670 Prof. Bragg and Dr. Madsen : An Experimental 
the secondary radiation due to /3 rays. Properly speaking, 
the secondary radiation produced by 7 rays, or rather from 
7 rays, is proportional to the density of the substance traversed 
(cf. Wigger's table) ; and this is only another form of the 
law of absorption of 7 rays. 
The relative importance of the two parts of the incidence 
radiation just mentioned must depend on the circumstances 
of the experiment*. The researches of Kleeman (Phil. Mag. 
Nov. 1907) show very well how the second part, which is 
influenced by previous screening, modifies the effect of the 
first part, which is not so influenced, but which follows the 
law of /3 rays strictly. 
It is easy to show, by comparing corresponding strata at 
the front and back of one plate, that the incidence radiation 
should be somewhat less than p times the transmitted radia- 
tion, — somewhat less, because the cathode radiation which 
is turned back is scattered and softened in the process. 
To sum up : — 
On the sether-pulse theory we ought to find perfect 
symmetry in the secondary radiations from the two sides ot 
a plate; but experiment shows nothing of the kind. 
On the material, or neutral pair theory, the " incidence " 
radiations should follow the /3-ray law. This is known to 
be the case. The " emergence ,s radiations should not follow 
the /3-ray law ; and experiment shows that they do not. If 
the density law held for both ft and 7 rays, and if the 7 rays 
were homogeneous, the emergence radiations should all be 
equal. As alread} r explained, experiment shows that the 
observed inequalities give promise of ready explanation on 
the ground that no one of these suppositions is quite true. 
It is perhaps better not to extend the preliminary account 
of these experiments by any lengthy discussion of the issues 
arising from them. Many points that invite consideration 
have been discussed already in the papers first referred to. 
Moreover, our own further experiments are incomplete ; and 
their full interpretation is not yet certain. We will therefore 
confine ourselves to one or two questions which seem to be 
of special interest. 
The X rays resemble the 7 rays so closely that it is 
practically inconceivable that the two radiations should be 
essentially different. The secondary cathode radiations which 
are set free when X rays impinge on any material must 
* In a recent letter addressed by one of us to 'Nature,' too much 
stress Tvas laid on the part played by the first part under all 
circumstances. 
