134 Action of Wood on Photographic Plate in the Dark 



does when it has not been so exposed. Again, old printing which is 

 now nearly inactive becomes much more active after exposure to sun- 

 light. Bodies other than those which may contain resin or allied 

 substances are not affected in this way by light, for instance, flour, 

 sugar, porcelain ; metals are not rendered active by sunlight. 



The next point was to ascertain which of the constituents of light M^as 

 most active in producing these effects, and the first experiments were 

 made by simply placing strips of different coloured glass on wood sections, 

 exposing them to sunlight and afterwards putting them up with the 

 photographic plate in the usual way. Pictures of the results are given 

 in the paper. Ked glass entirely prevented any increase in the 

 activity of the wood, in fact, it acted in the same way as a band of 

 black paper or tin foil would act, and a green glass acted much in the 

 same way, but under a blue glass the activity of the wood was increased 

 to much the same extent as under colourless glass or under no glass. 

 Plate 7, fig. 2, shows what happens when a red glass and a white glass 

 are placed upon it and exposed to sunlight. On the right of the 

 figure there was no glass. 



Further experiments were made by placing similar pieces of deal in 

 light which had passed through different coloured solutions. Three 

 double-cased bell jars were taken, one was charged with a solution of 

 potassium bichromate, another with copper ammonium sulphate solution, 

 and the third with pure water, and all were exposed to sunlight for 

 4 hours. The deal in the red light gave only a faint picture, that in 

 the blue light a dark picture, and that with the pure water was only a 

 slightly darker picture. Eesin, guiacum, copal varnish, white oil paint 

 and resin sized paper all acted in the same way and gave similar 

 results. . c 



The light from an arc lamp when passed through a red glass and 

 allowed to fall on a wood section for 1 J hours produced no effect, but 

 when the same light was passed through a blue glass and fell on a 

 similar wood section for only 1 hour it produced a dark picture. With 

 liquids this same increase of activity by the action of blue light is 

 produced. Turpentine, which has been exposed to blue light, is more 

 active than when in its ordinary condition. 



