Investigation of the Nature of the y Rays. 665 
The plates used were aluminium and lead ; the thickness of 
each plate was a little less than 2 mm. Inversion of the top 
plates A and B made a difference in favour of Al of less 
Fiff. 2. 
-A 
than 1 per cent. ; i. e., the current was slightly larger when 
the Al was next the chamber. On the other hand, inversion 
of the bottom plates made a difference of 44 per cent, in 
favour of Pb ; i.e., the current was 44 per cent, larger when 
the Pb was on top. The details are shown in the figure. 
Allowance was made for all radiation other than that which 
proceeded down the conical opening in the lead block. 
It may be well to point out that this effect cannot be 
ascribed to any complication due to secondary or tertiary 
rays. No doubt the radiation in the chamber is very com- 
plex ; but the fact is immaterial. Provided that the chamber 
is symmetrical in the first place, then the secondaries must 
be symmetrical also if the sether-pulse theory is correct, and 
therefore the tertiaries and so on. Nor is it necessary to 
consider whether the secondary radiations are /5 rays or 
scattered <y rays. 
Also it must be remembered that the secondary radiations 
which enter the chamber have their origin almost entirely in 
a very few millimetres of material bordering on the chamber. 
Therefore the <y rays are in almost exactly the same condition, 
both as to quality and as to quantity, when they excite 
secondary radiations from the top plate as they enter the 
chamber, and secondary radiations from the bottom plate as 
they leave. 
The details of the experiment may be varied greatly ; but 
