THE PHOTOGRAPHY OF COLOUR 351 



Herschel had repeated Seebeck's experiment ; Daguerre and 

 Fox Talbot had both observed that in some of their photographs 

 a red object produced a red colour in the picture ; and Robert 

 Hunt had succeeded in obtaining colours on paper impregnated 

 with various silver salts. Hunt put, for example, a prepared 

 sheet under red, yellow, green, and blue glasses, and exposed 

 the arrangement for a week to diffused light. The silver 

 chloride "became red under the red glass, dirty yellow under 

 the yellow glass, a dark green under the green, and a light olive 

 under the blue." But with a sheet "differently prepared" he 

 tells us he obtained "a very beautiful picture," in which the sky 

 was crimson, stucco-fronted houses came out slaty blue, and the 

 green fields of a brick-red tint. 



Becquerel used silver plates and produced a layer of silver 

 chloride on the surface of the metal, at first by the action of 

 cupric chloride, but afterwards by the electrolysis of dilute 

 hydrochloric acid with the silver plate as the positive electrode. 

 The plates were treated until the very thin layer of the accumu- 

 lating chloride on their surfaces had passed through certain 

 definite colour-changes, and then washed, dried, and warmed. 

 With these plates he reproduced, with a considerable measure 

 of success, the colours of brightly dressed dolls and highly 

 coloured designs besides the solar and electric arc spectra. 



There were many others who busied themselves with 

 experiments of this character at this time and for many years 

 afterwards, making little variations and meeting with more 

 or less success, sometimes propounding strange theories and 

 endeavouring to justify them, and now and again expressing 

 their hopes and almost their convictions that what they sought 

 for was just coming within their reach. But their hopes were 

 ill founded, for the work was purely empirical; they had no 

 sound principles to guide them, and no real progress was ever 

 made. Even the colours obtained were not permanent, and 

 though some when considerable care was taken to preserve 

 them lasted longer than others, the difficulty of "fixing" them, 

 or rendering them even moderately stable, was never overcome. 

 We shall shortly see something of the nature of the colours 

 produced in these experiments. 



If light waves are similar to the waves produced by dis- 

 turbing the surface of water, as they are supposed to be, it 

 follows that if a continuous series of waves is reflected back 



