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72 THE ADVANCE OF PHOTOGRAPHY
Taking into consideration the effects which light is known to produce in inanimate substances, Dr EdridgeGreen's theory would seem much more consistent with modern ideas, than the postulation of three distinct types of nerve fibre, as is done in the Young-Helmholtz theory.
Pseud o-Photographic Effects
A considerable amount of research has from time to time been carried out upon the effects produced when certain substances, such as wood, resin, various metals, etc., are placed in contact with a sensitive film. Perhaps of all English chemists the late Dr W. J. Russell took the greatest interest in this branch of the work.
In one set of experiments, the results of which were published in the Photographic Journal, Nov. 1908,1 Dr Russell found that resin and various kinds of coal have the power of fogging a photographic plate when placed on it in the dark. The action of these substances under ordinary conditions of temperature is extremely slow. On the other hand, quite a noticeable effect takes place in some three or four hours when their temperature is between 30° and 40° C. In such cases absolute contact is by no means necessary.
Various explanations of this action have been given at different times, some scientists attributing it to chemical action, others to radiations given off by these substances, while some have even tried to account for it by the presence of emanations, like those emitted by the radioactive substances.
It now seems as if the question has been decided in favour of the first-named process.
Russell found that the shadows thrown by the resin were not bounded by straight lines, but were able to curve round a screen. Also the action was not capable of
1 See also Roy. Soc. Proc, B, Vol. 80. 72