the initial effect was so large compared with that which was 

 to be measured. By placing a thin Al foil at G and then a 

 thick Pb plate, a measure was obtained of the incident and of 

 the maximum return radiation for that substance, from the 

 effects measured separately in the chambers A and B. This 

 enabled the readings for the incident scattered radiation to 

 be reduced to their correct values relatively to those of the 

 emergent rays, using the maximum return radiation from 

 Pb as a standard of reference. 



§ ni. 



Results of Expebiments. 



Fig. iii. shows the results of experiments performed with 

 the apparatus of fig. i., using Al screens. 



Curves D and E give the currents for different thicknesses 

 of screen, with the screens in the positions A and C respect- 

 ively. The abscissas represent grammes per square cm. from 

 which the thickness of screen may be immediately deduced, 

 knowing its density. 



Curve C is obtained by subtracting the values of D and 

 E corresponding to any screen, and is a measure of the total 

 amount of emergent scattered radiation. 



It will be seen from fig. i. that the whole of the scat- 

 tered radiation is not quite included, as the effects are some- 

 what iuterfered with by geometrical conditions. When, for 

 example, the screen was brought nearer the Ra than C a 

 slight rise was observed in the reading. The intensity of 

 the radiation falling on the screen was slightly increased 

 owing to some of the more oblique rays from the Ra being 

 now able to fall upon the screen. 



Curves A and B represent the results of subtracting 

 readings with the screen at B and C and A and B respect- 

 ively (fig. i.), and are measures of the amount of radiation 

 slightly deflected, and of that which has suffered much larger 

 deflection. 



Curve F represents the returned radiation from alumin- 

 ium screens of different thicknesses. Similar curves to the 

 above are shown in fig. iv., for Au screens. 



§IV. 



Discussion of Results. 



In fig. iii., from, curve C , it is seen that the total emer- 

 gent scattered radiation increases rapidly to a maximum, 

 and then steadily decreases as the thickness of screen is in- 

 creased. The maximum occurs at about 013 cm. 



