and of the Analysis of the Solar Atmosphere. 257 



in the solar spectrum reveal the constituents of the sun's atmo- 

 sphere. 



Angstrom, in his " Optischen Unter such un gen "*, states the 

 proposition that a body "in the state of incandescence must 

 emit exactly all those kinds of light which it absorbs at the 

 common temperature." Then follow these words : " The proof 

 of the truth of the above proposition is accompanied by great 

 difficulties, because the elastic relations of a glowing body are 

 quite different from those under which its power of absorption 

 is examined." These words convey no meaning as they stand ; 

 they become, however, intelligible if we suppose that Angstrom 

 really meant his proposition to imply that a body in the glow- 

 ing condition must emit exactly all those kinds of light which 

 at the same temperature it absorbs. Such an interpretation is, 

 however, by no means favoured by the statement which imme- 

 diately follows ; for he adds : — ■ 



" An indirect proof of the truth of this proposition is, how- 

 ever, given by the fact discovered byM. Niepce de Saint- Victor 

 of the relation existing between the colour which a body imparts 

 to a flame of alcohol, and that which light produces upon a silver- 

 plate which has been treated with the chloride of the body in 

 question. Thus a plate of silver treated with chlorine alone, 

 assumes all the colours of the spectrum, but treated at the same 

 time with a body capable of producing colour, it exhibits almost 

 exclusively the colour which the body produces ; this can only 

 be explained by the prepared plate absorbing exactly the colour 

 which the body in question imparts to a flame." Without en- 

 deavouring to follow this "proof" further, we see at once that 

 the radiation of a soda-flame, for example, is here compared 

 with the absorption effected by a cold plate of silver treated with 

 common salt. 



The meaning which is to be attached to this proposition is, 

 however, rendered altogether doubtful by a remark which Ang- 

 strom makes in page 143 of the memoir above cited. He here 

 states, " It is needful to observe that a medium not only absorbs 

 the vibrations which it can most easily take up, but likewise 

 those which stand in a simple relation to them, such as octave, 

 third, &c." In order to see how these statements contradict each 

 other, let us suppose a body which can take up certain vibrations 

 with equal facility, but is incapable of taking up certain others ; 

 according to Angstrom's proposition, found in page 144 of his 

 memoir, this body can only absorb the first kinds of vibrations ; 

 according to the remarks found in page 143, it must exert an 



* Pogg. Ann. vol. xciv. 1853, p. 144. [Phil. Mag. vol. ix. p. 329.] 

 Phil. Mag. S. 4. Vol. 25. No. 168. April 1863. S 



