166  Prof.  E.  Rutherford  on  some 
view  was  illuminated  with  yellow  light  ;  in  this  colour  the 
lines  were  seen  to  be  doubled  in  number  in  those  parts  of  the 
field  where,  in  the  former  experiment,  they  were  obliterated. 
Both  of  these  effects  are  immediately  explained  by  the  absence 
of  yellow  light  from  both  spectra  of  the  first  order. 
12.  In  the  experiments  which  have  been  described,  the 
microscope  was  used  under  what  would  be  considered  normal 
working  conditions,  with  central  illumination,  and  circular 
diaphragms  centred  on  the  optic  axis.  Nevertheless,  when 
certain  relations  existed  between  the  aperture  of  the  lens  and 
the  coarseness  of  structure  of  the  object,  images  were  formed 
which  were  utterly  false  in  their  smaller  details,  and  other 
images  were  profoundly  modified  by  the  presence  of  structure 
lying  entirely  beyond  the  focal  plane.  It  therefore  seems 
that  a  working  knowledge  of  the  phenomena  and  laws  of 
diffraction  might  well  form  a  part  of  the  equipment  of 
everyone  who  uses  the  microscope  and  attempts  to  interpret 
its  indications. 
XIII.  Some  Properties  of  the  u  Hays  from  Radium.  By 
E.  Rutherford,  F.R.S.,  Macdonald  Professor  of  Physics, 
McGill  University,  Montreal*. 
[Plate  IV.] 
[Second  paper.) 
IN  the  July  number  of  the  Philosophical  Magazine  I 
described  some  experiments  which  showed  that  the 
a  particles,  emitted  from  a  wire  made  active  by  exposure  to 
the  radium  emanation,  diminished  in  velocity  after  passing 
through  matter. 
The  active  deposit  of  radium,  rather  than  radium  itself, 
was  chosen  as  a  source  of  radiation  in  order  to  obtain  a 
homogeneous  pencil  of  rays.  The  active  deposit  on  the  wire 
is  of  extreme  thinness,  and  consequently  the  rays  emerge 
from  the  wire  without  any  alteration  by  their  passage  through 
matter.  About  15  minutes  after  removal  from  the  emana- 
tion, the  activity  of  the  wire  is  almost  entirely  due  to  the 
product  radium  C.  Using  the  photographic  method,  the 
rays  were  found  to  be  all  equally  deflected  by  a  strong 
magnetic  field,  i.  e.  they  consisted  of  a.  particles  projected  at 
the  same  speed. 
The  velocities  of  these  particles,  deduced  from  the  de- 
flexion of   a   narrow  beam   of  the  rays  by  a  magnetic  field, 
*  Communicated  by  the  Author. 
