﻿510 Prof. R. W. Wood on Fluorescence and 



the large concave grating will be found at the bottom of the 

 chart : the former is a positive. At the top I have given a 

 composite drawing which represents a superposition of all 

 the simple fluorescent spectra thus far obtained. Immediately 

 below it will be found the spectra excited by the complete 

 radiation of the cadmium and zinc tubes. In each case there 

 are three different exciting radiations simultaneously applied, 

 yet it is almost impossible to find two fluorescent lines which 

 coincide. The other spectra are excited for the most part 

 by a single monochromatic radiation, the wave-length of 

 which is indicated by the arrow. I have not yet obtained 

 photographs of the fluorescent spectra excited by the separated 

 radiations of the copper arc or by the separated zinc lines 

 4~68 and 472 ; consequently these have been drawn together. 

 It is possible, however, by comparing the spectrum excited 

 by copper with the one excited by zinc 4811, to make a guess 

 as to which lines belong together. 



We will begin by a study of the spectra excited by the 

 cadmium radiations. 



Cadmium Stimulation. 



Photographs of the fluorescent spectrum obtained with the 

 cadmium-arc excitation are shown on PL XIL, A, i,j, k. 



Of these, i and k are excited by all three cadmium lines ; 

 the former taken with a much narrower slit than the latter. 

 Spectrum h was taken when the sodium vapour was excited 

 by the line 480 ; observed visually, it consists of twelve bright 

 lines, in groups of two and four, as shown on the chart 

 immediately above the complex spectrum at the bottom. 

 Midway between the groups very faint lines can be per- 

 ceived if the eye is carefully rested. The strong lines are 

 arranged thus : two, absent line, four, absent line, two, 

 absent line, four. The absent lines, or, more correctly 

 speaking, the faint lines, evidently belong to the same series, 

 and taken collectively the lines will be found to be very nearly 

 equidistant, measured along a normal spectrum. 



The wave-lengths of the lines in this series, as determined 

 from measurements of the plates obtained with the prism- 

 spectrograph, are as follows : — 



A. X differences. A. A differences. 



4760 oa h 38 



4800 it 5095 38 



4838 •••••• 38 5133 



j 35 - I 37 



*2* 38 5207 ,. 



4946 "" r 9 , k 38 



4083 % Hf s 38 



5019 ° 9 532i 38 



