﻿520 Prof. R. W. Wood on Fluorescence and 



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 fiud, 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 XIII. 

 fig. 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 vapour. Its spectrum was photo- 

 graphed with the prism spectrograph, and is reproduced on 

 Plate XIJ ., 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 eliminate them entirely. 

 Very few experiments have been made on the electrical exci- 

 tation, but some very curious phenomena have been observed. 

 In some cases, by looking into the tube in an oblique direction^, 

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

 the mass of vapour there was a bright green spot of fluor- 

 escent 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 consider a very 

 remarkable circumstance, and a spectroscopic study of the 

 two spots of light will undoubtedly yield very fruitful results. 

 Unfortunately the condition is a difficult one to keep fixed, 

 for the phenomenon only appears when the density of the 

 sodium vapour is just right and the surrounding vacuum high. 

 As I have shown in tbe paper on the dispersion of sodium 

 vapour, we can have a dense mass of the metal vapour,, 

 bounded on each side by a very high vacuum, a very ano- 

 malous condition from the point of view of the kinetic theory 

 of gases. My impression is that the green spot will show the 

 fluorescent spectrum, and the yellow spot the lines of the 

 principal and subordinate series, as found in the sodium arc,, 

 but as yet I have not found time to make even a visual 

 examination. Several attempts have been made, but by the 



