Mr. G. Shaw on some PhotograpJiic 'Phcenomena. 451 



of the prepared surface was at all impaired by these repeated 

 exposures to light, the camera obscwa was resorted to. A 

 series of plates was prepared with the utmost attention to uni- 

 formity; some of these were exposed in the camera obsctira, 

 and pictures obtained by the subsequent exposure to vapour 

 of mercury; the time requisite for the proper development 

 of the picture was noted ; others were first exposed to the 

 direct rays of the sun, and afterwards to the mixed vapour, 

 and these were exposed in the camera ohscura for the same 

 length of time as those which had not been exposed to light. 

 On treatment with mercurial vapour perfect pictures were 

 produced, which could not be distinguished from those taken 

 on plates prepared by the ordinary method. So completely 

 does the mixed vapour restore the sensitiveness of prepared 

 plates after exposure to light, that the most beautiful impres- 

 sions were obtained in the camera obscura in two seconds on 

 plates which had previously been four times exposed to the 

 direct light of the sun, and after each such exposure treated 

 with the mixed vapour. 



As the plates experimented on, to this stage of the inquiry, 

 had been wholly exposed to the sun's light previous to ex- 

 posure in the camera obscura^ it was thought that possibly 

 some slight eifect was produced, which, from being the 

 same on all parts of the plates, escaped observation; and in 

 order to avoid the possibility of error from this cause, the 

 impressions of light which it was intended to destroy by 

 bromine were afterwards made in the camera obscura. Pre- 

 pared plates were impressed with virtual images of different 

 kinds, the camera obscura being pointed first at a house, after- 

 wards to a bust, next to a tree, and finally to a living figure, 

 the plates after each impression, excepting the last, being 

 momentarily exposed to the mixed vapour. In every in- 

 stance the most perfect impressions of the objects to which 

 the camera obscura was last directed were obtained, and no 

 trace of the previous impressions was left. 



Experiments were next instituted for the purpose of ascer- 

 taining if the prepared surface, after the process of mercu- 

 rialization, could be made to receive another impression by 

 treatment with mixed vapour. Impressions were taken 

 with the camera obscura^ and after the full developmerit of 

 the picture by vapour of mercury, the plates were exposed to 

 bromine and again placed in the camera obscura, the instru- 

 ment being directed in the different experiments to different 

 objects; on exposure to mercurial vapour other pictures made 

 their appearance, and although confused from superposition 

 on the first pictures, could be clearly traced, and were found 



2 G2 



