and the Resonance Radiation of Electrons. 



521 



Stimulation ivith Monochromatic Light. 



Having described briefly the general appearance of the 

 spectrum when the fluorescence is provoked with white light, 

 we will now consider the case of monochromatic stimulation. 

 In all of the earlier work with Mr. Moore, and during the 

 first stages of the present investigation, no trace of any 

 emission-band at the D lines was observed, neither was any 

 fluorescence whatever observed when the prisms of the mono- 

 chromatic illuminator were adjusted so that the instrument 

 delivered light of wave-length approximately that of the D 

 lines. A bright band at the D lines had, however, been 

 observed by Wiedemann and Schmidt, and independently by 

 the author in the case of sodium vapour formed in exhausted 

 glass bulbs, which observations have been confirmed by 

 Pucianti, who has recently published an interesting paper 

 on the subject *. 



It seemed probable that in the case of a large mass of 

 vapour in a steel tube, the radiations capable of exciting the 

 D line fluorescence were absorbed in the outlying mass of 

 comparatively rare vapour before coming to a focus in the 

 dense mass. As it was found difficult to observe the pheno- 

 menon in glass bulbs on account of the formation of a sodium 

 dew on the walls, and the speedy discoloration of the glass, 



Fijr. 3. 



to pump 





the arrangement shown in fig. 3 was adopted. A small piece 



of clean sodium was placed in a large test-tube, hermetically 



* Accad. Lincei, Atti, xiil pp. 430-440 (1904). 



