56 
MR. G. W. WALKER ON THE PROBLEM OF FINITE FOCAL DEPTH, ETC. 
The matter is further complicated by the circumstance that for a certain range of A 
the reflexion of S waves vibrating in the diametral plane is complex. Some attempts to 
estimate e for S waves were given in the paper referred to, p. 388, but they are isolated 
results, and what we require is a systematic investigation for a large range of A. 
In the absence of such data we may tentatively proceed a little way in the problems 
arising from finite depth of focus by adding to the time curve obtained from the Pulkovo 
data the values of S — P given by Zoppritz, which are known to be not seriously in error 
over the middle range. Although I have made some calculations in this direction, one 
cannot proceed very far. and it is an obviously unsatisfactory method. 
Summary. 
Observations of the emergence angle of P waves at Pulkovo suggest that the depth of 
focus is of order one-fifth of the earth’s radius. It is shown that important modifications 
would have to be made in the interpretation of seismograms and in the attempt to deter¬ 
mine how speed of propagation depends on depth. It is further shown that an important 
test of the accuracy of the Pulkovo values can be made by a careful scrutiny of seismo¬ 
grams for distances > 11,000 km. Further progress cannot be made until this research 
has been carried out, and until we have corresponding measures of the angle of emergence 
of S waves by means of three component seismometers. 
jUU-bCNiil 
2 6 SEP. 1921 
