560 M. ARAGO ON THE CHEMICAL ACTION OF LIGHT. 
violet are respectively accompanied by similar rays, but invisible 
from deficient or excessive velocity. To a greater degree of 
velocity corresponds a less degree of refraction, as less velocity 
is attended with a greater refraction. Thus each visible red ray 
is accompanied by obscure rays of the same nature, some of 
which are refracted more, some less than it: thus there exist 
rays in the black lines of the red portion of the spectrum: the 
same thing should be said of the lines situated in the yellow, 
green, blue and violet portions. Experience having shown that 
the rays contained in the lines are without effect on sensitive 
substances, it is established that all increase or diminution of 
velocity takes away from the luminous rays the photogenic pro- 
perties with which they were before endowed; that the solar 
rays cease to act chemically at the same instant when by a 
change of velocity they lose the faculty of producing luminous 
sensations on the retina. I need not point out how much there 
is that is curious in a chemical mode of action of light depending 
on the velocity of the rays. 
The same Monday on which M. E. Becquerel presented the 
Academy with the result of the experiment which I had proposed 
two years and ten months before, I publicly invited him to re- 
commence, imposing upon himself new conditions, which seemed 
as if they must help to explain the manner in which the 
velocity modifies the chemical action of light. I made the re- 
mark, that the solar rays moving quicker and quicker, according 
as the media through which they pass are more refractive, some 
useful result would be attained by studying, comparatively and 
simultaneously, the action of the spectrum on the iodized 
plate, the halves of it being plunged into two very dissimilar 
media, for example into water and into air. M. E. Becquerel 
readily complied with this idea. The following is the letter which 
he wrote to me on the 25th November, 1842 :— 
“When you were so kind as to present my ‘ Memoir on the 
Constitution of the Solar Spectrum’ to the Academy of Sciences 
last June, you pointed out an experiment to be made for the 
purpose of knowing if, when a substance sensible to the action 
of the solar rays is plunged in any other medium than air, the 
change of velocity of the solar rays, at the transition from the 
air into this medium, did not alter the position of the ssicanaebiaw 
lines of the spectrum of the chemical rays. 
