﻿330 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  what 
  kind, 
  and 
  with 
  what 
  other 
  classes 
  of 
  objects 
  the 
  

   improvement 
  will 
  be 
  too 
  slight 
  to 
  be 
  valuable. 
  

  

  The 
  pro-telescope 
  in 
  the 
  author's 
  apparatus 
  is 
  a 
  small 
  

   laboratory 
  telescope 
  of 
  one-inch 
  aperture 
  and 
  eight-inch 
  

   focal 
  length, 
  intended 
  for 
  reading 
  scales 
  upon 
  instruments 
  

   without 
  having 
  to 
  go 
  close 
  to 
  them, 
  but 
  here 
  used 
  to 
  view 
  

   the 
  object 
  z 
  f 
  , 
  placed 
  at 
  E. 
  We 
  make 
  use 
  of 
  the 
  letter 
  z 
  to 
  

   signify 
  any 
  feature 
  we 
  picture 
  as 
  possibly 
  existing 
  on 
  the 
  

   planet, 
  and 
  the 
  conditions 
  for 
  seeing 
  which 
  satisfactorily 
  we 
  

   want 
  to 
  investigate. 
  The 
  z' 
  in 
  our 
  apparatus 
  is 
  a 
  pro-£, 
  that 
  

   is 
  an 
  object 
  at 
  E 
  of 
  the 
  same 
  shape 
  as 
  z, 
  and 
  of 
  the 
  size 
  

   which 
  will 
  in 
  the 
  experimental 
  apparatus 
  correctly 
  represent 
  

   the 
  dimensions 
  of 
  z 
  upon 
  the 
  planet. 
  It 
  can 
  easily 
  be 
  made 
  

   of 
  this 
  size, 
  inasmuch 
  as 
  such 
  dispositions 
  have 
  been 
  made 
  

   in 
  setting 
  up 
  the 
  apparatus 
  that 
  one 
  millimetre 
  at 
  z' 
  repre- 
  

   sents 
  length 
  e 
  upon 
  the 
  planet, 
  viz.: 
  the 
  length 
  upon 
  the 
  

   planet 
  which 
  subtends 
  standard 
  angle 
  e 
  as 
  seen 
  from 
  the 
  

   earth. 
  We 
  already 
  know 
  that 
  this 
  length 
  is 
  also 
  represented 
  

   by 
  one 
  millimetre 
  upon 
  the 
  standard 
  eidolon 
  of 
  the 
  planet, 
  

   viz. 
  : 
  the 
  eidolon 
  furnished 
  by 
  an 
  astronomical 
  telescope 
  

   magnifying 
  400 
  times. 
  When, 
  as 
  usually 
  happens, 
  the 
  

   object 
  z 
  upon 
  the 
  planet 
  is 
  one 
  of 
  which 
  we 
  only 
  

   expect 
  to 
  see 
  the 
  outline, 
  it 
  will 
  be 
  sufficiently 
  represented 
  

   by 
  a 
  hole 
  in 
  copper-foil 
  at 
  z' 
  of 
  the 
  proper 
  shape 
  and 
  size, 
  

   illuminated 
  by 
  light 
  coining 
  from 
  beyond. 
  When 
  this 
  light 
  

   is 
  diffused 
  light 
  such 
  as 
  that 
  from 
  a 
  sheet 
  of 
  white 
  paper 
  or 
  

   from 
  the 
  reflexion 
  of 
  a 
  cloud, 
  and 
  when 
  the 
  aperture 
  of 
  the 
  

   pro-telescope 
  T 
  / 
  has 
  been 
  adjusted 
  to 
  correspond 
  to 
  the 
  

   aperture 
  of 
  the 
  astronomical 
  telescope, 
  which 
  it 
  will 
  be 
  when 
  

   the 
  one 
  aperture 
  is 
  made 
  the 
  hundredth 
  part 
  of 
  the 
  other, 
  

   then 
  will 
  the 
  vision 
  of 
  z' 
  which 
  we 
  shall 
  obtain 
  through 
  the 
  

   pro-telescope 
  be 
  precisely 
  the 
  same 
  as 
  that 
  which 
  the 
  astro- 
  

   nomer 
  can, 
  at 
  best, 
  secure 
  of 
  the 
  corresponding 
  object 
  z 
  if 
  it 
  

   exists 
  upon 
  the 
  planet. 
  By 
  " 
  at 
  best 
  " 
  is 
  meant 
  if 
  the 
  three 
  

   following 
  conditions 
  are 
  secured: 
  — 
  

  

  1. 
  If 
  the 
  '*' 
  seeing 
  " 
  is 
  perfect 
  ; 
  

  

  2. 
  If 
  a 
  magnifying 
  power 
  which 
  corresponds 
  to 
  that 
  on 
  

  

  the 
  pro-telescope 
  (viz. 
  : 
  a 
  magnifying 
  power 
  100 
  

   times 
  that 
  employed 
  on 
  the 
  pro-telescope) 
  can 
  be 
  

   with 
  advantage 
  employed 
  on 
  the 
  astronomical 
  

   telescope 
  ; 
  

  

  3. 
  If 
  z 
  upon 
  the 
  planet 
  is 
  as 
  adequately 
  illuminated 
  as 
  z' 
  

  

  in 
  the 
  experimental 
  apparatus. 
  

  

  The 
  pro-telescope 
  is 
  mounted 
  upon 
  a 
  separate 
  stand, 
  easily 
  

   removed 
  from 
  before 
  the 
  rest 
  of 
  the 
  apparatus, 
  so 
  as 
  to 
  allow 
  

  

  