﻿Magnetic Rotation Spectra of Sodium Vapour. 521 



time the image of the spot was thrown upon the slit in the 

 proper direction to pass the light through the prisms, and 

 the eye brought to the instrument, the conditions in the tube 

 changed. 



It is difficult to account for the absence of luminosity of 

 the centre of the mass and the two bright spots. Perhaps the 

 condition under which the rays excite fluorescence exists only 

 where the vapour mass is in contact with the vacuum, L e. in 

 the region where the hypothetical clusters of molecules are 

 breaking up and flying to the cooler walls of the tube. Even 

 assuming this to be the fact, the difference in the colour of 

 the two spots is still to be accounted for. Possibly the 

 cathode rays excite the green spectrum, while the canal rays 

 travelling towards the cathode excite the orange-yellow 

 luminescence. I have made one experiment with a similar 

 tube arranged so as to deliver a stream of canal rays against 

 the vapour. The luminescence was bright yellow, but the 

 tube cracked before a spectroscopic examination was made. 



On the other hand, it may be that whatever causes the 

 green luminescence is removed from the ray-bundle by ab- 

 sorption, the residue exciting the yellow luminescence at the 

 point of exit. If this is the case, we should expect the same 

 amount of yellow light in each spot ; and I am of opinion 

 that the green light is much too pure for this to be the case. 

 Further experimenting will be necessary before it is possible 

 to draw any very definite conclusions. 



In the spectrum excited by the cathode rays the D lines 

 are of immense brilliancy, running together into a single 

 band of light. On each side of this are seen three or four 

 symmetrically placed bands, decreasing in brilliancy as they 

 recede from the D lines in each direction. No trace of these 

 bands appears in the magnetic spectrum, which in this region 

 shows only fine lines arranged in narrow groups, which 

 do not coincide with the bright bands of the cathode 

 luminescence. 



A photograph of these bands is reproduced in PL XIV. fig. 2. 

 They have some connexion with the D lines, I feel sure, for 

 they are symmetrically arranged on each side of them. If 

 the photograph had been made with a grating, ^ve should of 

 course call them ghosts. It may be that they are analogous 

 to satellite lines ; but if they are, we are certainly dealing 

 with the phenomenon on a grand scale, for the fourth one is 

 not far from the sodium doublet at 5688 ! All of these points 

 will be more fully investigated during the coming year. 



