﻿504 Prof. P. W. Wood on Fluorescence and 



energy in the fluorescence spectrum o£ sodium vapour wLen 

 the wave-length of the exciting light is changed. With white- 

 light stimulation the general appearance of the spectrum is 

 shown in PL XIV. fig. 1, A. There is, in addition, a broad double 

 band at the position of the D lines, and a red-orange spectrum 

 which, when the vapour is dense, is distinctly banded. In 

 the present paper we shall be concerned chiefly w s ith the 

 portion figured, for it is in this region that most of the re- 

 markable changes occur. As will be seen, it is comprised 

 between wave-lengths 460 and 570, and is devoid of any 

 apparent regularity in the distribution of its lines, except in 

 the region above A = 505 where we have lines spaced with 

 considerable regularity, the spacing becoming less as the 

 wave-length increases. The distribution of intensity in this 

 portion of the spectrum is such as to give it a fluted appear- 

 ance, the flutings being most conspicuous in the region 

 between A, =s505 and A = 535. With white-light stimulation 

 the flutings cannot be made out above 540, as can be seen from 

 PL XIV. fig. 1, B, in which the upper limit of this part of the 

 spectrum is shown. The fine lines are present in this region, 

 becoming, however, less and less distinct as the upper limit 

 of the spectrum is approached. If, now, instead of stimulating 

 the vapour with white light, we employ blue light in the 

 region 460-465 obtained from a spectroscope for the excitation 

 the fluorescent spectrum presents a totally different appearance 

 (fig. 1, C). The blue region, corresponding in its range to 

 that of the exciting light (indicated by a double arrow), 

 appears as before, and the upper limit of the spectrum 

 between wave-lengths 540 and 565, the intermediate portion 

 being entirely absent, as shown in the lower spectrum of 

 fig. 1, C. Furthermore, at the upper or yellow end, there 

 now appear the flutings which were absent when the fluor- 

 escence was stimulated with white light. 



If, now, we gradually^ increase the mean wave-length of 

 the exciting light, the region of maximum intensity in the 

 fluorescence spectrum moves down from the yellow^ into the 

 green, as is shown by the remaining photographs in fig. 1 ? C 

 (PL XIV.) . Moreover, as I pointed out in the previous paper, 

 the positions of the fluted bands change slightly, the positions 

 of the individual lines which make up the bands remaining fixed 

 however, the shift resulting from a change in the distribution 

 of intensity among the lines. The reason of this curious 

 phenomenon will appear when we come to the study of 

 the fluorescence spectrum excited by strictly monochromatic 

 radiations. 



