M. ARAGO ON THE CHEMICAL ACTION OF LIGHT. 559 
mond Becquerel brought this same subject before the Academy 
at the meeting of the 13th of June, 1842. Sir John Herschel 
not having been able to fit up a heliostat, thought that he ought 
not to state positively the existence of the lines in the photo- 
graphic image of the spectrum. M. EK. Becquerel, on the con- 
trary, projected on his iodized plate a stationary spectrum, 
and saw distinctly, after the experiment, in the region of the 
plate which this spectrum occupied, transversal lines, along 
which the chemical matter had remained untouched, or at least 
had not received any perceptible modification. He also recog- 
_ nised that these lines corresponded exactly to the dark lines of 
_ the luminous spectrum. 
At first sight, the experiment which I have just mentioned 
_ might have seemed superfluous; in short, was not the result 
_ obtained necessarily true? How should we expect photogenic 
actions where light is entirely wanting ? 
My reply is this: It is by no means proved that the photo- 
genic modifications of sensitive substances result from the action 
of solar light itself. These modifications are perhaps engen- 
dered by invisible radiations mixed with light properly so called, 
_ proceeding with it and being similarly refracted. In this case 
_ the experiment would prove, not only that the spectrum formed 
_ by these invisible rays is not continuous, that there are solutions 
of continuity as in the visible spectrum, but also that in the 
two superposed spectra these solutions correspond exactly. This 
would be one of the most curious, one of the most strange re- 
sults of physics. 
Let us introduce into the discussion an element depending on 
i = velocity of light, and the consequences of the observation 
will not be less interesting. 
_ Many years ago I showed that the rays of the stars towards 
Be nich the earth advances, and the rays of the stars from which 
the earth recedes, are refracted exactly in the same degree. Such 
a result cannot be reconciled with the theory of emission, but by 
} the help of an important addition to be made to this theory, 
the necessity of which presented itself formerly to my mind, 
and which was generally well received by philosophers, we 
_ must admit that luminous bodies emit rays of every degree of 
velocity, that rays of a determined velocity alone are visible, and 
that they alone produce the sensation of light in the eye. In 
the theory of emission, the solar red, yellow, green, blue and 
VOL. III. PART XII. 2P 
