﻿Telescopic 
  Vision. 
  333 
  

  

  the 
  reader 
  to 
  imagine 
  an 
  immense 
  lens 
  placed 
  in 
  front 
  of 
  the 
  

   planet 
  as 
  functioning. 
  L',- 
  the 
  pro-L, 
  is 
  a 
  two-inch 
  objective 
  

   of 
  only 
  55 
  cm. 
  focal 
  length. 
  A 
  longer 
  focal 
  length, 
  some- 
  

   thing 
  near 
  a 
  metre, 
  would 
  have 
  been 
  preferred, 
  as 
  it 
  would 
  

   have 
  made 
  1/ 
  function 
  more 
  exactly 
  like 
  L 
  in 
  the 
  cosmical 
  

   apparatus. 
  But 
  the 
  shorter 
  focal 
  length 
  of 
  1/ 
  in 
  the 
  author's 
  

   apparatus 
  works 
  well 
  enough 
  : 
  it 
  only 
  necessitates 
  the 
  sub- 
  

   stitution 
  in 
  one 
  instance 
  of 
  a 
  resolution 
  into 
  spherical 
  undu- 
  

   lations 
  with 
  the 
  centres 
  of 
  the 
  spheres 
  the 
  several 
  points 
  of 
  

   surface 
  YV, 
  where 
  in 
  using 
  the 
  cosmical 
  apparatus 
  the 
  

   resolution 
  would 
  have 
  been 
  into 
  flat 
  undulations. 
  

  

  22. 
  The 
  whole 
  of 
  the 
  apparatus 
  from 
  B 
  to 
  E 
  is 
  mounted 
  

   upon 
  a 
  board, 
  made 
  movable 
  at 
  its 
  B 
  end 
  under 
  sufficiently 
  

   delicate 
  control, 
  both 
  sideways 
  and 
  up 
  and 
  down. 
  This 
  is 
  in 
  

   order 
  to 
  be 
  able 
  to 
  change 
  at 
  will 
  the 
  inclination 
  of 
  the 
  optic 
  

   axis 
  of 
  the 
  part 
  of 
  the 
  apparatus 
  to 
  the 
  left 
  of 
  E, 
  relatively 
  

   to 
  the 
  position 
  of 
  the 
  optic 
  axis 
  of 
  the 
  apparatus 
  to 
  the 
  right 
  

   of 
  E, 
  which 
  latter 
  remains 
  in 
  one 
  fixed 
  position. 
  When 
  the 
  

   pro-telescope 
  is 
  used 
  it 
  is 
  to 
  be 
  set 
  up 
  before 
  B, 
  and 
  its 
  optic 
  

   axis 
  is 
  to 
  be 
  adjusted 
  to 
  coincide 
  with 
  that 
  of 
  the 
  apparatus 
  

   attached 
  to 
  the 
  board. 
  

  

  23. 
  The 
  next 
  part 
  of 
  the 
  apparatus 
  to 
  be 
  noticed 
  is 
  the 
  

   pro-star 
  at 
  s 
  f 
  . 
  This 
  is 
  a 
  clean 
  round 
  hole, 
  J 
  of 
  a 
  millimetre 
  

   in 
  diameter, 
  made 
  by 
  a 
  very 
  fine 
  needle 
  in 
  copper-foil. 
  It 
  is 
  

   to 
  be 
  set 
  up 
  as 
  far 
  off 
  as 
  convenient 
  to 
  the 
  right 
  of 
  z\ 
  the 
  

   pro-planet, 
  and 
  this 
  distance 
  in 
  the 
  author's 
  apparatus 
  is 
  

   about 
  1^ 
  metre, 
  which 
  was 
  found 
  to 
  be 
  sufficient. 
  Beyond 
  

   it 
  again 
  is 
  a 
  simple 
  lens 
  G 
  and 
  the 
  heliostat 
  H. 
  The 
  heliostat 
  

   reflects 
  a 
  sunbeam 
  horizontally, 
  and 
  lens 
  G 
  condenses 
  this 
  

   into 
  an 
  image 
  of 
  the 
  sun 
  upon 
  the 
  sheet 
  of 
  copper 
  foil 
  IS', 
  at 
  

   the 
  place 
  where 
  the 
  minute 
  hole 
  s 
  f 
  has 
  been 
  made 
  in 
  it. 
  The 
  

   light 
  which 
  then 
  passes 
  will 
  produce 
  the 
  same 
  optical 
  effect 
  

   upon 
  a 
  sufficiently 
  small 
  object 
  at 
  z\ 
  as 
  would 
  a 
  single 
  undu- 
  

   lation 
  of 
  flat 
  luminous 
  waves 
  reaching 
  that 
  object. 
  As 
  this 
  

   is 
  a 
  very 
  important 
  optical 
  statement, 
  and 
  as 
  upon 
  its 
  being- 
  

   true 
  depends 
  the 
  working 
  of 
  our 
  apparatus, 
  we 
  shall 
  give 
  in 
  

   an 
  appendix 
  to 
  the 
  present 
  Chapter 
  an 
  exceptionally 
  simple 
  

   proof 
  of 
  it 
  ; 
  and 
  will 
  add 
  some 
  observations 
  which 
  will 
  it 
  is 
  

   hoped 
  help 
  the 
  student 
  of 
  this 
  branch 
  of 
  Nature's 
  work, 
  to 
  

   judge 
  correctly 
  how 
  far 
  the 
  conditions 
  he 
  produces 
  in 
  his 
  

   experiments 
  may 
  be 
  trusted 
  to 
  represent 
  the 
  process 
  pursued 
  

   by 
  Nature. 
  

  

  24. 
  Attempts 
  to 
  use 
  artificial 
  light 
  focussed 
  by 
  a 
  lens 
  upon 
  

   ,s', 
  instead 
  of 
  the 
  sunbeam, 
  have 
  been 
  more 
  or 
  less 
  unsatis- 
  

   factory. 
  The 
  Nernst 
  lamp 
  is 
  not 
  sufficiently 
  intense, 
  and 
  

   the 
  arc 
  light, 
  while 
  abundantly 
  bright, 
  produces 
  an 
  unsteady 
  

  

  