5.18 Prof. Wood on the Fluorescence of Sodium Vapour 



bands, which lie close together in the vicinity of the D 

 lines, widening, however, as the blue region is approached. 

 Coincident with the D lines there appears a hazy band, the 

 surrounding region being nearly devoid of light, which, if 

 the vapour is not too dense, can be resolved into a double 

 line, the components of which coincide with D ± and D 2 . 

 These lines only appear when the vapour is stimulated with 

 light of the wave-length of the sodium lines, i. e. the fluor- 

 escent light can be regarded as an emission of light by the 

 electrons, in virtue of the vibrations excited by the incident 

 light. This was proven in two ways : first, by illuminating 

 the vapour with the light of a very intense sodium flame, 

 which provoked a bright yellow fluorescence ; secondly, by 

 illuminating it with light from a spectroscope, and varying 

 the wave-length continuously. The yellow band only 

 appeared when the spectroscope furnished light of the wave- 

 lengths of the D lines. Further work, it is hoped, will show 

 whether the D 1 vibration is independent of that giving rise 

 to D 2 - To solve this interesting question, it will be necessary 

 to illuminate the vapour with the light of D 2 only, and 

 ascertain whether both lines are present or not in the fluores- 

 cent spectrum, a difficult observation, but one which can 

 doubtless be made. There is in addition a red fluorescence, 

 the spectrum of which Wiedemann and Schmidt believed to 

 be devoid of bands or lines. This, however, is not the case, 

 for the spectrum in the red was found to be crossed by a large 

 number of dark bands. They are much more difficult to 

 observe than the green bands, and are only very conspicuous 

 when the density of the vapour is considerable, which accounts 

 for their having been overlooked previously. The general 

 appearance of the fluorescence spectrum stimulated by white 

 light is shown in PI. VIT. fig. 1. In this photograph the 

 red region and the double band at the I) lines are not shown. 

 In the blue and green-blue reoion the lines and bands of 

 the fluorescent spectrum are irregularly spaced. From 

 A,:=5050 to A,= 5071 we have a regularly spaced system of 

 lines, which get closer and closer together as we approach 

 the less refrangible end of the spectrum. These lines col- 

 lectively form a system of bands, the spectrum having a 

 fluted appearance. The spacing between the bands also 

 becomes less as we approach the more refrangible end. 

 With white-light illumination the bands cannot be distin- 

 guished in the yellow region, but by exciting the fluorescence 

 with blue-violet light, the bands can be traced up to the 

 very end of the spectrum (\ = 571). The bands change their 

 position slightly as the wave-length of the exciting light is 

 altered, but the lines of which they are composed remain 



