Pressure. 



L 2 . 



L ] 



035 mm. 



7-9 cm. 



2-78 



0056 „ 



5-0 „ 



2-57 



o-io „ 



315,, 



2-58 



of Hydrogen Positive Mays. 563 



In the table are given " the mean free paths " L x and L 2 

 as they are derived from Wiea's experiments : — 



(i) (i) 



1-70 1-89 



2-31 2-33 



381 2-91 



The effective length of the magnetic field is put equal to 

 that of the pole-pieces (4*2 cm.). Now we observe the 

 light at a distance of about 0*5 cm. from the end of the 

 narrow glass tube, and, further, some of the rays that 

 acquire a positive charge during the passage of the last 

 centimetre of the glass tube may come into the field of view. 

 This will be equivalent to a diminution of the distance I. 



On the other hand, the spreading of the magnetic lines 

 will produce some increase of the effective length I. To 

 diminish these errors, the distance I ought to be somewhat 

 longer than in my experiments. 



Comparing the calculated and observed values, we see 

 that they show a fairly good agreement for the smaller 

 pressures. 



In the case of the highest pressure the value taken for 

 " the mean free path " is uncertain, and when L 2 becomes 

 small errors in the distance I will have a comparatively 

 great effect. 



At any rate, we can conclude from these numbers that the 

 greater part of the moved intensity is emitted from the rays in 

 the neutral state. 



Some moved intensity may also be emitted when the 

 positive rays get neutralized ; but this quantity should be 

 small as compared with that produced through the bombard- 

 ment of the neutral rays with the gas molecules. 



When we remember that the light-intensity measured by 

 means of the spectrograph is proportional to the intensity 

 per unit length of the ray bundle, the measured intensity 

 should be given by an expression of the form : 



T 7 / U l 7l 2\ 



Is= *V*Li + * f xJ 



e~» l . 



\ is the mean distance which a neutral ray has to pass between 

 successive collisions which excite the ray to light emission. 



- 2 is then the number of such collisions which the neutral 



A, 



rays suffer in unit time per unit length. k. 2 measures the 

 probability that such a collision shall result in the emission 

 of the particular spectral line considered. 



