﻿Telescopic 
  Vision. 
  

  

  329 
  

  

  about 
  to 
  be 
  described 
  in 
  the 
  present 
  chapter 
  — 
  an 
  apparatus 
  

   easily 
  put 
  together 
  and 
  which 
  the 
  author 
  ventures 
  to 
  suggest 
  

   ought 
  to 
  be 
  an 
  adjunct 
  of 
  every 
  astronomical 
  observatory 
  

   which 
  is 
  occupied 
  with 
  the 
  study 
  of 
  what 
  can 
  be 
  seen 
  upon 
  

   celestial 
  objects 
  of 
  visible 
  size, 
  whether 
  sun, 
  or 
  moon, 
  or 
  

   planet, 
  or 
  comet, 
  or 
  nebula. 
  

  

  16. 
  The 
  essential 
  parts 
  of 
  this 
  apparatus, 
  the 
  ease 
  with 
  

   which 
  it 
  can 
  be 
  constructed, 
  and 
  the 
  many 
  convenient 
  

   arrangements 
  that 
  can 
  be 
  secured 
  in 
  using 
  it, 
  will 
  be 
  under- 
  

   stood 
  from 
  a 
  description 
  of 
  the 
  specimen 
  which 
  the 
  author 
  

   was 
  able 
  to 
  set 
  up 
  for 
  his 
  own 
  use 
  out 
  of 
  materials 
  that 
  

   happened 
  to 
  be 
  at 
  at 
  hand. 
  The 
  apparatus 
  as 
  a 
  whole 
  may 
  

   be 
  designated 
  an 
  Observatory-experiments 
  Apparatus, 
  or 
  

   OE 
  Apparatus, 
  or, 
  still 
  more 
  briefly, 
  OEA. 
  The 
  essential 
  

   parts 
  are 
  represented 
  in 
  the 
  diagram 
  on 
  Plate 
  XII. 
  

  

  The 
  proposed 
  apparatus 
  is 
  a 
  working 
  model 
  of 
  the 
  Cosmical 
  

   Apparatus 
  described 
  in 
  the 
  last 
  chapter, 
  and 
  of 
  so 
  efficient 
  a 
  

   kind 
  that 
  all 
  the 
  experiments 
  that 
  could 
  be 
  made 
  with 
  the 
  

   Cosmical 
  Apparatus, 
  if 
  the 
  Cosmical 
  Apparatus 
  were 
  avail- 
  

   able, 
  can 
  be 
  made 
  with 
  our 
  model 
  of 
  it. 
  In 
  the 
  Cosmical 
  

   Apparatus 
  we 
  may 
  distinguish 
  four 
  planes, 
  two 
  of 
  which 
  have 
  

   already 
  been 
  mentioned 
  : 
  — 
  (1) 
  The 
  plane 
  X 
  within 
  the 
  tele- 
  

   scope, 
  erected 
  perpendicular 
  to 
  its 
  optic 
  axis 
  and 
  at 
  the 
  

   principal 
  focus 
  of 
  the 
  objective 
  ; 
  (2) 
  and 
  (3) 
  planes 
  Y 
  and 
  Z, 
  

   perpendicular 
  to 
  the 
  line 
  from 
  the 
  telescope 
  to 
  z, 
  the 
  object 
  

   on 
  the 
  planet 
  which 
  is 
  under 
  examination 
  — 
  Y 
  being 
  a 
  plane 
  

   erected 
  perpendicular 
  to 
  this 
  line 
  at 
  its 
  nearer 
  end 
  and 
  Z 
  the 
  

   plane 
  perpendicular 
  to 
  it 
  erected 
  at 
  its 
  farther 
  end. 
  Finally, 
  

   (4) 
  a 
  plane 
  that 
  we 
  may 
  call 
  plane 
  S, 
  to 
  be 
  located 
  at 
  the 
  

   star 
  s 
  that 
  we 
  have 
  asked 
  the 
  reader 
  to 
  imagine 
  behind 
  the 
  

   planet, 
  and 
  which 
  is 
  to 
  be 
  the 
  plane 
  through 
  that 
  star 
  

   perpendicular 
  to 
  the 
  line 
  from 
  z 
  to 
  s. 
  All 
  of 
  these 
  are 
  

   represented 
  in 
  the 
  experimental 
  apparatus 
  by 
  screens 
  or 
  planes 
  

   which 
  are 
  indicated 
  upon 
  the 
  diagram 
  in 
  PI. 
  XII. 
  by 
  the 
  

   letters 
  X 
  / 
  Y 
  / 
  Z 
  / 
  and 
  S'. 
  

  

  17. 
  T' 
  in 
  the 
  diagram 
  is 
  the 
  pro-telescope, 
  which 
  means 
  

   that 
  it 
  is 
  a 
  small 
  telescope 
  which 
  in 
  our 
  apparatus 
  can 
  be 
  

   made 
  adequately 
  to 
  exhibit 
  the 
  optical 
  performance 
  obtainable 
  

   from 
  any 
  astronomical 
  telescope, 
  T, 
  up 
  to 
  telescopes 
  of 
  two- 
  

   metre 
  aperture. 
  This, 
  moreover, 
  it 
  does 
  with 
  the 
  great 
  added 
  

   advantage 
  that 
  provision 
  is 
  made 
  whereby 
  we 
  are 
  enabled 
  to 
  

   study 
  the 
  Concentration 
  Images 
  which 
  are 
  associated 
  with 
  

   the 
  telescopic 
  image, 
  and 
  can 
  ascertain 
  from 
  them 
  the 
  causes 
  

   of 
  the 
  effects 
  produced 
  by 
  varying 
  the 
  apertures 
  of 
  telescopes 
  ; 
  

   and 
  so 
  learn 
  with 
  what 
  classes 
  of 
  objects 
  an 
  increase 
  of 
  aper- 
  

   ture 
  will 
  give 
  improved 
  vision 
  as 
  well 
  as 
  how 
  much 
  and 
  of 
  

  

  Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  16. 
  No. 
  92. 
  Aug. 
  1908, 
  Z 
  

  

  