514 Prof. Wood on the Fluorescence of Sodium Vapour 



style resonance radiation, to distinguish it from fluorescence. 

 The intimate connexion between fluorescence and phospho- 

 rescence, and the almost indisputable evidence that the latter 

 phenomenon is associated with chemical changes produced in 

 the substance, makes it appear probable that fluorescence 

 and resonance radiation are two entirety different phenomena, 

 though the former is doubtless caused in some roundabout 

 way by resonance. 



In the present paper, however, I shall, for convenience, 

 use the term fluorescence, though we may eventually have 

 to distinguish between fluorescence spectra and resonance 

 spectra. 



The very remarkable fact has been ascertained that the 

 stimulation of the vapour with approximately monochromatic 

 light, furnished by the monochromatic illuminator (spectro- 

 scope), results in the emission of light, the spectrum of which 

 consists of a number of more or less regularly spaced bright 

 lines of different intensities. The slightest change in the 

 wave-length of the exciting light results in the disappearance 

 of these lines, and the appearance of another set; the pheno- 

 menon indicating that the molecule contains a number of 

 groups of electrons, the excitation of any one of which sets 

 the entire group in vibration, without however disturbing 

 the other groups. As the wave-length of the exciting light 

 is slowly changed, the fluorescent spectrum presents a most 

 remarkable appearance. The lines appear in rapid motion, 

 the luminous bands moving in a rippling manner, like moon- 

 light on water. The motion is of course an illusion, due to 

 the continual disappearance and reappearance of the bright 

 lines, the phenomenon reminding one forcibly of the scintil- 

 lations produced on a screen of zinc sulphide by the electron 

 bombardment produced by radium. 



In the earlier work on the fluorescence of the vapour,, 

 carried on by the author in collaboration with Dr. Moore, 

 the metal was vaporized in a tube of seamless steel. Owing 

 to the distillation of the metal to the cooler parts of the tube, 

 it was necessary to open the tube and introduce a fresh 

 charge after two hours' heating. This was a great gain on 

 the exhausted glass bulbs used by Wiedemann and Schmidt,, 

 which blackened, and became useless in half a minute or less, 

 but something better was obviously necessary for the large 

 number of long exposures necessary for a careful study of 

 the remarkable changes in the fluorescence spectrum, which 

 accompanied changes in the wave-length of the exciting- 

 light. For purposes of demonstration a plain tube of steel, 

 or even brass, is all that is required. Steel is preferable, for 



