Various  Gases  by  the  a  Particles  of  Radium.  b'31 
particles  have  to  pass  through  it  on  the  way  to  the  photographic 
plate.  M.  Becquerel  supposes  that  there  ought  therefore 
to  be  an  increased  displacement  o£  the  photographic  image. 
But  this  is  not  so.  The  path  of  any  one  a  particle  is  slightly 
deflected,  but  the  whole  trace  is  not  appreciably  disturbed.  The 
aluminium  diminishes  the  range  of  every  a.  particle  by  the 
same  amount,  but  the  only  result  is  to  cut  off  all  the  rays 
which  would  have  gone  past  a  certain  point,  say  Q,  and  to 
cause  them  to  take  the  places  of  those  rays  which  fell  short 
of  Q  ;  these  latter  being  further  shortened.  This  does  not  in 
the  least  affect  the  position  of  the  outer  edge  of  the  trace 
upon  the  photographic  plate ;  and  though  there  must  be  a 
slight  movement  of  the  inner  edge,  so  that  the  trace  is  some- 
what narrower,  the  change  is  so  small  that  it  could  not 
possibly  be  detected,  as  a  glance  at  the  photograph  will  show. 
Magnetic  dispersion  of  the  u  rays  does  exist :  it  has  been 
directly  shown  by  Rutherford,  and,  as  I  think,  indirectly  by 
M.  Becquerel' s  own  experiments,  in  the  peculiarities  of  the 
curvature  of  his  photographic  traces.  But  it  could  not  be 
shown  in  the  manner  of  the  experiment  which  M.  Becquerel 
now  describes.  That  would  be  analogous  to  the  search  for 
evidence  of  the  motion  of  the  stars  in  the  line  of  sight  in  the 
displacement  of  the  visible  spectrum  as  a  whole ;  whereas  the 
measurement  to  be  made  is  of  the  displacement  of  the 
Fraunhofer  lines  in  the  spectrum,  i.  e.  of  one  set  of  waves 
which  can  be  isolated  for  consideration.  It  is  here  that 
Rutherford's  experiment  is  differentiated  from  that  of 
M.  Becquerel.  The  former  employed  as  a  source  of  rays  a 
wire  coated  with  a  thin  layer  of  fta  C  emitting  a  particles  of 
uniform  velocity,  which  is  analogous  to  confining  one's 
attention  in  the  star  problem  to  waves  of  one  length.  More- 
over, Rutherford  passed  his  a  particles  for  some  considerable 
distance  through  a  vacuum  whilst  yet  under  the  influence  of 
the  magnetic  field.  Thus  the  evidence  of  the  increase  of 
curvature  in  their  paths,  originally  caused  by  the  loss 
of  velocity  in  penetrating  matter,  was  accumulated.  But  if, 
as  in  M.  Becquerel's  experiment,  the  path  is  in  the  air,  then 
any  appreciable  increase  of  curvature  closely  precedes  the 
cessation  of  all  evidence  of  motion,  and  the  result  must  be  in 
any  case  almost  beyond  detection. 
M.  Becquerel  remarks  that  there  is  no  evidence  in  his 
photographs  of  the  greater  precision  of  the  outer  line  of  the 
trace,  which  I  had  anticipated.  But  the  photograph  which 
he  now  publishes  show  that  there  is  too  much  penumbra  for 
such  an  effect  to  be  visible. 
