320 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



to strike off several thousand copies. The only objection to be 

 overcome is that of being able to pull off the impressions by 

 machinery, and independently of the skill of workmen. When 

 this is the case, these processes will be used for book Illustrations 

 more largely than they are at present. 



III. CHROMOHELIOGRAPHY. 



In chromoheliography we have a problem, the solution of which 

 is much less advanced than that of photographic engraving, but which 

 has nevertheless been the object of interesting experiments. We here 

 deal with the reproduction of colour in images with no intervention 

 save that of light, hence this particular application of the photo- 

 graphic art and of physics has been named chromoheliography. 



When we look on the screen of the camera obscura at the landscape 

 which is there reproduced in miniature, all the objects represented are 

 depicted as in a mirror, with all the variety of shades and colours 

 with which they are clothed in nature. It is natural that the wish 

 should have arisen to fix this faithful image ; but how ? Does there 

 exist a sensitive substance which not only can receive different im- 

 pressions according to the colour of the luminous rays which strike it, 

 but can retain this exact impression and give it to the eye the same 

 as i was received ? 



This is the whole extent of the problem. It is far from being 

 solved ; but what has been achieved in this direction encourages the 

 hope that the solution is not impossible. 



In 1848, M. Edmond Becquerel announced to the Academy of 

 Sciences that he had succeeded in fixing on a sensitive plate the solar 

 spectrum with all its colours. He took a silvered plate, on the sur- 

 face of which he formed a coating of sub-chloride of silver by immers- 

 ing it in a solution of hydrochloric acid, acted on by the galvanic pile. 

 When the colour of the sensitized coating attained, for the second 

 time, a violet rose tint, he submitted it to the light of a spectrum 

 obtained by the aid of a lens. " The sensitized coating was then 

 impressed with red on the red, yellow on the yellow, green on the 

 green, blue on the blue, violet on the violet. The reddish tint turns 

 to purple at the extreme red and even extends beyond the line A of 



