The  Dielectric  Strain  along  the  Lines  of  Force.       607 
"this  sphere  as  rigidly  fixed,  being  obtained  in  my  work  by 
using  a  law  more  general  than  ee'/r2.  Both  papers,  however, 
deal  with  a  spherical  atom  executing  small  vibrations  :  in 
both  the  principal  vibrations  are  found  to  correspond  to  sphe- 
rical harmonics.,  the  order  of  the  harmonic  being  the  n  of  the 
spectrum  series.  Lord  Rayleigh  obtains  only  a  single  series, 
falling  off  from  the  head  according  to  the  law  1/n,  and  also  a 
single  spectral  line.  Nothing  corresponding  to  this  series 
appears  in  my  work;  but  corresponding  to  the  single  line  I 
obtained  a  number  of  series  falling  off  according  to  the 
observed  law  1/n2.  Given  the  value  of  e/m  for  electrons  (or 
electric  fluid)  the  same  relation  can  be  obtained,  in  either 
case,  between  the  frequencies  and  the  radius  of  the  atom. 
Unfortunately  the  numerical  calculation  given  in  §  23  of  my 
paper  was  inaccurate :  corresponding  to  frequencies  of  the 
order  of  those  of  light  the  radius  must  be  about  3  x  10-7, 
which  is  not  good  as  regards  agreement  with  facts,  but  is 
perhaps  as  near  to  the  true  value  as  can  be  expected  in  a 
vague  calculation  of  this  kind. 
My  picture  of  the  atom  led  to  a  Zeeman  effect,  consisting 
of  a  splitting  into  three  or  more  sharp  and  fully  polarized 
lines.  Lord  Rayleigh \s  model  would,  I  believe,  do  the  same. 
If,  however,  the  atom  consisted  of  electrons  in  orbital  motion, 
the  planes  of  these  orbits  making  all  directions  with  the 
lines  of  magnetic  force,  it  appears  as  if  the  Zeeman  effect 
could  at  best  consist  of  a  widening  into  a  continuous  band,  a 
point  which  has,  I  think,  already  been  made  by  Lord  Kelvin. 
Princeton,  N.  J.,  J.  H.  JEANS. 
Jan.  24th,  1906. 
LIT.    The  Dielectric  Strain  along  the  Tines  of  Force.    n 
To  the  Kclitors  of  the  Philosophical  Magazine. 
Gentlemen, 
IN  the  issue  of  December  1005  you  published  a  paper 
by  Mr.  L.  T.  More,  "  On  the  Dielectric  Strain  along 
the  Lines  of  Force,"  in  which  our  paper  "  Ueber  die  Electro- 
striction  des  Glases"*  is  criticised.  We  should  be  much 
obliged  if  you  would  publish  the  following  reply. 
Our  experiments  have  shown  that  the  expansion  of  the 
volume  of  electrified  glass  tubes  and  spheres  was  in  some 
kinds  of  glass  in  agreement  with  the  mechanical  pressure  of 
the  armatures  and  the  elastic  constants  of  the  glass.  In 
other  kinds  of  glass  the  expansion  was  found  to  be  too  small. 
These  results  confirm  in  general  those  of  Quincke,  but  are 
*  Ann.  d.  Physik  (4)  ix.  p.  1217  (1902). 
