on the Intensity of spectrum lines 47 



one of veiy great intensity. Examining the efEect of the magnetic 

 fields on these four lines, it is very interesting to note that the 

 line 6152 alone increases about five times in brilliancy while the 

 others if they suffer any change at all, decrease in intensity. It is 

 also interesting to note that this line 6152 seems to be the same 

 line that becomes so greatly enhanced when the tube contains a 

 trace of helium as observed by* Collie. It seems very difl&cult to 

 excite this line unless at least a trace of helium is present in the 

 ajjparatus and at this stage it is not possible to suggest any 

 explanation of its abnormal behaviour. 



In addition to these very prominent changes there are also 

 many minor changes, among which is the disappearance of a faint 

 trace of continuous spectrum, as well as of some of the nebulous 

 bands and lines, the remaining lines being quite sharp on a dark 

 background. 



The abnormal behaviour of the mercury spectrum in the visible 

 region (the ultra violet spectrum has not yet been investigated) 

 'suggested the study of other spectra and the spectrum of helium 

 was next examined. 



The discharge tube contained hydrogen and a slight trace of 

 mercury vapour as impurity and the hydrogen lines and the 

 prominent mercury lines were also visible. The effect of the 

 magnetic field in this case was to enhance the helium lines very 

 considerably, leaving the hydrogen lines practically unaffected or 

 even slightly reduced in intensity. In this spectrum there were 

 also a few faint lines not yet identified definitely which remain 

 quite unaffected by the magnetic field. In the further study of 

 the helium spectrum, the gas was contained in a separate tube 

 from which any small quantity of it could be introduced into the 

 discharge tube. At a pressure of 1 mm. of mercury the addition 

 of a small trace of helium produced no perceptible effect on the 

 spectrum of residual air which showed the prominent hydrogen 

 lines and the nitrogen bands, but no trace of any of the helium 

 lines. But on switching on the magnetic field, the helium lines 

 flashed out prominently and disappeared again as soon as the field 

 was turned off. The effect is shown in the accompanying photo- 

 graphs (Plates I and II). In a plate taken with a greater percentage 

 of helium the lines are visible without the magnetic field, but a 

 great enhancement of these lines with the field is evident, and a 

 dense new fine at 49334 a.u. is also noticed which has not yet 

 been definitely identified. 



The spectrum of neon was also studied, and in a tube kindly 



lent to me by Dr Aston, there was a trace of hydrogen also present, 



showing the three principal hydrogen lines. Here also the effect 



of the field was to enhance very considerably the neon lines, 



* Proc. Roy. Soc. 71, 25, 1902. 



