PHOTOGRAPHY. 23 



tion of them as correct (though I believe it to be so), as the 

 theory is still in the course of examination by Dr. Vogel, 

 Captain Waterhouse, myself, and others. 



We must again travel forward a step, and now we shall 

 find it more easy to understand the production of photographs 

 of a certain kind which we find in the Exhibition. If we 

 take pure silver chloride and expose it to the action of the 

 spectrum sufficiently long to give a visible impression, we 

 find that it extends from the green to far beyond the. extreme 

 violet, but that no action takes place in the red. I exhibit 

 here a photograph of the spectrum taken on paper impregnated 

 with silver chloride ; the exposure necessary to produce this 

 print was three-quarters of an hour. Another similar piece of 

 paper was taken and slightly darkened in diffused light, and 

 placed in the spectrum. The impression of the blue rays 

 continued, and at the same time there was a browning action 

 taking place where the yellow rays were thrown, whilst at the 

 extremity of the red a decided pink was apparent. Two 

 such spectra I have here, one fixed and the other not fixed ; 

 you will perceive that in the former the red tint has been lost, 

 though there is an evidence of decided darkening beyond 

 that due to tint on the paper. In the latter the colours are 

 still extant. 



Now conjointly with these results I wish to bring to your 

 notice another experiment that any of you possessing a prism, 

 and a camera, and a looking-glass can repeat, if you have 

 even the smallest photographic knowledge. Take a glass 

 plate and prepare it as if you were going to take a picture, 

 expose it to the daylight and develop it, then "intensify" 

 till you have a perfectly opaque film of silver, and dissolve 

 away the unaltered silver salts, and wash well. Next 

 take some copper chloride and flood the film with it. It 

 will gradually seem to turn to a dirty white by reflected 

 light, and by transmitted light it will be a reddish brown, 

 for the copper will part with half its chlorine to the silver, 

 which there is every reason to think is converted into a 

 mixture of chloride and sub-chloride. Now expose such a 

 plate to the action of the spectrum for ten minutes. You 

 will find that where the violet, blue, and green rays come, 

 you have a darkening of the surface, whilst where the 

 yellow and red rays fall you have a bleaching action. 



Again, take another plate, similarly prepared, but give 



