258 M. G. Kirchhoff on the History of Spectrum Analysis 



absorptive action not only npon these, but also upon certain 

 other vibrations. 



It is seen that the proposition which forms the basis of the 

 chemical analysis of the solar atmosphere floated before Ang- 

 strom's mind, but only, indeed, in dim outline. The leading 

 idea in the theoretical considerations upon which Angstrom en- 

 deavours to base the subject is the same as that which Stokes * 

 has carried out more correctly on a later occasion when speak- 

 ing of my first publication respecting the reversal of the spectra 

 of flames. Stokes here compares the absorption which such a 

 flame exerts upon the kind of rays which it emits, to the reso- 

 nance which is excited in a body capable of taking up the vibra- 

 tions of sound by a wave of sound of the same pitch as that 

 which the body is capable of emitting. This comparison, if it 

 can be followed out, may lead to the most important conclusions ; 

 but, in the manner in which Stokes gives it, it is of interest 

 because it affords a good illustration of such an absorption, 

 although it does not yield a proof of the proposition that a 

 glowing body which emits only rays of certain wave-lengths 

 also absorbs only rays of the same wave-lengths. The theory of 

 resonance, and the theory of the production and absorption of the 

 rays of light and heat, are still not sufficiently advanced to 

 enable us at present to prove the proposition in question by any 

 such comparison. 



3. In the Transactions of the Royal Society of Edinburgh for 

 the year 1858, a paper was published by Balfour Stewart, in 

 which he describes very interesting experiments upon the radia- 

 tion and absorption of partially diathermanous plates. He finds 

 that a plate of rock-salt is less diathermanous for rays emitted by 

 another plate of rock-salt heated to 100° C, than for those 

 which are emitted from a surface of lampblack at the same tem- 

 perature. From these and similar phenomena which plates of 

 glass and mica exhibit, he concludes " that every body which sifts 

 heat in its passage through its substance is more opake with 

 regard to heat radiated by a thin slice of its own substance, than 

 it is with regard to ordinary heat." He then recalls the prin- 

 ciple first enunciated by Prevost, that a body placed in a medium 

 of the same temperature must absorb as much heat as it emits 

 itself; and then he says, " Considering, therefore, the heat of 

 any temperature to consist of heterogeneous rays, we may state 

 the law thus : The absorption of a plate equals its radiation, and 

 that for every description of heat " 



This proof cannot be a strict one, because experiments which 

 have only taught us concerning more and less, cannot strictly 

 teach us concerning equality. The proposition founded upon 



* Phil. Mag. March 1860. 



