516 Prof. Wood on the Fluorescence of Sodium Vapour 



metal being poured in through one of the apertures. It was 

 then introduced into the tube and pushed down to the centre, 

 after which the plate-glass ends were cemented on as shown 

 in the figure. This arrangement prevented the rapid diffu- 

 sion of the vapour, and enabled a large supply of metal to 

 be kept at the centre of the tube. The tubes used in the 

 earlier work required re-charging after two hours' continuous 

 operation, while the retort-tube could be operated for several 

 hundred hours on a single charge. 



The illuminating beam was focussed just inside one of the 

 oval apertures in the retort, being received by the opposite 

 wall just to one side of the other aperture. The end of the 

 large tube was covered with a black cloth, by which device 

 it was possible to view the fluorescent spot against the dead- 

 black background of the second aperture. 



A large three-prism spectrograph was constructed for 

 photographing the spectra. The prisms were of clear dense 

 flint 4 inches in height, and the focal length of the lenses 

 36 inches. 



The monochromatic illuminator, which furnished the ap- 

 proximately homogeneous light by which the fluorescence 

 was provoked, was arranged as shown in fig. 2. A small 

 strip of silvered glass 2 mm. wide was mounted in front of 

 the second slit of the instrument, by which the emergent 

 beam was thrown into the sodium tube. By means of a 

 split lens the light was focussed upon the aperture, and an 

 image of the fluorescent spot thrown upon the slit of the 

 spectrograph. It was found necessary to split the lens and 

 separate the two halves a trifle, as otherwise the image of 

 the fluorescent spot was thrown back upon the small mirror. 

 The arrangement figured was found to be the only one 

 capable of yielding an image of the fluorescent spot upon an 

 absolutely black background. It was extremely important 

 to accomplish this, as otherwise it would have been im- 

 possible to ascertain whether or not the fluorescent light 

 contained the same wave-lengths as the exciting light. For 

 the same reason it was important to get rid of all traces of 

 floating particles of oxide. These gave a good deal of trouble 

 in the earlier work, but in the new tube it was found that 

 after a half-hour's operation all trace of them disappeared. 

 To prove the purity of the atmosphere in the tube and the 

 blackness of the background, it was only necessary to adjust 

 the prisms of the monochromatic illuminator for some wave- 

 length incapable of exciting fluorescence. The brightest part 

 of the spectrum in the yellow region, not far below the 

 sodium lines, answered the purpose admirably. 



