694  Lord  Rayleigh  :  Some  Measurements  of 
across  the  line  of  vision  so  as  to  bring  different  parts  of  the 
plates  into  action,  and  for  this  purpose  it  may  be  desirable  to 
increase  the  aperture.  A  dilatation  of  the  rings  means  that 
the  corresponding  parts  of  the  plates  need  approximation  by 
additional  pressure.  The  aperture  employed  in  the  actual 
measurements  was  of  about  9  mm.  diameter. 
The  (achromatic)  object-glass  of  the  telescope  is  of  15  inches 
focus.  In  rigid  connexion  with  it  is  the  vertical  reference 
wire  accurately  adjusted  to  focus,  and  close  to  the  wire  a 
small  frame  suitable  for  carrying  the  horizontal  slits  (cut  out 
of  thin  sheet  zinc)  necessary  for  the  isolation  of  the  various 
colours  *.  The  eyepiece  is  a  single  lens  of  5  inches  focus. 
mounted  independently,  so  that  it  can  be  re-adjusted  without 
fear  of  disturbing  the  object-glass  and  reference  wire.  The 
change  of  position  required  for  the  best  seeing  in  passing  from 
red  to  blue  or  even  from  red  to  green  is  so  great  as  to 
occasion  surprise  that  good  results  can  be  attained  in  the 
absence  of  such  a  provision  f . 
The  separation  of  the  colours  was  usually  effected  by  direct- 
vision  prisms  held  between  the  eyepiece  and  the  eye.  Of  these 
two  were  available.  The  larger  containing  (in  all)  three  prisms 
was  usually  the  more  convenient,  but  sometimes  a  smaller 
and  more  dispersive  combination  containing  five  prisms  was 
preferred.  It  is  better  to  use  more  dispersion  than  unduly 
to  narrow  the  slit.  The  refracting  edges  of  the  prisms  are, 
of  course,  horizontal.  In  order  to  secure  that  the  proper 
parts  of  the  ring  systems  should  be  visible,  the  axis  of  the 
telescope  was  adjusted  in  the  vertical  plane  with  substitution 
for  the  slit  of  a  horizontal  wire  coincident  with  the  middle 
line  of  the  former. 
The  advantage  of  this  arrangement  is  that  the  ring  systems 
(or  at  least  so  much  of  them  as  is  necessary)  of  the  various 
radiations  emitted  by  one  source  of  light  are  all  in  view  at 
the  same  time. 
In  some  cases,  direct-vision  prisms  held  between  the  5-inch 
eye-lens  and  the  eye  do  not  suffice.  The  soda  lines,  for 
example,  require  a  high  dispersion.  Even  the  yellow  lines 
of  mercury,  which  are  about  three  times  as  far  apart  as  the  soda 
lines,  could  not  be  fully  separated  by  the  prisms  already 
spoken  of.  Here  a  good  deal  depends  upon  chance.  If  the 
rings  of  one  mercury  system  happen  to  bisect  approximately 
those  of  the  other  system,  both  can  be  measured  in  the  inter- 
ferometer-gauge, and  the  only  question  which  remains  open 
*  Fabry  and  Perot,  C.  JR.  March  27,  1904. 
t  Especially  in  using1  the  method  of  coincidences.     1  ought  perhaps  to 
mention  that  my  eyes  have  now  very  little  power  of  accommodation. 
