of Silver and of some other Metals to Light. 493 



by sheets of thin mica, which, as Dr. AY. J. Eussell, F.E.S., has shown, 

 stops the action of vapours upon a sensitive gelatine dry plate, and no 

 doubt would do so upon a silver plate. As a rule, the mica itself 

 makes no difference in the action of light, and is quite transparent to 

 it, though, in some cases, it may exercise a slight retarding action. It 

 was found, however, that when the silver surfaces were exposed to 

 light in hydrocarbons, such as fluid paraffin, benzene, turpentine, &c., 

 no action took place under the parts screened by mica, though the 

 fluids were perfectly clear and colourless. Under ordinary conditions 

 of exposiu^e in a printing frame, the mica appeared to exercise no 

 special influence upon the plates, except at the cut edges, as will lie 

 noticed further on. 



Effects of Pressure. — That the images were not produced by diff'erences 

 of pressure was proved by leaving the mica screen, wiih. the cut-out 

 black paper design at its back, in contact with a polished silvered glass 

 plate in darkness for twenty-four hours. There was no visible image, 

 but on development with mercury vapoiu" the mercury was deposited 

 fairly evenly all over the plate, except just where the outside edges of 

 the mica and the edges of some initials cut in it had been in contact 

 with the silver. Here there was very little deposit, and the edges 

 appeared as dark lines on a white ground. There was no sign of the 

 black paper design. The fact that the images were readily obtained 

 from ordinary black and white photographic negatives is further 

 evidence against the results being due to pressure. 



First Experiment with Silver Leaf. — My first experiment was made on 

 the 14th August last with a piece of silver leaf laid down on a plate of 

 glass coated with varnish, and exposed in the sun for a short time 

 under a plain cut-out cardboard screen. On developing with mercury 

 vapour a faint image was visible, the mercury having deposited on the 

 part exposed to light. 



Next day the experiment was repeated. A similar plate was exposed 

 under a black and white gelatine negative of some lace for half an hour 

 in bright sunshine. On developing with mercury, a very distinct, 

 though faint, image of the lace pattern was produced, the mercury 

 again depositing on the parts exposed to light, as in Moser's experi- 

 ment. 



Effect on Polished Silvered Glass. — Thinking that these results might 

 be partly due to some action of light upon the varnish underlying the 

 silver leaf, a piece of polished silvered glass was exposed under the 

 same negative for about an hour. Again a distinct image was 

 developed with the mercury, but partly positive and partly negative. 

 A somewhat similar result was obtained on another plate developed 

 with the ordinary acid iron and silver developer already noticed. 



Effect on Pure Silver Plate. — The next experiment was with a piece of 

 nearly pure silver plate, carefully cleaned with " Globe " polish, and 



