492 Major- General J. Waterhouse. The Sensitiveness 



bright sunlight on a plate of almost pure silver, as indicated by a very- 

 sensitive Rosenthal galvanometer, was to make the exposed half 

 positive to the unexposed, or as zinc to copper, which would seem to 

 point to some slight oxidising action. The observation, however, was 

 a difficult one, not readily repeated with certainty, and no very definite 

 result was obtained. The currents observed did not appear to be due 

 to unequal heating of the two halves of the plate, because the direct 

 application of heat to the exposed side produced at once a clearly 

 marked current in the opposite direction. This effect was always the 

 same, and could be readily repeated. 



In further observations of the same series, in which pairs of pure 

 silver plates were partly immersed in distilled water or good tap water, 

 one plate being exposed to light while the other v/as covered, as in 

 Becquerel's electric actinometer, the current in nearly all cases, though 

 small, was as above, the exposed plate being positive to the unexposed, 

 as it generally was when the silver plates were placed in dilute sul- 

 phuric, nitric, phosphoric, or hydrochloric acids. 



Confirmation of Moser's Observation. — Eecent observations of the action 

 of light upon silver, made in connection with the working of the 

 Daguerreotype process, have fully confirmed Moser's observation 

 quoted above, and shown that silver, when exposed under ordinary 

 conditions, shows distinct sensitiveness to light, and that not only can 

 an invisible developable image be obtained, as was done by Moser, but 

 by prolonging the exposure printed-out impressions are produced 

 which are clearly visible after exposure. The difficulty is to make 

 sure of obtaining a surface of pure silver, free from the presence of 

 condensed gases or other foreign matter which might aiFect the plate 

 and give it a sensitive surface. The silvered glass plates used have 

 generally been cleaned and polished with well-washed tripoli, and the 

 metal plates in the same way, sometimes after being well cleaned 

 with fine emery paper, and, in some cases, after making the metal 

 red hot. 



Various Silver Surfaces Sensitive to Light. — Repeated observations with 

 silver surfaces of different kinds, pure silver plates and foil, silver leaf 

 on varnished glass. Daguerreotype plates and silvered glass, have shown 

 that by an exposure in bright sunshine of about half an hour to one or 

 two hours, under an ordinary black and white negative, or a cut-out 

 design, photographic images can be obtained, which are sometimes 

 clearly visible after exposure, under favourable conditions. 



Development of Images on Plain Silver Flates. — Whether visible or 

 invisible, these images can be developed by the vapour of mercury, or 

 by ordinary physical development with acid solutions of ferrous 

 sulphate or pyrogallic acid, to which a little silver nitrate is added, as 

 in the old wet collodion process. The images were also produced when 

 the negatives or cut-out masks were separated from the silver surfaces 



