450 Mr. G. Shaw o» some Photographic Phccnomena. 



posure to light. This conclusion was, however, erroneous, as 

 the following experiments prove. 



A prepared plate was exposed to light, and afterwards to 

 the mixed vapour ; mercurial vapour was found to have 

 no effect upon it; the plate was then partly covered with a 

 metallic screen, fixed close to but not in contact with it, and 

 the whole was exposed to light. On placing the plate in the 

 mercury box, a broad white band, nearly corresponding to 

 the edge of the defended part, made its appearance ; the whole 

 of the defended part (excepting the band in question) was un- 

 affected, and the exposed part exhibited very little change. 

 By a careful examination of the plate after it was removed 

 from the mercury box, the white band in the middle appeared 

 to be produced by the feeble light which had passed under 

 the edge of the metal plate which had screened tiie light from 

 part of the prepared surface; and the very dark, and appa- 

 rently unaltered appearance of the exposed part, was occa- 

 sioned by an excess of action, for mercury was found to have 

 condensed on that part in large quantity, and to have pro- 

 duced the dark lead colour which is commonly called solari- 

 zation ; but which effect, in the case in question, was so ex- 

 cessive, that the colour of the part on which mercury had 

 condensed differed but very slightly from that on which no 

 light had fallen. It was now evident that the apparent ab- 

 sence of effect in the last experiment was in reality occasioned 

 by an excess of action ; and by repeating that experiment, 

 and making the time of the second exposure to light much 

 shorter than before, the plate assumed, under the action of 

 mercury, an intense and beautiful whiteness. 



From these experiments, then, it was perfectly clear that 

 the impression produced by light on a Daguerreotype plate is 

 wholly destroyed by the mixed vapour, and that its sensitive- 

 ness to light is restored. 



It now remained to discover to what extent the sensitive- 

 ness is restored by the treatment in question. It was not at 

 first expected that the sensitiveness to light was as great after 

 this treatment as after the original preparation of the plate; 

 but experiment afterwards proved that the surface lost none 

 of its sensitiveness by this treatment, nor even by numerous 

 repetitions of it. A prepared plate was exposed to light; the 

 impression was destroyed and sensitiveness restored by the 

 mixed vapour ; the plate was a second time exposed to 

 light and a second time to bromine, still its sensitiveness ap- 

 peared unimpaired ; for a fourth or fifth exposure gave, on 

 treatment with mercurial vapour, a vivid impression. In order 

 to determine with the greatest accuracy if the sensitiveness 



