16 



Prof. A. Gray and Dr. W. Stewart. Eadiatio.i of [Apr. 27, 



On the Eadiation of Helium and Mercury in a Magnetic Held. 

 By Professor ANDREW GRAY, FAS, and WALTER STEWAKT, 

 D Sc with EGBERT A. HOUSTOUN, M.A., and 13. B. MCQUISTAN. 

 M.A.,' Eesearch Students in the University of Glasgow, 

 ceived April 27, Eead May 14, 1903. 



The experiments described in the following paper had for their 

 primary object to test, for as many lines as possible of different 

 substances, the proportionality of the change dX of wave-length or 

 each of the components into which a single spectral line is resolved by 

 the application of a magnetic field, to the field intensity H, and to 

 deduce the corresponding values of the ratio e/m of charge to mass of 

 the electron. It has been found difficult to obtain a ready supply of 

 good discharge-tubes, and up to the present our observations have been 

 mainly of the spectra of helium and mercury, in the latter case chiefly 

 of the green line of wave-length approximately 5461 tenth-metres. 



With respect to this line we incidentally observed fully a year 

 ago, as we found afterwards had also been done a little earlier by 

 Zeeman,* that in its immediate neighbourhood there existed a number 

 of comparatively faint lines, so that what appeared in an ordinary 

 spectroscope as a single bright line was in fact a group of six or seven, 

 consisting of one bright line with a triplet of faint satellites on the side 

 next the violet end of the spectrum, and a triplet or at least a doublet 

 of faint lines on the other side. These faint lines were very difficult to- 

 observe and to determine, and the measurements of wave-lengths which 

 we obtained do not agree with those supplied to Zeeman by 

 MM. Fabry and Perot.* The lines, however, we found were only 

 visible in our tubes, when the discharge had been kept going for a long 

 time, and when, therefore, the vacuum had been considerably changed - f 

 and from glimpses that were obtained at times of what seemed to be 

 further lines, its appears probable that the full complexity of the green 

 line, when unaffected by a magnetic field, has not yet been disclosed, 

 and can only be investigated under peculiar conditions of the vacuum 

 in the tube, and possibly of the action of the discharging coil. An 

 important question is raised by these facts as to what it is that is 

 resolved by a magnetic field ; it may not be correct to regard the 

 complex of lines, which are plainly seen when a powerful field is applied 

 to this light, as a resolution of the simple central line. It will be con- 

 venient to defer further particulars of these observations until after a 

 short description of the apparatus employed. 



Astiophysical Journal,' vol. 15 (1902), p. 220. 



