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70 THE ADVANCE OF PHOTOGRAPHY
regenerated in just about the same time that the violet rays would have taken to form a developable impression.
A dry gelatino-chloride plate which has been exposed in a camera a full time can be regenerated by exposure to the action of red and infra-red light for about four hours.
The feebler the impression the more rapid the destructive effect of the red rays ; but, on the contrary, the slower is the continuation effect produced b}^ the green.
Sir W. de W. Abney 1 has also carried out experiments by by which he has shown that the latent image on a sensitive plate can be destroyed by a prolonged exposure to red rays.
A. G. de Moncetz 2 has stated that when blue light or Rontgen ra}Ts have been used for fogging a plate, the fog can be completely destroyed by the action of infra-red rays of wave lengths extending from 800 to 1000 /mfx.
Should the plate be fogged by the action of Rontgen rays alone, he finds that exposure to rays of wave lengths between 920 and 1350 jujul increases instead of destroys the fog.
No effect is produced by these rays on the plates if they are not previously fogged, nor yet if they have been fogged by exposure to ordinary light.
It is a remarkable fact that the gelatino-chloride printing-out papers (P.O. P.), which are much less sensitive to the action of light than the gelatino-bromide plates, blacken far more readily when exposed to the action of light. Of course, the amount of. exposure required to produce a developable image on the paper is enormously greater than that required by the plate.
A. P. H. Trivelli3 has recently offered a partial explanation of these phenomena based upon the relative sizes of the silver-haloid grains contained in the emulsion. The grain in the emulsion of the P.O. P. is of ultra-microscopic
1 Photo. Journ., 48, pp. 318-319, August 1908.
2 Compies Eendus, 146, pp. 1022-1024, 1908.
3 Zeitschr. Wiss. Phot. 9, pp. 1G8-172, Dec. 1910.