194 M. Foucault and Prof. Kirchhoff on the Simultaneous 
though published in L’Jnstitut, does not seem to have attracted 
the attention which it deserves. Having recently received from 
Prof. Kirchhoff a copy of a very important communication to the 
Academy of Sciences at Berlin, I take the liberty of sending you 
translations of the two, which I doubt not will prove highly in- 
teresting to many of your readers. 
I am, Gentlemen, 
Yours sincerely, 
G. G. Stoxzs. 
M. Foucault’s discovery is mentioned in the course of a paper 
published in L’Jnstitut of Feb. 7, 1849, having been brought 
forward at a meeting of the Plilomathic Society on the 20th of 
January preceding. In describing the result of a prismatic 
analysis of the voltaic arc formed between charcoal poles, 
M. Foucault writes as follows (p. 45) :— 
“Its spectrum is marked, as is known, in its whole extent by 
a multitude of irregularly grouped luminous lines ; but among 
these may be remarked a double line situated at the boundary 
of the yellow and orange. As this double line recalled by its 
form and situation the line D of the solar spectrum, I wished to 
try if it corresponded to it; and in default of instruments for 
measuring the angles, I had recourse to a particular process. 
“T caused an image of the sun, formed by « converging lens, 
to fall on the are itself, which allowed me to observe at the same 
time the electric and the solar spectrum superposed ; I convinced 
myself in this way that the double bright line of the are coin- 
cides exactly with the double dark line of the solar spectrum. 
“This process of investigation furnished me matter for some 
unexpected observations. It proved to me in the first instance 
the extreme transparency of the are, which occasions only a faint 
shadow in the solar light. It showed me that this arc, placed in 
the path of a beam of solar light, absorbs the rays D, so that the 
above-mentioned line D of the solar light is considerably strength- 
ened when the two spectra are exactly superposed. When, on 
the contrary, they jut out one beyond the other, the line D 
appears darker than usual in the solar light, and stands out bright 
in the electric spectrum, which allows one easily to judge of their 
perfect coincidence. Thus the are presents us with a medium 
which emits the rays D on its own account, and which at the 
same time absorbs them when they come from another quarter. 
“To make the experiment in a manner still more decisive, I 
projected on the are the reflected image of one of the charcoal 
points, which, like all solid bodies in ignition, gives no lines ; 
and under these circumstances the line D appeared to me as in 
the solar spectrum.” 
