﻿Telescopic 
  Vision. 
  331 
  

  

  the 
  observer 
  to 
  examine 
  an 
  iris-diaphragm 
  at 
  B, 
  which 
  is 
  

   what 
  limits 
  the 
  aperture 
  of 
  the 
  pro-telescope, 
  and 
  a 
  scale 
  

   behind 
  it 
  at 
  0, 
  which 
  enables 
  him 
  to 
  adjust 
  the 
  iris-diaphragm. 
  

   These 
  observations 
  are 
  conveniently 
  made 
  through 
  a 
  Steinheil 
  

   lens 
  magnifying 
  about 
  7 
  times 
  mounted 
  on 
  a 
  small 
  piece 
  of 
  

   wood 
  so 
  as 
  to 
  be 
  readily 
  placed 
  in 
  its 
  position 
  before 
  B. 
  

   The 
  scale 
  at 
  G 
  to 
  be 
  read 
  through 
  this 
  lens 
  is 
  an 
  ordinary 
  

   eyepiece 
  micrometer 
  scale 
  — 
  a 
  centimetre 
  graduated 
  into 
  

   100 
  parts 
  upon 
  glass 
  ; 
  and 
  both 
  it 
  and 
  the 
  iris-diaphragm 
  

   are 
  mounted 
  on 
  an 
  ordinary 
  substage 
  apparatus 
  belonging 
  to 
  

   a 
  microscope, 
  the 
  eyepiece 
  micrometer 
  being 
  sufficiently 
  

   tightened 
  by 
  a 
  slight 
  caulking 
  of 
  soft 
  twine 
  into 
  the 
  swing- 
  

   frame 
  intended 
  for 
  the 
  microscope's 
  sub-stage 
  stops. 
  The 
  

   swing-frame 
  enables 
  the 
  observer 
  to 
  turn 
  the 
  scale 
  out 
  of 
  the 
  

   line 
  of 
  sight 
  when 
  using 
  the 
  pro- 
  telescope. 
  This 
  has 
  to 
  be 
  

   done, 
  since 
  the 
  glass 
  on 
  which 
  such 
  scales 
  are 
  graduated 
  is 
  

   not 
  sufficiently 
  good 
  to 
  be 
  left 
  in 
  front 
  of 
  the 
  pro- 
  

   telescope. 
  

  

  18. 
  It 
  is 
  here 
  also, 
  and 
  through 
  the 
  Steinheil 
  lens, 
  that 
  

   observations 
  are 
  to 
  be 
  made 
  upon 
  the 
  concentration 
  images 
  

   which 
  will 
  be 
  brought 
  to 
  their 
  focus 
  upon 
  plane 
  Y', 
  the 
  

   representative 
  of 
  plane 
  Y 
  of 
  the 
  cosmical 
  apparatus. 
  We 
  

   are 
  to 
  regard 
  the 
  plane 
  at 
  C 
  perpendicular 
  to 
  the 
  optic 
  axis 
  

   as 
  plane 
  Y'. 
  The 
  concentration 
  images 
  will 
  then, 
  when 
  

   focussed 
  upon 
  this 
  plane, 
  coincide 
  with 
  the 
  graduated 
  scale 
  

   — 
  an 
  arrangement 
  which 
  enables 
  details 
  upon 
  the 
  concen- 
  

   tration 
  images 
  to 
  be 
  conveniently 
  measured. 
  Provision 
  was 
  

   made 
  when 
  setting 
  up 
  the 
  apparatus 
  whereby 
  each 
  millimetre 
  

   on 
  the 
  concentration 
  image 
  as 
  measured 
  in 
  the 
  experimental 
  

   apparatus 
  corresponds 
  to 
  one 
  decimetre 
  on 
  the 
  concentration 
  

   image 
  that 
  would 
  be 
  formed 
  in 
  front 
  of 
  the 
  astronomical 
  

   telescope 
  if 
  the 
  imaginary 
  lens 
  L 
  could 
  be 
  placed 
  before 
  the 
  

   planet. 
  In 
  fact 
  all 
  measures 
  made 
  on 
  the 
  concentration 
  

   image 
  as 
  seen 
  in 
  the 
  experimental 
  apparatus 
  have 
  only 
  to 
  be 
  

   multiplied 
  by 
  100 
  to 
  arrive 
  at 
  what 
  the 
  corresponding 
  

   measures 
  would 
  have 
  been 
  in 
  the 
  cosmical 
  apparatus, 
  if 
  the 
  

   cosmical 
  apparatus 
  could 
  have 
  been 
  made 
  available. 
  

  

  19. 
  Another 
  very 
  useful 
  numerical 
  relation 
  furnishes 
  us 
  

   with 
  the 
  means 
  of 
  dealing 
  with 
  spectra, 
  and 
  is 
  often 
  of 
  

   service 
  because 
  the 
  concentration 
  images 
  which 
  render 
  us 
  

   most 
  assistance 
  are 
  images 
  consisting 
  of 
  spectra. 
  If 
  we 
  

   could 
  set 
  up 
  the 
  cosmical 
  apparatus 
  described 
  in 
  Chapter 
  1, 
  

   these 
  images 
  would 
  present 
  themselves 
  on 
  plane 
  Y, 
  and 
  

   measures 
  upon 
  the 
  spectra 
  which 
  would 
  be 
  there 
  seen 
  are 
  

   most 
  conveniently 
  expressed 
  in 
  terms 
  of 
  a 
  standard 
  length 
  b, 
  

   which 
  symbol 
  stands 
  for 
  \/e, 
  where 
  \ 
  is 
  the 
  wave-length 
  of 
  

  

  Z2 
  

  

  