524 Prof. Wood on the Fluorescence of Sodium Vapour 



fluorescence. No one of the periods happened to be quite right : 

 and the electrons remained quiet. The light of the cadmium 

 spark, though much less intense, excited the fluorescence ; 

 but it has not yet been ascertained to what degree the " con- 

 tinuous background " of its spectrum was responsible for the 

 phenomenon. Work along these lines will be continued 

 during the coming year. 



The shifting of the bands, which results in all probability 

 from a change in the distribution of the intensity among the 

 individual lines, when the wave-length of the exciting light 

 is altered, has been already alluded to. In the earlier stages 

 of the investigation, it was necessary to work with the slits of 

 the monochromatic illuminator wide open. Later on, as the 

 methods improved, it was found possible to narrow them by 

 degrees, until finally a fluorescent spectrum of sufficient in- 

 tensity for observation was obtained with the slits not much 

 wider than a hair. A remarkable phenomenon was at once 

 observed : the slightest change in the wave-length of the 

 exciting light caused the spectrum-lines to flutter about in a 

 most extraordinary manner. It was quite impossible to follow 

 the changes by the eye, they were so complex. As a line 

 disappeared or appeared, the sharp and shaded edges often 

 changed place, the appearance reminding one of a flag flying 

 first on one side of the mast and then on the other. Obviously 

 a vast amount of information regarding the nature of spectra 

 is to be obtained by a careful study of these changes, which 

 can only be done by photography. I have estimated that 

 fifty or one hundred photographs will be necessary for a 

 complete record of the changes which occur when the wave- 

 length of the exciting light is shifted over the range 450 to 550, 

 and as each picture requires an exposure of four or five hours, 

 the task is seen to be no small one. If the spectra were then 

 printed in succession on the film of a kinematograph and 

 projected on a screen, it would be possible to follow at leisure 

 the wonderful changes which occur. 



At the time of writing this paper only half a dozen photo- 

 graphs have been obtained, three of which are reproduced in 

 fig. 6, PI. VII. Much of the detail is lost in the process of 

 printing and reproduction, and on this account a careful 

 drawing has been made on coordinate paper of all the details 

 which are to be made out on the original negatives, fig. 7. 

 PL VII. The white-light fluorescent spectrum A is recorded 

 on the lower line, and the spectra obtained with nearly homo- 

 geneous radiation above, as is the case in the photograph as 

 well. A number of the lines in the complete spectrum shown 

 in fig. 6 have been lettered A, B, C, D, etc. These letters 

 correspond with the letters in the diagram, fig. 7. The 



