1906.] 



on 



Electric Discharge and Spectroscopy. 



201 



will be visible. Let us now consider the lines in the spectrum corre- 

 sponding to the system A ; these will be visible if the energy in the 

 system reaches the critical value ; the conditions in this case are in 

 some respects more unfavourable for the supply of energy to this 



system than they were in the previous one. For in the first case the 

 system B got into the condition in which it radiated as much energy 

 as it received, and thus did not absorb any of the energy ; in the 

 second case, however, B became luminous before its radiation was 

 equal to the absorption ; it is thus taking in more energy than it 

 gives out, and this may result in a diminution in the rate of supply 

 of energy to A ; it would do so, for example, to a marked extent if 

 the conditions were such that A received a considerable portion of its 

 supply of energy from B. This diminution in the supply might be 

 great enough to prevent the internal energy in B reaching the critical 

 value. Thus the result of the increase in the rate of supply of the 

 electrical energy might be to weaken or even obliterate the lines of A, 

 and while with the smaller rate we had the lines of A and not those 

 of B, with the larger rate we might have the lines of B and not those 

 of A. Thus an increase in the rate at which the electric field is doing 

 work, such as would be produced by increasing the current through 

 the discharge tube, might result in an entire change of the spectrum. 

 We should expect that it would only be in exceptional cases that the 

 lines of A would be obliterated under the conditions holding in case A, 

 but in all cases the increase in the brilliancy of the lines of B would 

 be large compared with the increase of those in A. 



We see from the equations giving Ei and E2 that until the supply 

 of energy has lasted for a time comparable with 1/^2, ^2 is large 

 compared with Ej ; thus for electrical discharges which last for an 

 exceedingly short time we might easily have the lines of B visible 

 and not those of A. 



