6Q8 Prof. Bragg and Dr. Madsen : An Experimental 
negatives are set free in B'C X because the two strata are of 
equal weight. Of those set free in BG only a certain 
number emerge from the face D because of the absorption 
of the plate CD. Since CD and CD' are of equal weight a 
similar absorption occurs in the case of the particles set free 
in B'C. Thus the same number emerges from each plate. 
Integrating for all effective strata, the whole cathode radia- 
tions emerging from the two plates are equal. 
We thus find that if the absorptions of /3 and 7 rays both 
followed the density law, the secondary cathode radiation on 
the far side of a plate — we may call it the " emergence " 
radiation — would be the same for all materials. There should 
be no such relation between the amount of the radiation and 
the atomic weight of the plate as various observers have 
shown to be true for the secondary cathode radiation of 
""incidence," a relation which is closely parallel to that 
found in the case of /3 rays. 
Experiment is in agreement with this theory, for it shows 
that no such relation exists in respect to the emergence 
radiations : in marked contrast to what happens in the case 
of the radiation from the front sides of the plates of various 
materials — the incidence radiations. 
It is true that the emergence radiations are not all equal, 
but this is to be expected, because (1) the amount of secondary 
cathode radiation depends, as Kleeman has shown, on the 
previous screening of the 7 rays, (2) the /3 rays are not 
absorbed strictly according to a density law, (3) the 7 rays 
also depart from this law. We have made no serious attempt 
as yet to disentangle the effects of these various disturbing 
factors. In fact the task promises to be long and intricate, 
for it will be necessary to find out how much of the ioniza- 
tion in the chamber is due to each class of rays : to discover 
the law of distribution of the radiations in space so that the 
form of the chamber may be allowed for, if necessary : to 
find out the nature of the departures from the density law of 
those /3 and 7 rays which are in question, and so on. Never- 
theless the results are satisfactory, so far as we have gone. 
The amount of emergence radiation is found to depend on 
the previous screening of the rays. In one case the in- 
version of a C, Pb pair of plates from *- C, Pb to 
» *- Pb, C altered the current in the ratio 1 : 1*11 when 
the rays had been previously screened by Pb ; but in the 
ratio 1 : "96 when the screen was changed to C. Again, 
when the rays had previously passed through an iron screen, 
the inversion . 3==- Pb Fe to s*> Fe Pb changed the 
current in the proportion 1 : 1*12, but when a lead screen was 
