Mr Growther, On the Transmission of ^-rays. 451 



then much more rapidly, and finally tailing off into something not 

 very distinguishable from the ordinary exponential law. 



The experiments now to be described have been made with a 

 view to testing the various points briefly touched upon above; and 

 to ascertain as exactly as possible the true shape of the absorption 

 curve for some different substances. 



The apparatus used is sketched in fig. 4. The radium in a 

 glass tube is placed at R in the centre of a large block of lead. 

 A is one of the systems used for obtaining a homogeneous beam 

 of /3-rays, fully described in the previous section of this paper 

 and sketched in fig. 2. D is the ionization chamber into which 

 the /3-rays pass. It is made of a copper hemisphere 12 cms. in 

 diameter, with an inner electrode e of aluminium leaf, and a 

 large opening dd, closed with thin aluminium leaf to admit the 

 rays. The absorbing sheets can be introduced into the small 



Fig. 4. 



space left between the exit tube B and the window d, by means 

 of a metal slide not shown in the figure. G is the shutter com- 

 pensator described in detail in a previous paper, and TTan inclined 

 gold leaf electroscope which differs from the original design in 

 being only 1 cm. in depth. This variation has the advantage that 

 it allows of the use of fairly high power objective and thus of a 

 considerable degree of magnification in the reading microscope. 



G and D were charged to opposite potentials and their inner 

 electrodes were connected through the key K to the electroscope 

 W by wires passing through earthed metal tubes. These tubes 

 were filled with sulphur and the key K was made small in order 

 to reduce as much as possible the leakage from the electrodes 

 through aiv ionized by 7-rays from the radium. 



