664 Prof. Bragg and Dr. Madsen : An Experimental 
Electricity through Gases,' p. 406). Suppose, therefore, a 
pencil o£ 7 rays to pass normally through a plate so thin 
that its absorption may be neglected, the secondary radiation 
should be exactly the same on the two sides o£ the plate in 
amount, in quality, and in distribution ; and it ought not to 
be possible to discover, by any comparison o£ the secondary 
radiations on the two sides, which is the face of entry and 
which of emergence. 
Consider now the ionization-chamber represented in fig. 1. 
The two ends are closed by plates, of which A and A' are 
alike ; so also are B and B'. The material of A and A! is 
different to that of B and B'. The nature of the side walls 
is of no consequence. A pencil of 7 rays passes along the 
axis of the chamber, which is represented by a dotted line. 
The ionization current within the chamber is measured as 
usual by inserting a high-potential electrode connected to an 
electroscope. 
When the plates A and B are inverted there is a change 
in the amount of the current : so also when A! and B x are 
inverted. By an extension of the principle already stated 
it ought not to be possible, on the sether-pulse theory, to 
discover which way the rays are going (up or down in the 
figure) by comparing the consequence of inverting A and B 
with that of inverting A' and B'. 
As a matter of fact the direction can be discovered with 
ease ; the more easily the greater the difference between the 
atomic weights of A and B. 
For example, in one experiment of ours the chamber was 
of cylindrical form, 7*5 cms. high and 25 cms. diameter. 
