﻿514 Prof. P. W. Wood on Fluorescence and 



The vibrations may be ripples running over its surface, or 

 they may be unlike anything with which we are familiar. If 

 we had never seen a bell, it would be difficult to work out 

 the theory of its very complicated vibrations from a study of 

 a set of simple pendulums. Possibly stimulation at some 

 other points might give rise to the double lines. 



I attempted to do this with the monochromatic illuminator, 

 but without success. The band of exciting light cannot well 

 be made much narrower than the distance between the com- 

 ponents of the doublets. Even with the instrument set at 

 5086, I could detect no evidence of the doublets. I am 

 planning to investigate this matter further with a larger 

 monochromatic illuminator designed to furnish more nearly 

 monochromatic light. 



In addition to the eleven pairs of lines in the fluorescence 

 spectrum excited by cadmium 5086, there are two strong lines 

 at wave-leneths 5305 and 5341. These seem to belong to the 

 same series, and the former has a faint companion, the two 

 forming a doublet. The' line 5341 is also accompanied by a 

 companion, which, however, is so faint as to be barely 

 distinguishable. 



The spectrum excited by the more refrangible of the two 

 blue cadmium lines 4678 is reproduced only on the chart. It 

 consists of a regular series of five lines in the blue region, 

 and a large number of irregularly spaced lines of widely 

 different intensities in the yellow-green region. None of 

 these lines appear to be represented in the magnetic spectrum. 



This spectrum illustrates well the characteristic peculiarity 

 of the sodium fluorescence spectrum, that stimulation at the 

 more refrangible end excites powerful fluorescence at the 

 opposite end. The lines which form the regular series we 

 may call directly excited, the others in the yellow region 

 indirectly excited. The latter in all cases seem to be irre- 

 gularly spaced. The great problem to solve is to determine 

 the nature of the mechanism and find out how the low- 

 frequency vibrators are set agoing by the stimulation of the 

 high-frequency ones, while they remain quiescent when the 

 stimulation is at the middle of the spectrum. Speculations on 

 these points must be deferred for the present. 



Zinc-arc Excitation. 



The complete fluorescent spectrum excited by all three of 

 the zinc lines (i. e. the total radiation of the lamp) is shown 

 near the top of the folding chart, just below the cadmium 



