140 
MR. W. GEOFFREY DUFFIELD ON THE 
(3) The displacement of the lines increases as the pressure is increased. 
(4) The relation between the pressure and displacement is in general, but not quite 
rigorously, a continuous and linear one. (See pp. 145, 146.) 
(5) The rates of increase of the displacement with the pressure vary greatly for 
different lines. 
Diagram I. 
No. 5 suggests a means for the classification of the lines : for example, the lines 
fl,f2, h0 always exhibit a much greater shift than do lines 2, 3, a\, whose displace¬ 
ments are themselves greater than those of the lines 5, 6, aO, &c. Three groups can 
in this way be definitely determined by the different values of the shifts of their 
several members. 
In Group I. the lines are least shifted, and the wide range in the values of the 
