36 J. W. Draper—Distribution of 
(2.) “ Blackened paper under the influence of an iodie 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 paper. Tio be 
viewed also transparently.” 
These paper photographs I still preserve. They are as per- 
sk as when first made. e different colored spaces of the 
ectrum 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 maximum oceurs abruptly about the blue; 
(3) has the same upper spectrum as the others, a bleached dot in 
the center of the yellow, and a darkened space on the extreme 
red. e action has reached from the ultra-red to the ultra- 
et. 
In Herschel’s epee these effects in the less refrangible 
region are connected with the drying of the paper. It is well 
known that paper in a Sas condition is more sensitive than 
‘such as is dry. But obviously this condition 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experiment originally made by me, presented a communication 
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 
: P eelerana, the effect is greatly icrensed in all the region above 
line ©, and is neutralized in all that below C. They 
remarked the distinctness with which the atmospheric line A 
‘comes out, and saw the ultra spectrum heat-rays a, f, y, de- 
‘scribed by me some years previously. 
The interpretation given by them is, that the more refrangible 
rays promote the previous action of light; t the less neutralize it. 
‘The curve representing the chemical intensity of the different 
rays would cross the axis of abscissas about the boundary o 
the red and orange; below that point to the ultra-red the ordin- 
ates would have negative values; above it to the ultra-violet 
2a values would be positive (Comptes Rendus, No. 14, tome 
him bein vg while the more refrangible rays atin sensi- 
nation “ rayons exeitacout.” to the latter “ rayons continuateurs” 
(Comptes Rendus, No. 17, tome 28). 
