of Cathode Rays by Metallic Sheets. . 281 
The curves given below are typical of two classes of absorption 
which have been obtained. In these curves 1/v‘ is plotted along 
the y-axis while the corresponding values of log J,/Z are plotted 
1/v4 
Log I,/I 
along the a-axis. The simpler type is the lower one obtained 
with Ag and Au sheets. It is clear that »X « 1/v* for velocities 
greater than that corresponding to the point A. The value of v 
for the point A is given quite as accurately as could be expected 
by the expression quoted above—d = v‘/a, and thus depends on 
the thickness of the sheet. 
The other type has been obtained with Cu and AI sheets 
and shows in addition to the bend at A already explained another 
bend at Bb. The velocity corresponding to this point is in the 
ease of Cu 6.2 x 10° cm./sec., which is the velocity above which 
the fluorescent X-radiation of Cu is excited. This is the effect 
which was being looked for, but it is not possible just at present 
to ascribe it definitely to the excitation of the fluorescent radia- 
tion since Al also shows a similar effect though not at the same 
velocity. 
The experiments are being continued. 
