216 1>R. W. GEOFFREY DtfFFIELD ON THE 



into shorter wave-lengths diminish this value. The mean displacement per atmosphere 

 of the lead line is 6'7 thousandths of an Angstrom unit (a little more than half that 

 of the copper lines) ; its values at different pressures are plotted on the same diagram. 



(7) Displacement and Reversal of Lines. 



The displacement curves for iron suggested a departure from the linear relationship 



at those pressures at which a large number of reversals appeared. In the case of 

 copper no reversals have been found between X 4000 and X 4600, and the displacement 

 varies directly with the pressure within the accuracy of the experiment. 



(8) The Two- fold Value of the Displacement? 



Comparison of the values of the displacements given in Table IT. with those of 

 previous workers affords additional evidence in support of the reality of the 

 phenomenon to which the writer first drew attention in his discussion of the iron arc 

 under pressure,* where it was shown that at certain pressures two values sometimes 

 appeared to exist for the displacement of a line. In the case of iron all the lines 

 belonging to the same group were upon one plate sometimes displaced twice as much 

 as they were upon others at the same pressure. Sometimes all three groups showed 

 abnormally high values, sometimes Groups II. and III., and at others Group III. 

 alone showed them. Whatever the nature of the disturbing cause, Group III. and 

 then Group II. seemed most susceptible to it. The ratios of the values of the 

 displacements at 25 atmospheres for different plates were : 



Group III. . . . Reversed lines 2 - 0, 



Group III. . . . Non-reversed lines . . . 1'8, 



Group II. . . . Non-reversed lines . . . 2'0, 



Group I Reversed lines 1'8. 



In the iron spectrum the phenomenon made its appearance in the neighbourhood of 

 25 atmospheres, where, because the displacements are not very large, and because of 

 the unexpected nature of the occurrence, some doubt was felt about it. But the 

 remarkable fact that the displacements of the copper lines measured by HUMPHREYS 

 at 69 atmospheres are half the values of the displacements at 70 atmospheres in the 

 present investigation lends some support to the writer's original view that at certain 

 pressures some lines may have two values of the displacement : within the limits of 

 error of measurement one is twice that of the other. It is true that HUMPHREYS 

 has only measured each line twice and that only six lines have been measured in 

 common, but the magnitude of the discrepancy should be quite beyond the limits of 

 error of measurement. It must also be remarked that HUMPHREYS and the writer 



* DUFFIELD, 'Phil. Trans.,' A, 208, p. Ill (1908). 



