of Siher and of some other Metals to Light. 



497 



This experiment was repeated in January, two silvered glass plates 

 being exposed face to face for fifteen days, of which five or six were 

 sunny and the rest fairly bright, with the object of also seeing whether 

 an impression could be made through the upper plate on to the silvered 

 surface of the under one. On developing with mercury, a fairly clear 

 image of the cut-out design was obtained on the inner surface of the 

 exposed plate, dark on a lighter ground, but neither on the outer 

 exposed silver surface of the upper plate nor on the silvered surface of 

 the lower plate was there any trace of an image. The experiment was 

 repeated with a similar result. The only other case in which images 

 have been obtained through the silvered film was on a plate partly 

 fumed with hydrogen peroxide and developed with mercury, but much 

 overdone. As far as they go, the experiments show that the visible 

 image is not produced on the silver except more or less in contact 

 with the air. Further trials in bright sunshiny weather are required 

 to prove this. 



Images formed ow Both Sides of Exposed Glass Plate. — In connection 

 with this action through glass, a curious result obtained on a plain 

 glass plate may be mentioned. A piece of clean glass plate was 

 exposed for a day and a half in October under the same aluminium 

 screen, with another glass plate over it^ so as to protect the surface 

 from the air. It showed a clear breath image, but mercury vapour did 

 not produce any image. Iron development, however, brought out part 

 of the design very clearly, the silver depositing upon the unexposed 

 parts and giving an image darker and clearer than the ground. The 

 image of the design appeared by breathing on both sides of the exposed 

 glass plate, so that the action of light went through both plates. 



Effect produced on Varnished Silver Glass. — With the same object of 

 ascertaining whether the images could be produced when the silver 

 surface was protected from the air by varnish, a silvered glass plate 

 was coated over one half with photographic negative varnish and then 

 exposed in the usual way, under the cut-out paper mask and mica screen, 

 in October. After exposm^e for two days the image of the black paper 

 mask, the cut-out initials, and edges of the mica screen were not repro- 

 duced so clearly as usual, though they appeared readily enough on the 

 unvarnished half when breathed upon. The distinct heightening of 

 the effect under the varnished part is apparently due to some chemical 

 combination, probably oxidation, or the formation of an organic silver 

 compound sensitive to light. When the varnish is removed, the strong 

 image remains, and there is a distinct change of colour in the exposed 

 part of the silver surface, which takes a sort of yellowish olive-grey tint. 

 Recent repetitions of the experiment with silvered glass and with a 

 plate of nearly pure silver gave exactly the same results. It may be 

 noted that with iodised silver plates the same varnish has been found 

 to exercise a decidedly retarding effect on printed-oufc images. 



VOL. LXVI. 2 Q 



