150 
MR. W. GEOFFREY DUFFIELD ON THE 
between the values for the displacement, the photograph giving the higher value 
contains more reversed lines. 
The ratios of the displacements are also given, and the higher value will be seen to 
be in each case (except at 10 atmospheres, where the accuracy is probably not as 
great as in the other cases) roughly twice the lower value. 
Another peculiarity is that at 10 and 15 atmospheres pressure only Group III. is 
affected, that both Group II. and Group III. are affected at 20 atmospheres, and that 
at 25 atmospheres all the groups may show abnormal displacements. Whatever may 
be the nature of the disturbing cause, Group III. appears to be most susceptible to it. 
Note.— In this diagram the doubtful yalue for /I at 40 atmospheres has been omitted, 
