3 6 J- Prof. Wood and Mr. Moore on the Fluorescence 



tube closed with plate-glass carefully cemented on with 

 sealing-wax. The side tube was then closed in the same 

 manner and the whole exhausted to a pressure of about a 

 millimetre by means of a mercurial pump. The tube was 

 then sealed off from the pump, a small piece of glass tubing 

 having been cemented into the brass tube and drawn down 

 to a caj^illary. 



The tube was now mounted in a horizontal position and a 

 large Bunsen burner placed beneath the crucible, which was 

 soon raised to a red heat. The dense sodium vapour poured 

 out of the mouth of the crucible and gradually condensed on 

 the cooler portions of the tube. Sunlight from a heliostat 

 was sent down the large tube, a lens placed close to the glass 

 window forming an image of the sun immediately above the 

 neck of the crucible. On lookino- down the side tube an 

 intensely brilliant green cone of light was seen many times 

 brighter than anything that has ever been obtained with 

 glass bulbs. The floating specks of oxide which appear when 

 the tube is first heated, and which shine with a dazzling white 

 light, soon disappear and leave the fluorescence entirely un- 

 contaminated. It is apparent that with this arrangement 

 light enters at once into the densest vapour without suffering- 

 previous loss by absorption in vapour of less density. More- 

 over, the fluorescent light passes down the observation-tube 

 without having to traverse more than a very thin layer of 

 the vapour, a matter of considerable importance^ as w^e wish 

 to examine the fluorescent light unmodified by subsequent 

 absorption. The large amount of sodium which can be 

 stored in the crucible enables us to deliver a dense stream of 

 vapour in front of the observation-tube for a very long time, 

 which is absolutely essential if photographic records are to 

 be obtained. 



Spectrum of tlie Fhiorescent Light. 



The spectrum of the fluorescent light was first examined 

 with a two-prism Steinheil spectroscope. The spectrum con- 

 sisted of a red band and a green band, the latter appearing 

 distinctly fluted. No trace of any bright line or band at or 

 near the position of the D lines has ever been seen in any of 

 our experiments. Its presence in the spectrum described by 

 Wiedemann and Schmidt^ and also by one of us, may possibly 

 have been due to the fact that in both of these cases the 

 vapour was contained in a glass bulb heated by a Bunsen 

 burner. This flame coloured b}' the sodium of the glass may 

 have been responsible for the appearance of a bright line in 

 the place mentioned, a matter which can be very easily 

 settled by repeating the experiments with the bulbs. 



