660 Mr. R. 1). Kleeman on Different Kinds of y Rays 
resonance effect, we would expect that the absorption of the 
secondary rays would be, on the whole, greater than that of 
the primary, and that the selective absorption of the secondary 
rays would be more marked than that of the primary. It 
becomes of interest, therefore, to investigate what the results 
of these experiments indicate on the aether-pulse theory of the 
7 rays. 
According to this theory, one aether pulse may differ from 
another in three different ways, namely — thickness of pulse ; 
magnitude of the electric force at, say, midway between the 
edges of the pulse ; and the function which expresses the 
relation between the ratio of the force at any point in the 
pulse to that midway between the edges of the pulse and 
the distance of the point from one of the edges of the pulse. 
This function will be called throughout the paper the force- 
relation of the pulse. It follows also from the theory that 
the force-relation of a pulse and its breadth remain unchanged 
as the distance of the pulse from the source increases, but the 
force in the pulse is everywhere decreased. The selective 
absorption of some members of a number of different aether 
pulses must now be explained by differences between the 
pulses in the magnitude of one of the properties of an aether 
pulse. It was observed in these experiments that the selective 
absorption of the y rays is independent of the distance of the 
rays from their source. It follows, therefore, that selective 
absorption cannot be due to the different groups of rays 
differing from one another in the magnitude of the force in 
the pulse, for, if selective absorption were due to this, a group 
of 7 rays at a given distance would possess the same properties 
as some other group at some other distance. It is also im- 
probable that it is due to the groups of rays differing from 
one another in the breadth of the aether pulse. For there would 
be nothing in this case to distinguish a broad pulse from a 
narrow one, as far as the absorbing electrons are concerned, 
till both had passed completely over the electrons. One 
would expect then that an electron which absorbs a pulse of 
certain breadth well, would absorb to a greater or less extent 
other pulses also. It is difficult to see, therefore, if selective 
absorption is entirely due to differences in breadth of pulse, 
why a narrow pulse is better absorbed than a broad one by 
one substance, while the opposite is the case with some other 
substance. We conclude, therefore, that selective absorption 
is probably due to the existence of groups of rays which 
differ from one another in the force-relations of their pulses. 
Since the secondary rays possess the same properties of 
selective absorption as the primary, the form of the force- 
