WOOD. — SPECTRA OF SODIUM VAPOR. 241 



moves down from the yellow into the green, as is shown by the remain- 

 ing photographs in Figure 6, C. 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. 



The spectrum stimulated by white light I have named the "complex 

 fluorescent spectrum," for it has been found that it is a superposition of 

 a number of simpler spectra, any one of which can be independently 

 excited by suitably controlling the wave-length of the stimulating light. 

 Indications of something of this sort were found last year, and were 

 described in the preliminary paper. An insufficient number of photo- 

 graphs were obtained, however, at the close of the university year, 

 to make anything like a complete analysis of the complex spectrum 

 possible. 



During the past winter and spring a careful study has been made of 

 the relations existing between the complex fluorescent spectrum, the 

 absorption spectrum, and the bright-line rotation spectrum described 

 in the earlier paper. The fluorescent spectrum has at last been photo- 

 graphed with the twelve-foot concave grating, enabling a study to be 

 made of its more minute structure. 



Some very remarkable eff"ects have been observed with monochro- 

 matic stimulations obtained by the isolation of certain lines from 

 metallic arcs, which yield comparatively simple fluorescent spectra 

 made up of mdely separated sharp lines, placed in many instances 

 at nearly equal intervals along a normal spectrum. A given series 

 of lines can be brought out by stimulating with light of any wave- 

 length corresponding to that of some line in the series, but when the 

 stimulations occur at certain points, some of the lines may be absent, 

 gaps appearing in the series. The most conspicuous example is the 

 case of stimulation with the cadmium line 480, which will be considered 

 in detail presently. It will be remembered that certain lines are absent 

 in each series, in the magnetic spectrum. 



The apparatus employed in the experiments was essentially the 

 same as that used in the earlier work. It consisted of a seamless tube 

 of thin steel three inches in diameter and thirty inches long, with a 

 steel retort at its centre in which a large amount of sodium could be 

 stored. The retort was made by fitting two circular disks of steel to a 

 short piece of tubing, just large enough to slip snugly into the larger 

 tube. The circular ends of the retort were provided with oval aper- 



TOL. XLII. — 16 



