MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 421 



salt when no excess of nitrate of silver exists in the 

 paper." 



(3.) "Action of spectrum under iodic influence when 

 very little nitrate of silver remains in excess in the pa- 

 per. To be viewed also transparently." 



These paper photographs I still preserve. They are as 

 perfect as when first made. The different colored spaces 

 of the spectrum are marked upon them with pencil. The 

 appearances they respectively present are as follows: (1) 

 is bleached by the more refrangible rays, and blackened 

 deeply from the yellow to the ultra-red ; (2) is bleached 

 from the ultra invisible red to the ultra-violet (a maxi- 

 mum occurs abruptly about the blue) ; (3) has the same 

 upper spectrum as the others, a bleached dot in the cen- 

 tre of the yellow, and a darkened space on the extreme 

 red. The action has reached from the ultra-red to the 

 ultra-violet. 



In Herschel's opinion, these effects in the less refran- 

 gible region are connected with the drying of the paper. 

 It is well known that paper in a damp condition is more 

 sensitive than such as is dry. But obviously this con- 

 dition does not obtain in the case of the daguerreotype 

 operation, which is essentially a dry process. 



In 1846 MM. Foucault and Fizeau, having repeated 

 the experiment originally made by me, presented a com- 

 munication to the French Academy of Sciences to the. 

 effect that when a silver tablet which has been sensitized 

 by exposure to iodine and bromine, and then impressed 

 by light, is exposed to the spectrum, the effect is greatly 

 increased in all the region above the line C, and is neu- 

 tralized in all that below C. They remarked the dis- 

 tinctness with which the atmospheric line A comes out, 

 and saw the ultra spectrum heat-rays a, ]3, y described by 

 me some years previously. 



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



