Spectrum of Penetrating y Rays from Radium B and C. 269 



the slit, and the dark lines due to the reflected radiations 

 appeared symmetrically on either side. The experiments 

 were not complicated by the presence of absorption bands, 

 which were cut out by the lead screen. Since the dark bands 

 appeared on the photographic plate at points outside the 

 central beam, they were relatively more intense than in the 



Fig. 2. 



Up 



experiments where no lead screen was used to cut down the 

 main radiation. In this way, we were able to determine the 

 wave-length of the stronger lines due to the very penetrating- 

 rays *. This method has obvious advantages in determining 

 wave-lengths where only a small fraction of the total intensity 

 of the radiation is due to the waves under examination. 



In the following table are given the wave-lengths of the 

 stronger lines determined by the transmission method. 

 Column 1 gives the measurements made by means of ab- 

 sorption bands, and column 2 by means of the reflexion 

 bands. The results obtained by the two methods are seen to 

 be in very fair agreement. In column 3 the lines deter- 

 mined by the first reflexion method are given again. The 

 wave-lengths given in column 4 correspond to the mean 

 value of the reflexion angle. 



It will be seen that there is also a very good agreement 

 between the values obtained by the direct reflexion and by 

 the transmission method, but for the very penetrating rays 

 under examination, the results obtained by the transmission 



* At each edge of the dark band corresponding to the beam directly 

 transmitted through the lead slit appear close lines not due to beams 

 diffracted by the crystal, since they appear equally whether the crystal 

 is there or not. These "false'' lines render this method unsuitable for 

 investigating the lines reflected at the smallest angles. Their cause is 

 uncertain, and is now being investigated by one of us. 



