Wave-Lengths  with  a  Modified  Apparatus.  695 
is  the  distinction  of  the  two  systems.  For  this  purpose  a 
prism  of  moderate  power,  by  which  one  system  is  lifted  a 
little  relatively  to  the  other,  suffices.  If,  however,  the  two 
ring  systems  chance  to  be  nearly  in  coincidence,  a  much  more 
powerful  dispersion  is  required  in  order  to  measure  them 
separately. 
In  such  cases  recourse  was  had  to  a  special  direct-vision 
prism  of  glass  and  bisulphide  of  carbon  through  which  a 
selected  ray  of  the  spectrum  passes  without  refraction  at  all 
at  any  of  the  surfaces  *.  In  this  instrument  the  upper  edge 
of  the  beam  traverses  20  inches  of  glass  and  the  lower  edge 
20  inches  of  bisulphide  of  carbon.  This  prism  cannot  be 
inserted  between  the  eyepiece  already  described  and  the  eye, 
which  latter  must  be  placed  at  the  image  of  the  object-glass. 
Additional  lenses  are  therefore  required.  These  are  merely 
ordinary  spectacle-lenses  and  constitute  a  telescope  of  unit 
magnifying  power.  A  more  precise  description  is  postponed, 
as  I  am  not  sure  that  I  have  as  yet  hit  upon  the  best  arrange- 
ment. It  may  suffice  to  say  that  with  this  instrument  rings 
formed  of  spectral  rays  even  closer  than  the  soda  lines  could 
be  readily  separated,  and  that  without  too  great  a  contraction 
of  the  slit  limiting  the  visible  portion  of  the  rings. 
The  source  of  light,  sometimes  very  small,  was  focussed 
upon  the  diaphragm  at  the  gauge,  and  it  is  necessary  that  the 
aperture  be  completely  filled  with  light.  This  gives  the  ratio 
between  the  distances  of  the  lens  from  the  source  (u)  and 
from  the  gauge  (v).  Again,  the  angular  diameter  of  the 
field  of  light,  which  must  not  be  too  small,  fixes  the  ratio  of 
the  aperture  of  the  lens  to  v  ;  so  that  only  the  absolute 
scale  of  the  three  quantities  is  left  open.  It  is  desirable  that 
the  lens  be  achromatic.  I  have  used  a  one-inch  lens  from  a 
small  opera-glass,  and  this  worked  well  with  u  =  2J  inches 
and  v  —  4c  feet. 
As  sources  of  light  in  experiments  involving  high  inter- 
ference, vacuum-tubes  are  by  far  the  most  convenient,  and 
their  introduction  is  one  of  the  many  services  which  Optics 
owes  to  Prof.  Michelson.  At  the  head  of  the  list  stands  the 
helium  tube,  both  on  account  of  its  not  requiring  to  be 
heated  and  also  of  the  brilliancy  of  the  yellow  radiation. 
Hitherto,  however,  the  wave-lengths  have  not  been  measured 
with  the  highest  accuracy.  The  tube  that  I  have  employed 
was  made  some  years  ago  by  my  son  and  had  already  seen  a 
good  deal  of  service  in  experiments  designed  to  answer  the 
question  :  "Is  Rotatory  Polarization  influenced  by  the  Earth's 
*  Nature,  lx.  p.  64  (1899)  ;  Scientific  Papers,  iv.  p.  394. 
