Secondary  Rontgen  Radiation.  817 
spheric  pressure  before  passing  through  the  absorbing  plates 
into  the  electroscope.  This  distance  was  chosen  because 
the  rajs  previously  experimented  upon  had  travelled  an  equal 
distance  from  the  radiator.)  Then  the  absorptions  of  the 
secondary  rays  from  tin  in  aluminium  and  tin  were  determined. 
Absorbing  plates  of  thickness  to  diminish  the  ionization  by 
approximately  the  same  amount  were  used.  It  was  found 
that  the  ratio  of  the  two  absorptions  by  aluminium  and  tin 
for  the  radiation  from  lead  was,  within  a  small  possible  error, 
the  same  as  that  ratio  when  the  radiation  from  tin  was 
absorbed. 
No  evidence  then  of  special  absorption  for  the  radiation 
from  tin  by  tin  was  obtained. 
Variation  in  Intensity  of  Primary  Radiation. 
Experiments  were  made  to  determine  the  effect  on  the 
secondary  radiation  of  variation  in  intensity  of  the  primary. 
Calcium  was  chosen  as  the  radiating  substance,  because  it  was 
the  substance  of  lowest  atomic  weight  experimented  upon 
which  emitted  a  radiation  differing  considerably  from  the 
primary,  and  consequently  might  be  expected  to  be  specially 
sensitive  to  such  variations.  The  apparatus  used  in  these 
experiments  was  that  described  in  previous  papers  on  polarized 
Rontgen  radiation.  The  electroscope  receiving  the  vertical 
secondary  beam  was  used  to  standardize  the  intensity  of  the 
horizontal  secondary  beam,  while  plates  of  aluminium  were 
placed  in  the  path  of  the  latter  to  the  electroscope. 
The  ratio  of  the  rates  of  deflexion  of  the  two  electroscopes 
appeared  constant  under  definite  controllable  conditions,  so 
no  particular  period  of  discharge  was  taken.  Variation  in 
intensity  of  the  primary  was  obtained  by  varying  the  distance 
of  the  X-ray  tube  from  the  calcium  radiator. 
The  results  are  given  below  : — 
Conditions. 
No  absorbing  plate  
Aluminium  plate  ('01  cm 
thick)  before  lower 
electroscope  , 
No  absorbing  plate  
Distance  of 
Anticatbode 
of  X-ray  tube 
from 
Eadiator. 
28  centims 
■28      „ 
78      „ 
Deflexion  of 
Upper 
Electroscope 
(standard). 
44-81 
54-3  j 
55-4 
700 
9-5 
146 
49-2  1 17 
Deflexion  of 
Lower 
Electroscope 
65-3 
12-1 
16-8 
32-4 
34-8  }  „ 
42-4 
18-8 
2V3 
25-8 12 
4(V0  J  ~ 
156 
r6 
Eatio  of 
Deflexions. 
10  to  164 
10  to  5-2 
10  to  5 
10  to  KV7 
