WOOD. — SPECTRA OF SODIUM VAPOR. 247 



the following points of the spectrum form a series analogous to the 

 series obtained with the cadmium 480 series. 



This we have called the first series. 



The wave-length differences are, in this case, much more nearly con- 

 stant, and decrease progressively. 



The lines of this series are especially conspicuous in the magnetic 

 rotation spectrum (Plate l. Figure 1, c), hence I have mentioned it first ; 

 they appear in the fluorescence excited by the lead line 5001, as will 

 be seen by reference to the chart, Plate 5. 



If now we try to fit one of the magnetic series to the cadmium 480 

 fluorescence series, we find that the third magnetic series coincides 

 with it between 5019 and 5134, while in the violet region it coincides 

 with the fourth magnetic set. I do not feel sure whether this peculiarity 

 is due to slight errors in the determination of wave-lengths or not. 

 I think not, however, for I have very carefully superposed the two 

 negatives (cadmium fluorescence and magnetic rotation spectra), both 

 taken with the same instrument, and find the same disagreement. 

 We cannot be sure of anything, however, until the cadmium series 

 has been photographed with the grating. During the coming year I 

 expect to photograph the fluorescence spectrum excited by cadmium 

 and zinc radiations with the large concave grating. It will then be 

 possible to determine the wave-lengths of the lines to within a tenth 

 of a unit. 



One of the most remarkable facts connected with the appearance 

 of the lines of a series is that the distribution of energy among 

 the individual lines depends upon the point of excitation. Unfortu- 

 nately there are very few arc lights bright enough to excite fluor- 

 escence. It was found, however, that the silver line 5207, which 

 coincides with one of the fluorescent lines of the cadmium 480 series, 

 was bright enough for the purpose. The silver was carefully fi-eed from 

 copper, as the neighboring copper lines are powerful exciters ; in tlieir 

 absence, no prismatic separation was necessary, as the rest of the silver 

 lines were inoperative. A photograph of the fluorescence spectrum 



