from a Plate exposed to Pays from Radium. 185 



Secondary Radiation from the Bach Surface of a Lead Plate. 

 " Transmitted, Rays" 



The plate was put in position T of the diagram. The 

 position of the bottom surface of each plaie was always the 

 same. The results are in the following Table. 



Table IT. 



Thickness of reflector 

 T in mm. 



•020 



•C60 



095 



i 

 •190 225 "45 1-00 



1-80 







309 



3-60 







2-87 



5 60 

 2-62 



9-20 

 2-38 

 203 



15-G 



xS o plate 

 at A. 



154 mm. 

 lead at A. 



1 



s 



M 

 © 



a 



© 



© 



6 + y 

 rays. 



38-9 



337 



19-5 

 216 



11-9 101 5-10 3-30 



222 



? 205 2-10 

 rays. | | 



223 2*25 227 2 - 27 2-24 

 1 



249 2-16 



1-99 

 •23 



Differ- 

 ence. 



^ 36-8 316 

 rays. 



17-3 



97 7-8 2-83 1-03 



■ 1 1 



•85 



■68 



•46 



•35 



1-8 mm'. 1 







333 ... 



... 



2-87 



2-59 

 2-74 







231 



2-35 



2-18 



le<td at A. 







"225 mm. 













... 



lead at A. 











An examination of these numbers discovers a rather 

 surprising result. The ft rays which ceased giving "re- 

 flected " rays after the thickness of lead traversed reached 

 ^ mm., now give " transmitted " rays up to the limiting 

 thickness used, 15*6 mm., and the intensity even for this 

 thickness is relatively great. The result was so unexpected 

 that it was considered it must be due to the more absorbable 

 7 rays cut out by the 15*4: mm. of lead. To test this some 

 measurements were made with a thickness of the absorbing 

 plate A only 1*8 mm., which would keep back only the more 

 absorbable ft rays. The results are appended to Table IV., 

 and show that these ft rays which had been cut out must 

 give ii very decided stream of u transmitted 3i rays. For 

 instance, with a plate T of thickness 9*2 mm. the leak when 

 all the ft and y rays struck the plate was 2*38, and when the 

 rays which could not pass through 1*8 mm. of lead were cut 

 out Ihe leak was only 2"31, showing that a leak of '07 was 

 due io the easily absorbable ft rays. That is 7 the rays which 

 cannot pass through 1*8 mm. of lead will yet start up rays- 

 from the far side of a plate of lead 9*2 mm. thick, when they 

 strike the near side of it. Even more astonishing is the 

 evidence of the last row of numbers in Table IV-, which 

 shows that even the softest ft rays (those totally absorbed by 



