52 
MR. GEORGE W. WALKER ON THE PROBLEM OF 
the investigation might proceed by aid of the tables and diagrams in ‘ Phil. Trans. 
Roy. Soc.,’ vol. 218, A, p. 373, etc. We shall only point out that, on account of the 
large angle of impingence in some regions, some of the P effects would only be shown 
on vertical component seismographs. 
The preceding discussion on elementary lines of the effect of finite depth of focus has 
led to inferences which differ in a very marked degree from those we are accustomed to 
draw from actual seismograms. 
Naturally our results are qualitative in the first instance, and we must be prepared 
to find quantitative alterations when data for the earth are available. For example, 
we may be prepared to find that the epicentral distances at which PR, SR or PS start are 
less than the values we have calculated. In the case of SR or PS this may be so (we 
have not yet the required data to decide the matter), but in the case of PR, Galitzin’s 
data, which we have taken as our basis, settle this at once. Anticipating the proof 
which will be given later, we find that PR starts at 11,000 km., which is only slightly 
less than the 103 degrees calculated. This, however, implies that the effects hitherto 
interpreted as PR a , PR 2 , etc., cannot be so described, but in place we may be able to 
interpret them as Sl\, SP 2 , etc. Their capricious occurrence in practice favours this 
suggestion, and direct test can be made by means of the vertical component 
seismograph. 
There are other possibilities as well as serious difficulties when a deep focus is con¬ 
sidered, but we cannot discuss the problem with advantage until we have the requisite 
data. Hence we now proceed to show what may be deduced from Galitzln’s data 
themselves and what additional data are wanted before proper tests can be applied. 
Fig. 6 shows in graphical form the Pulkovo observed angle of emergence e for various 
epicentral distances A. No data are given for A < 2500 km., and the dotted part 
from A = 0 to A = 2500 km. is hypothetical. It is certain that e must be 90 degrees 
at A — 0. 
