1903.] Helium and Mercury in a Magnetic Field. 21 



components found to be proportional to the field strength. In this 

 case, however, it was more difficult to obtain reliable results, owing 

 probably to the disturbance of the field produced by the hollow 

 pole. The following table gives the numbers obtained : 



Substance. Wave-length. 5A./H". SA./HA 2 . elm. 



Helium 5016 l'75xlO~ 5 6'95 x 10~ 5 13-1 x 10 6 , 



5876 2-24 xlO~ 5 6'50 x 10~ 5 12-3 x 10, 



6678 3-13 xlO~ 5 7-01 x 10~ 5 13-2 x 10 6 , 



Mercury 5461 l-88xlO~ 5 6-31 x 10~ 5 12'0 x 10 6 . 



The results here given for helium are the means of six, seven and 

 five determinations respectively, those for the green line of mercury are 

 the means of eight determinations. 



The observations of Zeeman on the green line of mercury, when 

 the field was zero, are referred to above. They followed an observa- 

 tion of Perot and Fabry,* that the line appeared to be a triplet 

 consisting of the main line with a fainter companion on each side at 

 about the same distance. Our observations gave three companions 

 on the violet side, and two (we seemed at times to see three) on the 

 red side. The values of dX for these are for the first time - 0-208, 

 -0-096, - 0-059, and for the other two +0'032, +0'067. But, as has 

 been stated, we have had what seem unmistakable indications that 

 this line system is of hitherto unexpected complexity, which is only 

 disclosed under special conditions of the discharge tube. Hence, 

 though these values of dX do not agree in several cases with those 

 observed by Perot and Fabry, it is not impossible that both sets of 

 observations are correct. 



It ought here to be noticed that Eunge and Paschenf have 

 obtained a resolution of the green mercury line into three triplets. 

 This observation is entirely confirmed as to the side triplets by ours 

 (which were made before Messrs. Runge and Paschen's paper came to 

 hand), but we have not been able to verify their result for the middle 

 group, which appears to us to be a doublet. But Messrs. Runge and 

 Paschen's instrument was a large Rowland grating of 6:5 metres 

 diameter of circle, and the spectrum was photographed, so that their 

 observations were, no doubt, more certain than ours. 



' Comptes Kendus,' vol. 126 (1898), p. 409. 

 f * Astrophjsical Journal,' vol. 35 (1002), p. 235. 



