﻿Magnetic Rotation Spectra of Sodium Vapour. 517 



the total copper radiation. Only the three green lines are 

 operative in stimulating the vapour. The lines in the fluor- 

 escence spectrum appear to bear no very definite relation to 

 the lines of the magnetic spectrum, as will be seen by the 

 chart. There are many coincidences, however, with lines in 

 the spectrum excited by zinc 4811, and by zinc 468 and 472. 

 By comparing the three spectra I have made a provisional 

 determination of the lines which belong together in the 

 spectrum excited by the copper radiation. These lines are 

 indicated by crosses and vertical dashes placed above them ; 

 other lines, which do not appear in the zinc spectra, have not 

 been marked. I suspect that excitation with copper 5152 

 will produce a doublet at this point, and probably at other 

 points, just as does cadmium 5086. An attempt will be made 

 to verify this surmise. I have on one or two occasions, when 

 trying to stimulate the vapour with this isolated line, been of 

 the impression that I saw doublets distinctly, but at the time 

 I attributed it to incomplete separation of the exciting lines. 



Lead Excitation. 



A lead arc, operated in a manner similar to the one described 

 for bismuth, was used for exciting the vapour. The only 

 line operative was the one at 5001, and it gave rise to a well- 

 marked series of fluorescence lines, which coincided exactly 

 with the first series of the magnetic rotation spectrum. It is 

 worthy of remark that one of the extra lines of the magnetic 

 spectrum lies very close to the exciting line, yet none of these 

 lines appeared in the fluorescence spectrum. The fluorescence 

 excited by lead was very feeble; and even with an eight-hour 

 exposure the lines were very faint. 



Helium Excitation. 



A large " end on " helium tube with a 3 mm. bore was con- 

 structed for the investigation. This tube could be run conti- 

 nuously with an induction-coil yielding a heavy 10-inch spark. 

 An exposure of about twelve hours was given. Two of the 

 helium lines are operative : line 5015 gives a well-marked 

 series, the lines of which fall exactly midway between the 

 lines of the second and third magnetic series ; line 4713 gives 

 a good series in the blue and at least six distinct lines in the 

 yellow-green, the wave-lengths of which can be seen from the 

 chart. The exciting line in this case coincides with one of 

 the lines in the spectrum excited by zinc 468; and there is 



