EFFECT OF PRESSURE UPON ARC SPECTRA. 



217 



may differ by 50 per cent, in judging which is the most intense portion of a line and 

 that the discrepancy may possibly be explained in this way. The probability that 

 this is the explanation is not great, because measurements of the writer's photographs 

 made by different observers do not differ materially from one another. 



The following table gives all previous measurements of the copper lines under 

 discussion, together with a comparison with the writer's values. It will further be 

 seen that at 7 atmospheres on HUMPHREYS' plates the rate of displacement is twice as 

 great as it is at 69 atmospheres, the only exception being that of line q (A. = 4587'19), 

 but the values at low pressures are not always very reliable, and the writer does not 

 wish to lay too much stress upon this line of argument. 



TABLE V. 



The displacements are in thousandths of an Angstrom unit. 



Upon what factor the value of the displacement is most directly dependent it is 

 difficult "to determine. In the writer's own experiments with the iron arc the chief 

 variables were the current strength and the length of the arc which, on account of 

 the necessity for continually striking the arc, could riot be kept constant. The 

 variables dependent upon these are the temperature, potential gradient, the quantity 

 and, perhaps, the density of the material in the arc. The nature of the poles used by 

 HUMPHREYS in obtaining his copper arc at 69 atmospheres is not specifically stated nor 

 is the current strength,! but the voltage of his supply was 220 whereas that used for 



* HUMPHREYS, ' Astrophysical Journal,' VI., p. 169 (1897). 

 t HUMPHREYS, 'Astrophysical Journal,' XXVI., p. 18 (1907). 



[I No differences in the displacements at 70 atmospheres were found when the voltage of the supply 

 was 100, 200, or 400 volts. Added October 23, 1908.] 

 VOL. CCIX. A. 2 F 



