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THE CHEMICAL ACTION OF LIGHT 69
lights. In ten hours the print will be found to have come up with all its details and clear in the high lights. The presence of a soluble salt of silver is absolutely essential for the success of this experiment. Bromide or chloride papers or plates will show this effect if they are exposed to yellow or green light in a bath of nitrate of silver or of silver chloride or bromide dissolved in ammonia or hyposulphite of soda, but the darkening is never as great as when printing papers are used. The effect can be strengthened with printing papers if they are soaked in a dilute solution of oxalic acid, after the very short exposure under the negative. This action is not equivalent to a longer exposure under the negative, but to a physical development or intensification of the print. Dry plates show no such action.
If we print very faintly on printing-out paper under a negative, and then continue the printing under green and yellow glass, the contrasts in the print are greatly increased. The continuation action due to the green and yellow rays is proportional to (1) the deposit already formed, and (2) the amount of these rays locally passing through the negative. The effect produced therefore depends, as regards densities, on the square of the relative transparency of different parts of the negative.
Platino-cyanide of barium browned by Rontgen rays can be regenerated by exposure to light, particularly in the green, yellow, and red. As stated elsewhere, a long exposure to these rays brings about a kind of solarisation effect, but it is always possible to annul this effect by exposure to light ; the longer the plate has been exposed to the X-rays the longer must be the exposure to light in order to bring about its regeneration.
Suppose a dry plate has been exposed to Rontgen rays for just sufficient length of time to produce a visible effect on developing, then by exposing that plate to the action of red and infra-red rays before developing, it can be