ABSTRACTS 327 



On the Structure of the Wave-length X = 6708 A.U. of the Isotopes of 



Lithium 



By Professor J. C. McLennan, F.R.S., and Mr. D. S. 



AiNSLIE, M.A. 



(Read May Meeting, 1922) 

 Abstract 



In this investigation a study has been made of the structure of 

 the lithium red line X = 6708 A.U. by using a vacuum arc in the vapour 

 of the metal and by using both Lummer plates and a 30 plate Echelon 

 grating crossed with a Lummer plate to eflfect the resolution. 



It has been shown that with strong arcs the wave-length X = 6708 



o 



A.U. consists of two doublets, with a separation of the doublet com- 



o o 



ponents of 0. 128 A.U. and 0. 165 A.U. respectively. The mean dis- 

 placement of the two doublets was found to be 0.32 A.U., which is 

 between 3 and 4 times the displacement demanded on Bohr's theory 

 for isotopes of lithium having atomic weights sax and seven. 



Attention is drawn in the paper to the results obtained by Merton 

 and by Aronberg in studying the wave-length X=4058 A.U. in the 

 spectrum of ordinary lead and in that of lead having a radioactive 

 origin. In these experiments it was found that the observed difference 

 in wave-length v/as between 80 and 90 times as great as the difference 

 to be expected on Bohr's theory. 



With both lead and lithium it seems that in what would appear 

 to be isotopic spectral displacements the value found by observation is 

 about the atomic number times the value obtained by calculation on 

 the basis of Bohr's theory. 



