ON THE THEORY OF EXCHANGES. 105 



seem to have been impressed with the idea that the same physical cause 

 which produces the dark bands of the solar spectrum, produces also the bright 

 bands in the spectra of incandescent bodies. 



In a paper by Angstrom* (a translation of which will be found in the 

 ' Philosophical Magazine' for May 1855), the author refers to a conjecture 

 by Euler, that a body absorbs all the series of oscillations which it can itself 

 assume ; " and it follows from this, says Angstrom, that the same body when 

 heated so as to become luminous must emit the precise rays which at ordi- 

 nary temperatures are absorbed ;" after which remarkable conjecture, now 

 amply verified by experiment, he goes on to say, " I am therefore convinced 

 that the explanation of the dark lines in the solar spectrum embraces that of 

 the luminous lines in the electric spectrum." 



In connexion with this subject it may not be out of place to introduce the 

 following extract of a letter from Prof. W. Thomson to Prof. KirchhofF, 

 dated 1860. Professor Thomson thus writes : — " Professor Stokes mentioned 

 tor me at Cambridge some time ago, probably about ten years, that Professor 

 Miller had made an experiment testing to a very high degree of accuracy 

 the agreement of the double dark line D of the solar spectrum with the 

 double bright line constituting the spectrum of the spirit-lamp burning with 

 salt. I remarked that there must be some physical connexion between two 

 agencies presenting so marked a characteristic in common. He assented, 

 and said he believed a mechanical explanation of the cause was to be had 

 on some such principles as the following : — Vapour of sodium must possess 

 by its molecular structure a tendency to vibrate in the periods correspond- 

 ing to the degrees of refrangibility of the double line D. Hence the pre- 

 sence of sodium in a source of light must tend to originate light of that 

 quality. On the other hand, vapour of sodium in an atmosphere round a 

 source, must have a great tendency to retain in itself, i. e. to absorb and to 

 iiave its temperature raised by light from the source, of the precise quality 

 in question. In the atmosphere around the sun, therefore, there must be 

 present vapour of sodium, which, according to the mechanical explanation 

 thus suggested, being particularly opake for light of that quality, prevents 

 such of it as is emitted from the sun from penetrating to any considerable 

 distance through the surrounding atmosphere. The test of this theory must 

 be had in ascertaining vvliether or not vapour of sodium has the special 

 absorbing power anticipated. I liave the impression that some Frenchman 

 did make this out by experiment, but I can find no reference on the point." 



The experiment alluded to by Professor Stokes in this conversation was 

 made by M. Foucault, who in July 184'9 communicated to the Institute the 

 result of some observations on the voltaic arc formed between charcoal poles. 

 He found, to use his own words, that this arc, placed in the path of a beam 

 of solar light, absorbs the rays D, so that the dark line D of the solar light is 

 considerably strengthened 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 arc, he continues, 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, Foucault pro- 

 jected on the arc the reflected image of one of the charcoal points, which, 

 like all solid bodies in ignition, give no lines ; and under these circumstances 

 the line D appeared as in the solar spectrum. 



* Poggcndorff's ' Annalen,' vol. xciv-. p. 141. 



