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ROXTGEN-RAY PHOTOGRAPHY 347
black cardboard, and yet he found that the rays could penetrate, in fact from his experiments he came to the conclusion that all substances are in varying degrees transparent to the rays emitted under such conditions. He further discovered that these rays have the power to affect a photographic plate in the same manner as does ordinary light, and a somewhat brief notice was given by him of their use in obtaining a photograph of the bones of the hand.
To obtain a shadow of the bones of the hand, all that is necessary is to place the hand between the prepared screen and the Rontgen tube. Since the flesh is less dense than the bones, the rays can the more readily pass through the former, and the shadow thrown by the latter appears dark by comparison.
Other materials for use on the screen have been tried from time to time. Thus, Edison obtained good results by using tungstate of calcium, mixed with a small quantity of tungstate of manganese ; hydrated potassium platinocyanide has been used by others, but the barium-platinocyanide is still the substance most generally used.
The first attempts to use Rontgen-rays for photographic purposes were greatly handicapped by the want of a suitable tube. The tubes used in the earlier work had such a wide region from which the rays were generated that the photographs obtained by their use were all more or less lacking in definition.
The Focus-tube. — This defect was removed by the invention by H. Jackson of the " focus-tube," in which the kathode or negative electrode is concave in shape, and the rays sent out from this strike upon a platinumfaced aluminium anode or positive electrode, arranged at an angle of 45° with their path from the kathode. The anode is placed a short distance beyond the centre of curvature of the kathode. The reason for placing it in that position is because, when working with the lower