334 M. A. J. Angstrom's Optical Researches. 



not well isolated lines. "What pronounces particularly against 

 the assumption referred to, is the fact that these luminous lines 

 seem to proceed from the edge of the prism, and to lose them- 

 selves in the centre, while the ground on which they exhibit them- 

 selves does not appear to be in the slightest degree changed. 



In the same manner as we assume that the almost homoge- 

 neous light emitted by a name of alcohol is due to the fact that 

 its oscillations belong almost exclusively to the yellow light, 

 although a trace of luminous lines appears on the rest of the 

 spectrum also, we seem, on the other hand, to be forced to assume 

 that the bright lines belonging to a metal in the electric spec- 

 trum, the number of ♦which increases with the strength of the 

 electricity, correspond to the vibrations of the metal in the 

 gaseous condition. 



9. In connexion with the foregoing, I will here mention an 

 observation which I have made on the flame of a solution of 

 iodine in alcohol. The alcohol flame alone gives a spectrum 

 shown in the following: figure : — 



where a denotes the vivid yellow line, b a weaker one in the 

 extreme blue, and c and d traces of two other lines. The solu- 

 tion of iodine produces, on both sides of d, several bright lines 

 with equal intervals; their distance apart amounts to about 10'. 

 Twice, when the wick was furnished with iodine, I saw dark 

 bands formed between b and c, which appeared to be at about 

 the same distance from each other. The phenomenon seems to 

 be the same as in the case of the absorption by iodine ; the bands 

 were not sharply defined, but resembled real bands of inter- 

 ference. 



10. The study of the spectra of the electric spark, it seems to 

 me, can also contribute to the solution of a question equally in- 

 teresting to both astronomy and optics. Doppler has endea- 

 voured to explain the colour of the double stars by the assump- 

 tion that the velocity of a body has an influence upon its colour. 

 On the other hand, Petzval, in a discourse before the Academy 

 of Sciences in Vienna, has proved analytically that the motion 

 of a medium can have no influence on the time of oscillation, on 

 which alone the colour depends. Although this proposition is 

 probable in itself, still a practical proof of it is not, I think, 

 without interest. 



From Wheatstone's researches on the velocity of electricity, 



