[December, 
742 Spectrum Analysis . 
me matter for some unexpected observations. ... It showed 
to me that this arc . . . absorbs the rays D, so that the 
above-mentioned line D of the solar speCtrum is consider- 
ably strengthened when the two speCtra are exadtly super- 
posed. Thus the arc 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 moie 
decisive, I projected on the arc the reflected image of one 
of the charcoal points, which, like all solid bodies in igni- 
tion, gives no lines ; and under these circumstances the line 
D appeared to me [dark] as in the solar speCtrum.” 
In 1855 M. Angstrom published a memoir, remarkable in 
many ways, and to which we have already had occasion to 
refer. In speculating upon the origin of the absorption of 
light by bodies, the author observes : — “ As according to the 
principle of Euler [Quemadmodum (ergo) corda tensa a 
sono ei, quern ea edit, sequali vel consono concitatur, ita 
particulae illae minimae in superficie corporis opaci sitae, a 
radiis ejusdem vel similis indolis, contremiscere, pulsusque 
undique diffundendos producere valebunt — as correCtly ap- 
plied by M. Angstrom] a body absorbs all the series of oscil- 
lations which it can itself assume ; it follows from this that 
the same body, when heated so as to become luminous, must 
emit the precise rays which, at its ordinary temperature, are 
absorbed.” This proposition, it was added, is in so far true 
as the diversity in “the condition of the heated body, as 
regards elasticity, Twhich] is altogether different from the 
state in which the light is supposed to be emitted,” may be 
neglected ; but then it would have been more correCt at once 
to refer the emitted light to the power of absorption actually 
obtaining under the same conditions, instead of at common 
temperatures. Adverting further to the speCtra of the elec- 
tric spark as compared with the solar speCtrum, M. Angstrom 
observes that, “ regarded as a whole, the first produced the 
impression of being the reverse of the other. I am there- 
fore of opinion that the explanation of the dark lines in the 
solar speCtrum embraces that of the luminous in the eleCtric 
speCtrum.” Thus, by the observations of M. Foucault, con- 
joined with the above explanations, the origin ot the dark 
lines in the solar speCtrum was accounted for, supposing the 
two had been brought mutually to bear. Like M. Angstrom, 
Prof. Kirchhoff was not aware of M. Foucault’s experi- 
ments when, in 1859, he published the following: — “ On the 
occasion of an examination of the speCtra of coloured flames 
... I made some observations which disclose an unexpected 
