566 Stoney — Cause of Double Lines in Spectra. 



The first step to connect these sethereal oscillations with motions in the mole- 

 cules was I believe taken by the author when, in 1870, he pointed out the 

 harmonic relation which exists between the lines a, /3, and 8 of the line spectrum 

 of hydrogen. These are the lines C, F, and h of the solar spectrum. Their 

 periodic times are inversely as the numbers 20 : 27 : 32. This gives evidence 

 that these three lines have their source in some one event in the molecules of 

 hydrogen. The next step was taken by Professor J. Emerson Reynolds and the 

 author working in conjunction in the laboratory of the Royal Dublin Society in 

 1871, when, on a careful examination of the spectrum of chlorochromic anhydride 

 (CrOsClj), it was ascertained that the sequence and intensities of a very long series 

 of lines in the absorption spectrum of that ruddy vapour, stand in a close relation- 

 ship to the sequence and intensities of the series of harmonics emitted by a violin 

 under definite circumstances, viz. when the string is bowed at a point nearly, but 

 not quite, two-fifths of its length from the bridge.* From this, and from the regu- 

 larity in the spacing of the lines, it appears that all the lines of this long series have 

 their source in some one event occurring in the molecules of the vapour. It was 

 also ascertained by mixing air with the vapour that this event is one which ia 

 independent of the encounters that must then occur between molecules of the 

 vapour and molecules of the air. It is therefore probably a periodic event excited 

 and maintained by the incident light during the comparatively long periods of 

 flight of the molecules, which, in the experiments that were made, lasted over some 

 hundreds of jots, and not during the very much briefer periods when the molecules 

 are now and then grappling with one another in struggles, no one of which 

 probably can last more than some very few jots. During these brief encounters 

 we must presume that the motions excited by the incident light are, on the con- 

 trary, in part obliterated, since some of the energy which is absorbed from the 

 sether increases the pressure of the vapour. 



The next notable event was the discovery by Dr. Huggins, that the four 

 hydrogen lines of the solar spectrum are part of a much longer series of Hues 

 arranged in a conspicuous pattern, which is evidence that they are due to some 

 common cause. Dr. Huggins found the additional lines in the spectra of white 

 stars. They are absent from our Sun. This discovery was shortly followed by 

 a laboratory investigation, confirming the opinion suggested by the telescopic 

 observation, viz. that the whole series is due to hydrogen. 



Then followed the very important discovery of " Balmer's Law." Professor 

 Balmer, in 1885, showed that the law connecting three of the hydrogen lines, to 

 which the author had called attention in 1870, is part of a more comprehen- 

 sive law which includes the whole series. This comprehensive law is that 



* See Phil. Mag. for July, 1871, p. 47. 



