422 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 



frangible rays promote the previous action of light ; the 

 less neutralize it. The curve representing the chemical 

 intensity of the different rays would cross the axis of 

 abscissas about the boundary of the red and orange ; be- 

 low that point to the ultra-red the ordinates would have 

 negative values ; above it to the ultra-violet those values 

 would be positive (Comptes-Rendus, No. 14, tome xxiii.). 

 Hereupon M. Becquerel communicated to the same 

 Academy a criticism on this interpretation, the opinion 

 maintained by him being that while the more refrangi- 

 ble rays excite sensitive surfaces, the less refrangible, far 

 from neutralizing, continue the action so becmn. To the 



O' O 



former he gave the designation "rayons excitateurs," to 

 the latter "rayons continuateurs " (Comptes-Hendus^o. 

 17, tome xxiii.). 



In 1847 M. Claudet communicated a paper to the 

 Royal Society, subsequently published in the Philosoph- 

 ical Magazine (Feb., 1848), on this subject. His atten- 

 tion had been drawn to it by observing that the red im- 

 age of the sun, during a dense fog, had destroyed the 

 effect previously produced on a sensitive silver surface, 

 and that this destruction could be occasioned at pleasure 

 by the use of red and yellow screens. A surface which 

 has been impressed by daylight, and the impression then 

 obliterated by the less refrangible rays, had recovered its 

 primitive condition. It was ready to be impressed again 

 by daylight, and again the resulting effect might be de- 

 stroyed. Claudet found that this excitation and neutral- 

 ization might be repeated many times, the chemical con- 

 stitution of the film remaining unchanged to the last. 



These facts seem to be inconsistent with Herschel's 

 opinion, that positive and negative pictures may succeed 

 each other by the continued action of a radiation, on the 

 principle of Newton's rings. 



On a collodion surface such negative neutralizing or 



