of Silver and of some other Metals to Light. 495 



vapour, i.e., the mercaiiy deposited upon the unexposed parts instead 

 of upon the exposed. In oae plate the image thus produced from the 

 negative of lace, exposed for two hours in the bright August sunshine, 

 has quite the appearance of an ordinary Daguerreotype picture on an 

 iodised silver plate, though no halogen or other sensitiser was used. 



Fiu-ther trials of prolonged exposures of pure silver foil or plates 

 carefully cleaned with dry tripoli powder have given very distinct 

 printed-out images on the metal, so that there is no doubt about the 

 fact that visible images can be produced on clean plain silver surfaces by 

 light. 



Blue Rays found to he Active. — In order to ascertain, if possible, what 

 rays were active in producing these visible images upon the silver, and 

 as it was hopeless to expect to obtain satisfactory results from obser- 

 vations with the solar spectrum, a slip of silvered glass was exposed 

 under a small artificial spectrum of seven coloured glasses. After 

 forty-five minutes' exposure in sunshine, very faint images of the 

 violet and cobalt blue glasses were distinguishable, but showed more 

 distinctly when breathed upon. A similar result was obtained upon a 

 pure silver plate, and on developing with acid iron and silver the 

 space exposed under the cobalt blue glass developed out quite clearly, 

 with traces of the violet and blue-green glasses. This result quite 

 agrees with an observation made by Moser that only the blue and 

 violet rays have any influence on pure silver, for he obtained very 

 clear images by means of glasses of these colours, while only traces 

 could be rendered visible when red glasses were employed, although 

 they transmit more light and heat. 



On another silvered glass plate, exposed under a similar colour 

 screen or artificial spectrum, consisting of fifteen coloured glasses, for 

 three hours in bright sunshine, the image has apparently reversed by 

 over-exposure, the mercury being deposited on the spaces exposed 

 under the red, orange, yellow, and yellow-green glasses, but not on 

 those which were under the blue-green, blue, and violet glasses. 



Developable Images produced on Silver hy Heat. — As we have seen, 

 Robert Hunt was inclined to attribute Moser's results to the effect 

 of heat or differences of relative temperature, rather than to light 

 or solar radiations, but his experiments were mostly carried out on 

 copper, which is, as I have found myself, much more sensitive to 

 rays of low refrangibility and to heat than silver. 



Towards the end of September, when the weather was much cooler 

 than it had been at the commencement of my experiments in August, 

 I found that there was a distinct falling off in the sensitiveness of the 

 silver surfaces, and it seemed that Hunt's \dew might, at any rate to 

 some extent, be correct. I therefore tried an experiment to see if the 

 same developable images could be obtained by heat as hy light. A 

 silvered glass plate was polished and put into a printing frame with 



