on the Nature of the Light emitted by a Substance. 229 



become widened. For their width at a given height is 

 chiefly determined by the number of incandescent particles 

 at that height in the direction of the axis of the tube. Although 

 this explanation still leaves some difficulties, certainly some- 

 thing may be said for it. 



9. The explanation of the widening of the lines attempted in 

 § 8 is no longer applicable to the following variation of the 

 experiment, in which an unglazed tube is used. The inner 

 diameter of the tube, about 1 mm. thick, was 10 mm. The 

 poles of the electromagnet could be moved till the distance 

 was 14 mm. The tube now was heated by means of the 

 blowpipe instead of with the Bunsen burner, and became in 

 the middle part white hot. The blowpipe and the smaller 

 diameter of the tube make it easier to bring the upper and 

 lower parts to the same temperature. This is now higher than 

 before (§ 7), and the sodium lines remain visible conti- 

 nuously"*. One now can wait till the density of the sodium 

 vapour is the same at various heights. By rotating the tube 

 continuously round its axis I have still further promoted this. 

 The absorption-lines now are equally broad from the top to the 

 bottom. When the electromagnet was put on, the absorption- 

 lines immediately widened along iheir whole length. Now 

 the explanation in the manner of § 8 fails. 



10. I should like to have studied the influence of magnetism 

 on the spectrum of a solid. Oxide of erbium has, as was 

 found by Bunsen or Bahr, the remarkable property of giving 

 by incandescence a spectrum with bright lines. With the 

 dispersion used, however, the edges of these lines were too 

 indistinct to serve my purpose. 



11. The different experiments from § § 3 to 9 make it more 

 and more probable that the absorption- and hence also the 

 emission lines of an incandescent vapour are widened by the 

 action of magnetism. Now if this is really the case, then 

 by the action of magnetism on the free vibrations of the 

 atoms, which are the cause of the line-spectrum, other 

 vibrations of changed period must be superposed. That it is 

 really inevitable to admit this specific action of magnetism is 

 proved, I think, by the rest of the present paper. 



12. From the representation I had formed to myself of the 

 nature of the forces acting in the magnetic field on the atoms, 

 it seemed to me to follow that with a band-spectrum and with 

 external magnetic forces the phenomenon I had found with a 

 line-spectrum would not occur. 



It is, however, very probable that the difference between a 

 band- and a line-spectrum is not of a quantitative but of a 

 * Pringsheiru, /. c. p. 450. 



