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Screen-Plate Practu e 525
sure, or too bright a light, may produce this condition, with the result that verv little is reduced to silver, which should block out those screen elements which are not required to form the colored image ; so that the result ma*' appear pale from admixture of white light.
It is obvious that any actinic light may be used, arc, magnesium, electric or incandescent gas, etc., so that it is unnecessary to record all the details of particular exposures to particular lights that have been suggested by workers. J. Husnik84 was of the opinion that the second time of development, as advised by the makers, which was about 2 minutes, was insufficient, and that it should be continued for at least 6 or 7 minutes, particularly in dull weather.
F. Torchon85 warmly supported the use of 5 per cent solution of ammonium sulfide, instead of the second developer, and stated that the image was pure black. Subsequent intensification was unnecessary, the plates were extremely transparent, and the results obtained perfectly permanent. A. Damry86 contended that the images, thus obtained, were weak and required intensification, and for this purpose proposed mercuric iodide in sulfite solution. Red stains were also found to be formed by Torchon's method ; but R. Ceiller87 pointed out that these were due to traces of manganese left in the film by the reversing solution, and that a bisulfite bath was a complete remedy. This method did not find general adoption in practice.
C. Gravier88 proposed to treat the plate after reversal, with bisulfite solution and then to omit all further operations, merely washing and drying. This method, if sound, would eliminate five operations; but it is extremely doubtful whether the results are as good. Gravier claimed that the pictures were more brilliant ; but as pointed out by the authorS9 this is hardly correct. That there might be greater luminosity of the plate as a whole is obvious, as in the original method, in which the second development reduced the primarily unattacked silver salt, the final result was a colored image plus black silver, which stopped out the light where it was not wanted. In the Gravier method there was a colored image plus silver bromide, and the latter was neither colorless nor opaque. Therefore, there was added to the colored light passing through the screen elements, that transmitted through the silver salt, so that the colors were actually paled.
Intensification. — In order to render the black stopping-out silver grains quite opaque and free from any greenish tinge, and also to improve overexposed plates, it was recommended by the makers that intensification should be resorted to. Although this is not always requisite with correctly exposed plates, it will be found as a rule that there will be some improvement in the apparent brilliancy of the pictures.
The method first suggested was with nascent silver, for which the following solutions were required :