On the Ionizing Potential of Sodium Vapour. Ill 



A simple relation exists in the general case between the 

 distances OH4 and OH 2 . Let 



u=- v = — f—i^-— - ii, 



fll flj J fl 2 Pi 



/*! and fj 2 being the refractive indices of the first and last 

 media. 



Then 



0H 1= ~-{^-^)f 



u 



or OHj _ a 



— (^3— mi). 



Also, OH 2 a , N 



therefore OH s OH, , /l l\/»i— /* 2 



OH 2 OH! 



or 





V 



W 



=^ 



~ ^2- 





If 



f*i : 



= /*25 















OH! 



= 3/' 



and 



OH 2 = 



5£ 



XXI. On £/i£ Ionizing Potential of Sodium Vapour. 

 By R. W. Wood and S. Okano*. 



THE experiments of Frank and Hertz, M c Lennan and 

 others have shown that, when the vapours of mercury, 

 cadmium, zinc, and magnesium in vacuo are bombarded by 

 electrons from a hot cathode, a single line spectrum is 

 emitted, provided the kinetic energy of the electrons does 

 not exceed a certain critical value. 



In the case of mercury, Frank and Hertz showed that the 

 single line \= 2536*7 appeared when a potential difference 

 of 4*9 volts was applied between the hot cathode and the 

 anode wire, this being the voltage required by the quantum 

 relation for the frequency of the line 2536*7. 



It is of course very important to ascertain whether the 

 quantum relation holds in the case of all metallic vapours, 

 as it has been found to do in the case of the four above 

 enumerated, and we have accordingly carried out an expensive 

 series of experiments with the vapour of sodium, which does 

 * Communicated by the Authors. 



