194 



Mr. J. Norman Lockyer. 



" II semble difficile que la reproduction d'un pareil phenomene 

 soit un effet du hasard : n'est-ii pas plus naturel d'admettre que ces 

 groupes de raies semblables sont des harmoniques qui tiennent a la 

 constitution moleculaire du gaz lumineux ? II faiidra sans doute un 

 grand nombre d'observations analogues pour decouvrir la loi qui regit 

 ces harmoniques." 



I wrote thus on this subject in 1879 : — 



"I am at present engaged in investigating this question of rhythm, 

 and I have already found that many of the first order lines of iron 

 may probably arise from the superposition or integration of a num- 

 ber of rhythmical triplets. All this goes to show how long the 

 series of simplifications is that we bring about in the case of the so- 

 called elementary bodies by the application of a temperature that we 

 cannot as yet define. 



" Indeed, the more one studies spectra in detail, and especially under 

 varying conditions of temperature which enable us to observe the 

 reversal now of this set of lines, now of that, the more complex be- 

 comes the possible origin. Some spectra are full of doublets ; others, 

 again, are full of triplets, the wider member being sometimes on the 

 more, sometimes on the less, refrangible side. Doublets and trip- 

 lets, as a rule, reverse themselves more freely than the irregular 

 lines in the same spectrum — which particular doublet or triplet will 

 reverse depending upon the temperature."* 



A series of spectral lines may be defined as a sequence of lines, the 

 intensity of which decreases with the wave-length, and the number 

 of vibrations of which may be determined by the formula — 



where n is given the integers from three upwards, and the constants 

 A, B, and C are determined for each element separately. 



The fact that lines must close up to one another, as the violet end 

 of the spectrum is reached, indicates that the character of a " series " 

 is best brought under notice in the ultra-violet end of the spectrum. 

 In the visible part of the spectrum the lines forming " series " are 

 too far apart to be recognised as belonging to series. 



As soon as it becomes apparent that a set of lines in the violet 

 seems to form a series, computation will at once give the lines that 

 belong to it in the visible part of the spectrum. 



So far there has been no definite pronouncement touching the 

 question of temperature or the possibility that each series may repre- 

 sent vibrations of similar molecules, but this question has recently 

 been advanced in an instance which to many will carry conviction 

 with it. 



* "On Young's List of Chromospheric Lines," 'Roy. Soc. Proc.,' vol. 28, March, 

 1879. 



