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76 THE ADVANCE OF PHOTOGRAPHY
The photographic action of the positively charged particles (a-rays) emitted by these substances has been studied in some detail by S. Kinoshita1 a further consideration of this question is, however, beyond the scope of this book.2
It may in passing be stated that a large amount of useful information with respect to radioactive substances has been obtained by the aid of photographic effects produced by the radiations they emit.
Phosphorescent Substances. — A somewhat curious effect produced by the action of phosphorescent substances has been noted by J. H. Player.3 He prepared a phosphorescent surface by coating a uniformly translucent sheet of paper with luminous calcium sulphide. This coated surface Avas then exposed for such a time to the action of full daylight until when taken into a dark room it was seen to glow with a bluish light. Some fifteen to twenty seconds' exposure was found to be sufficient for this purpose. The picture to be copied is laid perfectly flat on a piece of glass and the prepared surface is placed on the picture, care being taken to ensure uniform contact between the two. In an otherwise darkened room a strong source of j'ellow light (an ordinary gas flame will do) is then used to illuminate the picture, the light for this purpose acting for five to ten minutes and passing through the phosphorescent paper on to the picture. The prepared paper will at the end of this time be only faintly luminous. It is next removed from contact with the picture and placed in a frame in contact with a sheet of bromide extra rapid paper, the phosphorescent surface being face to face with the sensitive surface of the paper. This frame is then placed in a light tight box and heated to about 120° F. for about
1 Roy. Soc. Proc, Ser. A., pp. 432-453, March 1910.
2 See also "The Radioactive Substances," International Scientific Series pp. 48, 113, 129 etseg.
3 Photo. Journ., Nov. 1904.