﻿7)22 Prof. R. "YV. AYood on Fluorescence and 



Other Possible Excitations. 



It has occurred to me during the preparation o£ this paper 

 that very interesting results would be obtained by exciting 

 the fluorescence with the light selectively rotated by the 

 vapour in a magnetic field, i. e. by the magnetic bright-line 

 spectrum. This light is fairly intense; and it would be 

 interesting to see whether the intensity distribution among 

 the excited fluorescence lines was the same as in the magnetic 

 spectrum. 



What I most need, however, is a set of screens which will 

 enable me to separate lines such as those of copper without 

 resorting to the systems of prisms and lenses. A good 

 collection of solutions of the rare earths would probably be 

 very useful in the work. Erbium, praseodymium, and 

 neodymium I have, but I should feel very grateful for the 

 loan of any others which might prove serviceable, or for any 

 suggestions regarding other possible sources of monochromatic 

 light. As I have said before, the instrument most needed is 

 a light siren ! 



Composite Excitation. 



At the top of the chart just below the magnetic series will 

 be found a spectrum containing about two hundred lines. 

 This is a composite drawing made by superposing all of the 

 drawings made of the simple spectra excited by monochromatic 

 stimulation. It contains many lines not found in the complex 

 spectrum excited by white light. In the latter, between 

 wave-lengths 5000 and 5100, we find but ten or a dozen 

 lines, while in the composite spectrum there are at least 

 twenty. This circumstance is of interest in connexion with 

 the periodic dark regions of the complex spectrum, w T hich 

 give it a fluted appearance. The formation of these flutings 

 requires further study, as their position shifts as we alter the 

 wave-length of the exciting light, which in this case is a rather 

 broad band isolated from the continuous spectrum with the 

 monochromatic illuminator. The phenomenon was more fully 

 described in the earlier paper, but requires further study. 



The Series in the Magnetic Spectrum. 



As we have seen, the complex fluorescent spectrum is made 

 up of six or more series of lines, the individual lines of each 

 series being about 38 Angstrom units apart, the spacing 

 becoming less as we pass from yellow tow-ards violet. The 



