434 Dr. J. "W. Draper on the Distribution of 



the effect is greatly increased in all the region above the line C, 

 and is neutralized in all that below C. They remarked the di- 

 stinctness with which the atmospheric line A comes out, and saw 

 the ultra-spectrum heat-rays a, (i, y, described by me some years 

 previously. 



The interpretation given by them is, that the more refran- 

 gible rays promote the previous action of light, the less neutral- 

 ize it. The curve representing the chemical intensities of the 

 different rays would cross the axis of abscissae about the boundary 

 of the red and orange; below 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, 

 vol. xxiii.) 



Hereupon M. Becquerel communicated to the same Academy 

 a criticism on this interpretation, the opinion m-aintained by him 

 being that, while the more refrangible rays excite sensitive sur- 

 faces, the less refrangible, far from neutralizing, continue the 

 action so begun. To the former he gave the designation '' rayons 

 excitateurs;^' to the latter, ^^ rayons continuateurs ^^ (Comptes 

 Rendus, No. 17, vol. xxiii.). 



In 1847 Mr. Claudet communicated a paper to the Royal 

 Society, subsequently published in the Philosophical Magazine 

 (February 1848), on this subject. His attention had been drawn 

 to it by observing that the red image 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 

 had been impressed by daylight, and the impression then obli- 

 terated by less refrangible rays, had recovered its primitive con- 

 dition. It was ready to be impressed again by daylight; and 

 again the resulting effect might be destroyed. Claudet found 

 that this excitation and neutralization might be repeated many 

 times, the chemical constitution of the film remaining unchanged 

 to the last. 



These facts seem to be inconsistent with HerschePs 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 reversing 

 actions cannot be obtained by the less refrangible rays. The 

 spectrum-impression, developed in the usual manner by an iron 

 salt, presents a sudden maximum about the line G, and con- 

 tinues thence to the highest limit of the spectrum. In the 

 other direction it extends below F. From E to the ultra-red 

 not a trace of action can be detected. The lines a, /9, y cannot 

 be obtained on collodion. There is therefore a difference be- 



