﻿328 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  features 
  upon 
  a 
  planet 
  can 
  be 
  adequately 
  seen 
  through 
  a 
  tele- 
  

   scope, 
  and 
  what 
  modification 
  their 
  appearance 
  will 
  undergo 
  

   when 
  those 
  conditions 
  can 
  be 
  only 
  partly 
  secured. 
  In 
  

   prosecuting 
  this 
  inquiry 
  an 
  efficient 
  working 
  model 
  o£ 
  the 
  

   cosmical 
  apparatus 
  described 
  in 
  the 
  foregoing 
  pages, 
  will 
  be 
  

   found 
  exceedingly 
  useful. 
  This, 
  accordingly, 
  we 
  shall 
  next 
  

   describe. 
  

  

  CHAPTER 
  2. 
  

  

  How 
  to 
  construct 
  an 
  adequate 
  working 
  model 
  of 
  the 
  apparatus 
  

   described 
  in 
  the 
  last 
  Chapter, 
  with 
  which 
  the 
  necessary 
  

   experiments 
  can 
  be 
  made. 
  

  

  15. 
  In 
  the 
  preceding 
  pages 
  the 
  reader 
  has 
  been 
  asked 
  to 
  

   consider 
  what 
  optical 
  effects 
  would 
  follow 
  if 
  a 
  lens 
  L 
  could 
  

   be 
  placed 
  close 
  to 
  the 
  object 
  which 
  the 
  astronomer 
  is 
  

   examining 
  through 
  his 
  telescope. 
  If 
  we 
  are 
  content 
  to 
  

   confine 
  ourselves 
  to 
  the 
  deductive 
  treatment 
  of 
  telescopic 
  

   vision, 
  the 
  intrusion 
  of 
  this 
  imaginary 
  lens 
  may 
  be 
  dispensed 
  

   with, 
  since 
  the 
  conclusions 
  arrived 
  at 
  can 
  be 
  reached 
  without 
  

   that 
  machinery. 
  It 
  would 
  be 
  legitimate 
  and 
  sufficient 
  to 
  

   resolve 
  the 
  light 
  emitted 
  by 
  the 
  planet 
  directly 
  into 
  undu- 
  

   lations 
  of 
  spherical 
  wavelets 
  subjected 
  to 
  the 
  condition 
  that 
  

   the 
  centres 
  of 
  these 
  spherical 
  undulations 
  shall 
  be 
  the 
  several 
  

   points 
  of 
  any 
  assumed 
  unfolded* 
  surface 
  of 
  one 
  sheet 
  

   enveloping 
  the 
  whole 
  of 
  that 
  aspect 
  of 
  the 
  object 
  which 
  is 
  

   turned 
  towards 
  us. 
  This 
  would 
  be 
  a 
  legitimate 
  procedure 
  ; 
  

   and 
  would 
  answer 
  our 
  purpose. 
  But 
  the 
  conceptions 
  which 
  

   the 
  investigator 
  must 
  be 
  able 
  to 
  form 
  in 
  order 
  to 
  prosecute 
  

   the 
  inquiry 
  in 
  this 
  way 
  can, 
  perhaps, 
  most 
  easily 
  be 
  grasped 
  

   after 
  he 
  has 
  pictured 
  to 
  himself 
  the 
  intervention 
  of 
  lens 
  L, 
  

   which 
  he 
  may 
  think 
  of 
  as 
  placed 
  either 
  immediately 
  in 
  front 
  

   of 
  or 
  immediately 
  behind 
  the 
  pro-Mars 
  ; 
  and 
  at 
  all 
  events 
  

   this 
  imagined 
  lens 
  suggests 
  and 
  serves 
  as 
  an 
  introduction 
  to 
  

   a 
  valuable 
  piece 
  of 
  apparatus 
  whereby 
  the 
  steps 
  of 
  the 
  process 
  

   by 
  which 
  the 
  telescopic 
  image 
  is 
  formed 
  can, 
  so 
  far 
  as 
  is 
  

   necessary, 
  be 
  submitted 
  for 
  our 
  inspection. 
  The 
  steps 
  of 
  that 
  

   process 
  can 
  no 
  doubt 
  in 
  many 
  cases 
  be 
  ascertained, 
  and 
  the 
  

   details 
  of 
  it 
  computed, 
  by 
  the 
  deductive 
  method 
  of 
  treatment 
  ; 
  

   but 
  it 
  is 
  in 
  some 
  respects 
  perhaps 
  even 
  more 
  instructive 
  and 
  

   it 
  is 
  certainly 
  much 
  more 
  impressive 
  actually 
  to 
  see 
  them. 
  

   On 
  this 
  account, 
  and 
  as 
  in 
  this 
  paper 
  deductive 
  reasoning 
  is 
  

   avoided 
  as 
  much 
  as 
  possible, 
  the 
  reader 
  will 
  be 
  invited 
  to 
  

   direct 
  his 
  attention 
  for 
  the 
  most 
  part 
  to 
  those 
  verifications 
  of 
  

   results 
  which 
  are 
  brought 
  within 
  his 
  reach 
  by 
  the 
  apparatus 
  

  

  * 
  By 
  the 
  term 
  " 
  unfolded 
  " 
  is 
  meant 
  that 
  no 
  vector 
  outwards 
  from 
  the 
  

   planet 
  will 
  pierce 
  the 
  surface 
  more 
  than 
  once. 
  

  

  