Mr Kaye , The Selective Absorption of Rontgen Rays. 237 
Arrangement of Apparatus. 
The general arrangement is shown in figure 1, which is drawn 
to scale. The discharge tube D, containing the cathode and, in 
a side tube, the anode, was constricted over its lower half, where it 
met the anticathode tube A. The central beam of cathode rays met 
the anticathode at 45° and a pencil of the Rontgen rays produced, 
passed along the tube B, which was mounted at right angles to 
the discharge tube. These relative directions were chosen because 
they are the usual ones in ordinary X-ray practice. 
Both the electrodes were of aluminium. 
The metals used as anticathodes were mounted on a car which 
ran along horizontal rails fastened by sealing wax to the bottom of 
the tube A. For rails one half of a piece of aluminium tube, split 
from end to end, was used. The car was also made of aluminium 
and was arranged to carry some eight metal discs in line. These 
were of diameter 2'5 cms., thickness 2 mms., and were kept in 
position by small central pins, which passed through corresponding- 
holes in the body of the car. The latter for convenience of space 
was slung from the axles of the wheels. Underneath each axle 
