﻿Men lactic Rotation Spectra of Sodium Vapour. 515 



fluorescence. It is scarcely possible to find a coincidence of 

 two lines. The two spectra placed side by side make a striking- 

 picture of the variation produced by different excitations or' 

 the same fluorescing medium. A photograph of the spectrum 

 is reproduced on Plate XII. c. 



Exciting with the zinc line 4811 alone gives us the spectrum 

 shown on Plate XII. b. The other two zinc lines are of course 

 present on account of diffused light. Referring now to the chart 

 (PI. XV.), we find that the violet end of the spectrum agrees 

 pretty well with the fifth magnetic series, though other lines 

 are present. The two strong lines at .518* and 5225 also 

 appear to belong to the same series. The lines on the whole 

 are much less regularly distributed than in the case of the 

 cadmium 480 excitation. The three wide pairs between 523 

 and 535 are peculiar to this excitation. 



The fluorescence excited by the other two zinc lines, 4680 

 and 4722. is also recorded on the chart. These lines are so 

 close together that it was found impossible to illuminate the 

 vapour with light from but one of them, and have at the same 

 time sufficient illumination to excite much fluorescence. I 

 have not yet found much evidence o£ regularity in the distri- 

 bution of the lines in this case, though there i> undoubted 

 evidence of two series in the immediate vicinity of the exciting- 

 lines. I have indicated with the letters A and B the lines 

 which appear to belong together (compare with helium 

 excitation). We have an enormous number of lines in the 

 yellow-green region, since we have a double stimulation at 

 the opposite end of the spectrum. There seems to be some 

 regularity here, but it is difficult to say which lines belong 

 together. 



Bismuth Excitation. 



The light of the bismuth arc makes a beautiful stimulus for 

 the fluorescence, since it contains but a single operative line, 

 the strong one at 4724. It gives rise to a very regular series 



O O c? 



in the blue- violet region, the lines appearing to fall midway 

 between the lines of the third and fourth magnetic series 

 (Plate XII. e. and Chart). Though the wave-length of the 

 exciting line is onlv two Angstrom units longer than that of 

 the zinc line 4722, the spacing of the series in the two cases 

 is quite different. The same thing has been noticed in the 

 case of the shortest cadmium and zinc lines, which makes it 

 seem possible that interesting results may be obtained by 

 altering the wave-length of the exciting line, either by 



2L 2 



