522 Prof. Wood on the Fluorescence of Sodium Vapour 



sealed with a glass plate, and furnished with a tube for ex- 

 haustion. Since a much more powerful D line radiation can 

 be obtained from a bright sodium flame than from a selected 

 portion of the spectrum, the monochromatic illuminator 

 was abandoned, and a small oxy-hydrogen flame, heavily 

 charged with sodium, substituted for it. An image of this 

 flame was formed by means of a lens, at the centre of the 

 tube immediately above the lump of metal. On gently 

 heating the tube and viewing the interior through the cover- 

 glass, a faint cone of yellow light was observed which, 

 however, only extended to the centre of the tube, owing to 

 the inability of the effective radiations to penetrate to a 

 greater depth. This stage is shown in the upper diagram of 

 fig. 3. As the temperature rose and the density of the 

 vapour increased the fluorescence increased in brilliancy, 

 the region retreating towards the wall, until only a bright 

 skin of yellow light remained, which lined the Avail of the 

 tube at the point where the exciting radiations entered, as 

 shown in the lower diagram. This experiment proved con- 

 clusively that sodium vapour is capable of re-emitting a 

 } T ellow light, when in the act of absorbing the light of the 

 soda flame. The appearance of the yellow cone and its 

 behaviour as the vapour-density increased proved that it was 

 a true re- emission of absorbed light, and not a scattering of 

 light by floating particles of. oxide. 



An attempt was next made to observe the same pheno- 

 menon in the steel retort-tube. On focussing an image of 

 the flame upon the window of the retort a distinct spot of 

 yellow light was observed, though it proved too faint to admit 

 of satisfactory observation in the large spectrograph. Re- 

 turning once more to the monochromatic illuminator, I was 

 surprised to find no difficulty at all in obtaining a bright 

 emission-band at the D lines, which proved to be double 

 when the vapour-density was not too great. The band was 

 of considerable brilliancy when direct sunlight was used. 

 The failure to observe this band in the previous experiments 

 may have been due to an insufficiently high vacuum, or to 

 insufficient density of the sodium vapour. It has been found 

 that long-continued work results in a steady improvement of 

 the spectrum as to brilliancy, though it is quite impossible 

 to ascertain the precise reason. It is probable that the 

 proper conditions are unconsciously found, and the detri- 

 mental ones discard ed, by what Prof. James would call • 

 the " Sub-conscious-self. " The band at the 1) lines only 

 appeared, when the light from a corresponding part of the 

 spectrum fell upon the vapour, showing that the electron 

 system which produces these lines is independent of the 



