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272 History of Three-Color Photography
with sodium hydroxide. If panchromatic plates were bathed for 4 minutes at 18° C. and rapidly dried, the speed to white light was increased 100 per cent in nearly all cases. The red-sensitivity was extended about 100 Angstrom units, and the red speed was increased in some cases about 400 per cent. If the alcohol was omitted, the speed was increased still more, but the plates should be used immediately after bathing.
In a circular, issued by the Bureau of Standards, the following procedure is advised for panchromatic plates, and it would seem of value in that stale plates may be thus rejuvenated. Bathe the plates for 4 minutes in 3 parts of the strongest ammonia (20 per cent), 25 parts of 95 per cent alcohol and 75 parts water, then rapidly dry in front of a fan. The increase of speed to white light may be from one-half to twice and even five times for the red. Greater speed can be obtained with a 3.5 per cent solution of the ammonia, without the alcohol. These plates fogged badly in development; but this could be ignored in some cases, particularly if development was not pushed too far. If the alcohol be omitted the plates must be used a few hours after treatment. See also the ultra-sensitizing of screen-plates with ammoniacal silver chloride solutions (p. 512), which is also applicable to ordinary plates. F. M. Walter and R. Davis113 also confirmed the improvement in speed by washing panchromatic films and plates, ascribing this to the loss of free bromide and chrome alum. The increase in speed was about 33 per cent, although the fog was doubled.
The Keeping Properties of Color-Sensitive Plates. — This has been a moot point ever since the introduction of orthochromatic plates, and very contradictory statements have been made, which it is not worth while to record. The differences are due, of course, to the different methods of manufacture and conditions of keeping. But it may be accepted as an established fact that color-sensitive plates will keep as well as ordinary under like conditions.
With the introduction of the isocyanins and the more general use of bathing methods, this question has become of great importance. E. Konig114 stated that at the commencement of 1906 different kinds of German and French plates were sensitized with pinachrom and pinacyanol in a semi-aqueous bath with ammonia, and that after 2 months the whole were good ; but at the end of 10 months nearly all were foggy and some utterly useless. Those treated with pinachrom still had full color-sensitiveness ; but the pinacyanol-bathed plates had lost a good deal of the same.
H. Luppo-Cramer115 stated that whilst bathed plates do not keep so well as those in which the dye is added to the emulsion, yet the degree of color-sensitivity was higher in the former. He had already showed that the color-sensitiveness could be increased by washing in water and drying.118 This it was stated to be due to the loss of soluble bromide by diffusion from the film, which occurs when the latter is bathed in water or in a bromide-free solution. By adding a small percentage of potassium bro