THEORY OF SEISMIC PROSPECTING 43 
For other directions than the one considered above, the directrices 
all pass through the point S; and they all make an angle 90+ <a with 
S’S. Hence the distance SS’’ remains fixed. Hence the parabolas are 
all of the same size. Furthermore, all of the parabolas pass through 
points of critical reflection like Ri. In fact the surface of revolution, 
formed by rotating the parabola, not about its own axis (unless? =o), 
but about SS’, is a surface of critical points. Where this surface inter- 
sects the surface of the earth is the curve of critical points that we 
wished to construct. 
It might be observed that this surface of revolution has an inner 
part (corresponding to Q’) and an outer part (corresponding to Q) and 
that for us the only significant part is the outer part. 
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