts which seem important and which cannot or by one ion pair. can only partly be explained by our picture. The ionic movements take some time and are There is .solarization, the fact that on prolonged dependent on temperature, and it is therefore not exposure development leads to a partly reversed surprising to find that the formation of the latent image ; desensitizing by dyes, whereby the material image depends not only on the number of quanta is made insensitive to further exposure while still absorbed but also on the rate at which they are remaining developable ; reversal by dyes, where fed in, and on the temperature. Much attention the latent image is destroyed by subsequent ex• has recently been paid to the 'reciprocity break• posure in the absorption band of the dye ; and down' of photographic materials. In Fig. 2 is many more. But our picture may be useful as a plotted for various temperatures the exposure, that working hypothesis for future experiments. is, the number of quanta necessary to produce a w. F. BERG.

© 1937 Nature Publishing Group