Radiation  from  Ordinary  Materials. 
211 
If  p  is  the  ionization  due  to  a   surface  S  of  an  infinite 
slab  of  the  material,  then 
T  0  ira-       da  .  ~ 
(±) 
But   d=  -0013/0,      .-.  p=-00016aAS.      We   shall   write   s 
f0V  I. 
We  have  now  to  consider  the  effect  of  placing  another 
similar  slab  parallel  to  AB  and  distant  x  from  it.  Both 
slabs  will  emit  rays,  but  all  the  rays  will  not  produce  their 
full  effect  before  they  strike  the  opposite  slab  and  are  ab- 
sorbed.    By  drawing  the  figure  (fig.  14)   we  see   that  the 
Fig-.  14. 
limits   within  which  we  have  to  integrate  for   an  element 
distant  I  from  the  surface  of  either  slab  are  : — 
0  =  0  to  2it,  r  —  l  sec#  to  a  —  (p  —  l)Zsec  0, 
tor  0  =  cos     --  to  cos   L , 
a  a 
and         0  =  0  to  2tt,  r=  I  sec  0  to  (x  +  l)  sec#, 
_-rI±^  to0. 
a 
Hence  the  effect    due  to   the    single   element  for  which 
a  —  x 
for  0  =  cos" 
l> 
T0  c/S  dl 
f>  1 
*a— (p""1)  I  sec 
I  sec  9 
*cos— l 
sin 
<9tf<9 
«-i  ?±£? 
.r  +  p/ 
p2rr  f(*  +  ')sec0        i*cos-i      - 
Jo  JjsecG  J0 
PL  -,loo-A'  +  ^W/.     .     (5) 
a     J 
P2 
-2rf0{*  +  p*log, 
