WOOD. — SPECTRA OF SODIUM VAPOR. 255 



the sodium flame. The sodium tube was provided with a quartz 

 window, and the light of the oxyhydrogen flame, heavily charged with 

 sodium, focussed upon the aperture of the retort with a glass lens. 

 White light from the arc was also used, as this excites the D line vibra- 

 tions in the fluorescence. The spectrum was photographed with a 

 small quartz spectrograph, and though the D lines were greatly over- 

 exposed, no traces ofany of the ultra-violet doublets were found on the 

 plate. Conversely, illumination with ultra-violet light in the region of 

 the first ultra-violet pair of lines failed to produce any visible fluores- 

 cence. It was hoped that a faint yellow fluorescence might be produced 

 in this way, due to emission in the region of the D lines. I have not 

 yet tried stimulation with Di and D2, alone, to see whether both D lines 

 appear in the fluorescence. This will be a very difficult experiment, 

 and I am saving it for the last. It will settle the question as to 

 whether the principal series of sodium is a series of doublets or two 

 series of single lines. 



Cathode-ray Excitation. 



The cathode rays, I find, excite a fluorescence similar to white light. 

 The lines of the principal and subordinate series appear as well, some 

 of them of overpowering intensity. The apparatus for the electrical 

 excitation is shown on Plate 3, Figure 3. 



It consisted of a steel tube 3 cms. in diameter and 35 cms. in length, 

 one end closed with a glass plate, the other cemented with sealing-wax 

 to a glass tube carrying the cathode. The mercury pump was kept 

 in continuous operation to remove the hydrogen liberated from the 

 sodium. On looking into the tube through the glass window a blazing 

 spot of yellow light 2 cms. in diameter was seen at the point where the 

 cathode rays entered the vapor. Its spectrum was photographed with 

 the prism spectrograph, and is reproduced on Plate 2, n. In addition 

 to the fluorescent spectrum, and the sodium lines of all three series, 

 the hydrogen lines come out strong. I have never been able to elimi- 

 nate them entirely. Very few experiments have been made on the 

 electrical excitation, but some very curious phenomena have been 

 observed. In some cases, by looking into the tube in an oblique direc- 

 tion, it was seen that at the point where the cathode rays entered the 

 mass of vapor there was a bright green spot of fluorescent, light, while 

 at the point of exit there was an orange-yellow spot, the intervening 

 space being non-luminous. Seen in a direction oblique to the direction 

 of the rays, the two spots were seen completely separated. This I con- 

 sider a very remarkable circumstance, and a spectroscopic study 

 of the two spots of light will undoubtedly yield very fruitful results. 



