Investigation of the Nature of the y Rays. 675 
Fig. 6. — Tho upper figure shows the general arrangement. 
The wall o£ the cylindrical vessel was of brass : a Pb or an Al 
lining could be inserted as shown. The lower figures are 
diagrammatic, and show the currents for different arrange- 
ments of Pb and Al at top, bottom, and sides. Inversion of 
the plates through which the y rays pass into the chamber 
makes little difference ; but there is a great change if the 
material is changed on which the 7 rays fall, or the " emer- 
gence " radiations from the top plate. The base is of less 
importance than in fig. 5 ; but the sides of more importance. 
This should clearly be so, for geometrical reasons. When 
the conical opening was filled by a Pb stopper the currents 
were all reduced considerably, but retained the same propor- 
tions pretty nearly. 
When a small pencil of /3 rays was admitted through a 
hole in the centre of the top plate a change of the material 
of the bottom became more effective, and of the sides less 
effective than before ; but this difference became smaller 
when thin Al sheets were so placed as to scatter the {3 rays 
on their entry into the chamber. 
In conclusion we should like to add that Wigger was the 
first, so far as we know, to show clearly that the secondary 
radiation of Al, on the far side of the plate, was greater than 
that of Pb. A comparison of the emergence radiations of 
different metals was made by Dawes (Phys. Rev. xx. p. 182), 
who showed that they did not follow the law of the incidence 
radiations. The same effect was indicated in the experiments 
of Eve (Phil. Mag. Dec. 1904). We have little doubt that 
the interesting experiments of Mackenzie (Phil. Mag. July 
1907) are to be explained on the lines indicated in this 
paper. In fact it is clear that this is the case in a broad 
sense ; but it is difficult to give a complete explanation until 
the laws are so completely worked out that they can be 
applied to the interpretation of experiments which are really 
very complicated, although at first sight they may seem to 
be simple. 
