Recently the rapid strides were made in all fields of the photographic chemistry, especially in the field of organic addition agents for photographic emulsions.
Amoung them, the hardeners found in patents from 1950 to 1956, are reviewed.

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Two kinds of silver bromide photographic emulsions were prepared by using silvernitrate solutions containing 0 and 0.02mol.% lead nitrate, respectively.
Characteristics of these two emulsions to light in the over-exposed region were investigated and compared with those to x-ray.
Results: By addition of lead salt, sensitivity to x-ray was increased and degree of solarization by light was decreased markedly. But this lead-sensitized emulsion, when kept at 55°C. for 48hrs., showed remarkable reduction in x-ray sensitivity and heavy solarization by light. By treating with halogen acceptor, such as NaNO₂, both leadsensitized and non-sensitized emulsions showed higher x-ray sensitivity and less solarization.
Conclusion: It is supposed that the reason why the degree of solarization is decreased by addition of lead salt is that the silver ion vacancies generated by addition of lead salt (c. f. 2nd raport) possibly act as traps for positive holes which would otherwise be trapped by latent image silver on surface of the crystal and make the crystal undevelopable.
By X-ray exposure, a large number of electron-hole pairs is produced in a very short interval of time. Under such conditions the utilization of the available electrons is probably ineffective because of recombination with positive holes and trapping at internal sites.
So it is supposed that the sensitivity to X-ray is increased in one or more of following cases, (1) the rate of neutralization of electrons by silver ions is increased, (2) the latent image can be formed with fewer electrons, and (3) the probability of recombination of positive holes with electrons is lowered.
Lead sensisization probably belongs to the last case, because silver ion vacancies mentioned above act as traps for positive holes and keep them from recombination with electrons.

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In this paper, the present state of four kinds of reciprocity sensitometers is reviewed, which have been improved in the Research Laboratory of Fuji Photo Film Co., Ltd.
Long exposure time reciprocity sensitometer:
The exposures are timed by a double falling type shutter which is operated electromagnetically. The time signal is given by areocise putmatic timer for second order, and by push-button operated manually with a stopwatch or an electro-magnetic pendulum clock for longer exposures than 16 seconds. The methods of varying the intensity are:(1) exchanging from one source tungsten lamp into another, (2) inserting into optical path a neutral opal glass of optical density value of 2 and a set of stops which varies the effective source area, and (3) shifting a light source on the optical bench over the range from one meter to ten meters. Thus exposure intensity covers the range from 10^-4 to 600 lux.
Deci-second reciprocity sensitometer:
The exposure is timed by a rotating sector with four-steps slot which is uniformly illuminated by a condenser. This slot light source is imaged on the film by a relay camera lens with magnified scale. By proper selection of the steps, exposure time can be varied over the range from 1 to 1/64th second. An auxiliary rotatiug secter with a wide slot revolves at a slower speed than the main sector. Thus exposures are given every 40 seconds repeatedly for continuous work. Step-wise variation of illumination is accomplished by a set of chronium-foiled neutral filters or by irises of relay camera lens. Thus exposure intensity can be varied from 20 to 2000 lux
Milli-second reciprocity sensitometer:
A strip of film is attached on the pendulum. A relay camera lens forms an image of the slit light source on the film. A series of slits are used, the widths of which are six kinds from 1 to 20 millimeters. Two kinds of relay camera lenses are used, the magnifications of which are 1: 1 and 1: 1/10. The exposures are timed by a pendulum with a film traversing the optical path at a constant speed. The periods of pendulum are stepwisely 1.68 and 4.49 seconds. Thus exposure time can be varied over the range from 64 to 0.1 milliseconds, and exposure intensity from 1500 to 10⁵ lux.
Micro-second reciprocity sensitometer:
A strip of film is wrapped around the outside surface of a drum. A relay camera lens forms a reduced image of a micro-slit light source on the film. The slit widths are changed over the range from 1 to 10 millimeters. The reduced scale of the relay camera lens is 1: 1/10. The exposure time is determined by the linear speed of the film and the width of the slit image on the film.
The rotation speed of the drum is varied stepwisely over the range from 31.2 to 500r. p. m. Thus exposure time can be varied over the range from 100 to 1 micro-seconds, and the exposure intensity from 10⁵ to 10⁷ lux. As a light source a high brightness Xenon lamp is used. The speed of rotating durm is controlled and calibrated by JJY standard radio wave. An auxiliary lens shutter which operates under the combination with revolution of the drum is used to prevent double exposure.

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Results of the basic and practical studies on the ammonium thiosulf ate fixer were already reported by authors. As this fixer consists of ammonium salt instead of sodium salt in common hypo fixer, it may occur that explosive substances such as silver fulminate or silver azide can be formed in the electrolytic recovering process. So, tests for electrolysis of this fixing solution were carrid out, varying current densities at various pH values. Then the silver deposits and the precipitated substances from these solutions were tested of the impacts and the ignition-sensitivity and analysed the precipitated substances to examine the presence of silver azide. Moreover, the analysis of the electrolysed liquid were carried out. Neither trace of silver fulminate nor silver azide could be found in the solution. Impact- and ignition-test showed that the silver deposits and the precipitated substances did not contain the detonating substances. So, it may be conculuded that the electrolytic silver recovery from the waste rapid fixer can be done in safety.

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