The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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which yields a much finer collodion. This is said to be prepared by immersing ordinary pyroxyline in pure nitric acid for ten minutes, washing and drying. Pyroxyline is insoluble in water, almost insoluble in alcohol, and ether, but readily soluble in a mixture of the two, and in glacial acetic acid. It is used for preparing collodion. (See Cellulose and Celluloid.)
Racks. Although many convenient forms are sold by dealers, one of the most convenient kinds (that designed by Mr. G. F. Williams) can be easily made by the amateur. A sketch of this rack is given under the heading Emulsion.
Radiography. When the intermittent discharge from an ordinary induction coil - which discharge it must be remembered has a prevailing direction, the impulse on breaking contact being much more powerful than that on making contact - is passed through a highly exhausted vacuum tube, radiations are- given off from the cathode (negative pole), and appear to be reflected from the anode (positive pole); these when they escape from the tube are found to be not only capable of affecting the gelatino-bromide film, but they also are able to penetrate many sub-stances opaque to ordinary light. Such radiations, as long as they are within the vacuum tube, are generally called Cathode rays ; and Hertz, who investigated this subject shortly before his death, which took place on January ist, 1894, came to the conclusion that cathode rays are fundamentally different in their nature from light rays, but that cathode rays differ among themselves much as the various degrees or wave lengths of light differ among themselves. About the beginning of 1896 Professor Rontgen realised that such radiations as actually escape from the vacuum tube, and which he prefers to rather call X rays than cathode rays, may be used in obtaining outline photographs of many objects invisible to the eye; and the " new photography," thus inaugurated, was one of the most popular of all scientific recreations during 1896. Metals and mineral substances are generally very opaque to the X radia-tions, while vegetable substances, water and the softer animal tissues, are fairly transparent even when coloured black with carbon. Hence it is that if a sensitive plate be wrapped up in ordinary black paper (film upwards), and the hand is laid over the package, an image of the bones of the hand can be obtained