﻿978 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  provide 
  round 
  holes 
  o£ 
  the 
  sizes 
  given 
  in 
  column 
  4 
  of 
  the 
  

   table 
  on 
  p. 
  ol)7. 
  With 
  a 
  little 
  care, 
  these 
  can 
  be 
  made 
  with 
  

   needles 
  in 
  softened 
  copper 
  Eoil 
  (see 
  § 
  26, 
  p. 
  331 
  ). 
  Of 
  these 
  

   holes 
  that 
  one 
  is 
  to 
  he 
  selected 
  which 
  corresponds 
  to 
  the 
  

   angular 
  size 
  of 
  Mars 
  at 
  the 
  time, 
  which 
  is 
  given 
  in 
  the 
  

   Nautical 
  Almanac 
  (see 
  column 
  2 
  o( 
  the 
  table). 
  On 
  looking 
  

   with 
  the 
  naked 
  eye 
  through 
  this 
  hole 
  at 
  the 
  hill 
  Moon, 
  we 
  

   the 
  Moon 
  under 
  the 
  same 
  optical 
  conditions 
  as 
  when 
  we 
  

   view 
  Mars 
  through 
  the 
  astronomical 
  telescope 
  armed 
  with 
  a 
  

   magnifying 
  power 
  equal 
  to 
  the 
  number 
  which 
  is 
  found 
  in 
  

   column 
  3 
  of 
  the 
  table. 
  W 
  we 
  wish 
  to 
  have 
  at 
  our 
  command 
  

   an 
  opportunity 
  of 
  using 
  other 
  power-, 
  we 
  must 
  provide 
  the 
  

   small 
  auxiliary 
  tele-cope 
  suggested 
  in 
  § 
  IN. 
  

  

  Note 
  2. 
  On 
  tin 
  .<<:, 
  of 
  the 
  Eidolon. 
  — 
  In 
  the 
  text 
  (see 
  § 
  13) 
  

   w.' 
  have 
  described 
  the 
  Eidolon, 
  or 
  object 
  which 
  appears 
  to 
  

   the 
  observer 
  to 
  In- 
  what 
  he 
  i- 
  Looking 
  at, 
  as 
  of 
  one 
  special 
  

   Bize. 
  In 
  this 
  ii 
  was 
  assumed 
  that 
  the 
  observer 
  had 
  focussed 
  

  

  hi- 
  telescope 
  BO 
  a- 
  to 
  he 
  able 
  to 
  -crutini-e 
  the 
  eikon, 
  the 
  

  

  image 
  presented 
  to 
  hi- 
  eye, 
  with 
  mosi 
  success. 
  By 
  adjusting 
  

   th" 
  focus 
  differently 
  the 
  eikon 
  could 
  he 
  made 
  to 
  give 
  the 
  

   impression 
  of 
  a 
  Larger 
  object 
  at 
  a 
  greater 
  distance 
  : 
  hut 
  it 
  is 
  

   best, 
  a- 
  in 
  the 
  text, 
  to 
  Eocus 
  in 
  the 
  way 
  that 
  enahles 
  the 
  

   ol 
  server 
  to 
  scrutinise 
  the 
  image 
  be 
  sees 
  to 
  most 
  advantage. 
  

  

  And, 
  when 
  focussing 
  his 
  tele-cope, 
  the 
  experienced 
  

   observer 
  will 
  judge 
  as 
  to 
  whether 
  it 
  i- 
  Eocussed 
  correctly 
  by 
  

   the 
  vision 
  he 
  obtains 
  of 
  whatever 
  is 
  the 
  most 
  distinctly 
  seen 
  

   feature 
  of 
  the 
  image 
  presented 
  to 
  him. 
  This 
  in 
  the 
  case 
  of 
  

   Mars 
  will 
  be 
  the 
  contour 
  of 
  the 
  planet 
  and 
  not 
  any 
  of 
  the 
  

   objects 
  upon 
  its 
  disk, 
  and 
  especially 
  not 
  any 
  phenako-spile. 
  

  

  Note 
  3. 
  On 
  lens 
  L 
  1 
  of 
  the 
  OE 
  apparatus. 
  — 
  Since 
  the 
  focal 
  

   length 
  of 
  lens 
  L' 
  is 
  Less 
  than 
  a 
  metre, 
  we 
  have 
  the 
  concen- 
  

   tration 
  image 
  affected 
  by 
  the 
  spherical 
  aberration 
  spoken 
  of 
  

   in 
  the 
  footnote 
  on 
  p. 
  960. 
  However, 
  it 
  is 
  found 
  that 
  this 
  

   defect 
  does 
  not 
  sensibly 
  impair 
  the 
  image 
  even 
  when 
  the 
  focal 
  

   length 
  of 
  1/ 
  is 
  as 
  short 
  as 
  it 
  is 
  in 
  the 
  author's 
  apparatus. 
  

   Neither 
  is 
  it 
  sensibly 
  the 
  worse 
  because 
  s 
  1 
  is 
  a 
  little 
  farther 
  

   from 
  z' 
  than 
  surface 
  W, 
  the 
  conjugate 
  of 
  plane 
  Y'. 
  If 
  on 
  

   the 
  other 
  hand 
  the 
  focal 
  length 
  were 
  longer 
  than 
  a 
  metre, 
  the 
  

   specks 
  of 
  the 
  concentration 
  image 
  which 
  are 
  formed 
  by 
  each 
  

   of 
  the 
  punctades 
  (or 
  stel 
  lades) 
  into 
  which 
  the 
  beam 
  is 
  

   analysed, 
  would 
  dilate 
  and 
  become 
  less 
  definite 
  as 
  seen 
  upon 
  

   plane 
  Y\ 
  and 
  if 
  the 
  focal 
  length 
  were 
  excessive 
  (wdiich 
  would 
  

   have 
  the 
  same 
  effect 
  as 
  removing 
  lens 
  1/ 
  from 
  the 
  apparatus) 
  

   the 
  dift'useness 
  of 
  its 
  specks 
  would 
  impair 
  our 
  vision 
  of 
  the 
  

  

  