﻿Telescopic 
  Vision. 
  385 
  

  

  opening. 
  The 
  object 
  z 
  will 
  then 
  come 
  into 
  its 
  proper 
  

   position 
  when 
  the 
  carpenter's 
  square 
  is 
  simply 
  slid 
  up 
  against 
  

   lens 
  L'. 
  

  

  27. 
  The 
  copper-foil 
  used 
  at 
  s' 
  is 
  of 
  the 
  same 
  kind 
  as 
  that 
  

   used 
  at 
  z 
  and 
  is 
  softened 
  in 
  the 
  same 
  way. 
  It 
  is 
  then 
  

   possible 
  to 
  pierce 
  it 
  with 
  the 
  thinnest 
  needle 
  that 
  can 
  be 
  

   procured, 
  with 
  which 
  a 
  clean 
  round 
  hole 
  about 
  \ 
  mm. 
  across 
  

   can 
  be 
  made. 
  This 
  small 
  hole, 
  which 
  subtends 
  less 
  than 
  1' 
  of 
  

   angle 
  at 
  z', 
  furnishes 
  the 
  opening 
  at 
  s 
  / 
  which 
  is 
  to 
  admit 
  the 
  

   light 
  that 
  is 
  to 
  be 
  our 
  pro 
  -star. 
  The 
  copper-foil 
  at 
  5' 
  is 
  

   fitted 
  into 
  the 
  swing- 
  frame 
  of 
  a 
  substage 
  apparatus 
  belonging 
  

   to 
  a 
  microscope 
  similar 
  to 
  that 
  employed 
  at 
  BC, 
  and 
  like 
  it 
  

   fitted 
  with 
  an 
  iris-diaphragm. 
  The 
  swing-frame 
  allows 
  the 
  

   copper-foil 
  with 
  the 
  small 
  hole 
  to 
  be 
  turned 
  aside, 
  and 
  the 
  

   iris-diaphragm 
  can 
  then 
  be 
  brought 
  into 
  action 
  to 
  limit 
  

   diffused 
  light 
  incident 
  upon 
  z 
  f 
  , 
  in 
  the 
  way 
  wdrich 
  is 
  required 
  

   for 
  one 
  of 
  the 
  experiments 
  which 
  are 
  about 
  to 
  be 
  described. 
  

  

  28. 
  One 
  other 
  appliance 
  remains 
  to 
  be 
  mentioned. 
  One 
  

   of 
  the 
  eyepieces 
  provided 
  for 
  the 
  pro-telescope 
  is 
  armed 
  

   with 
  an 
  eyepiece 
  micrometer 
  in 
  its 
  focus. 
  By 
  using 
  this, 
  

   and 
  varying 
  the 
  aperture 
  of 
  the 
  pro-telescope 
  by 
  the 
  iris- 
  

   diaphragm 
  at 
  B, 
  it 
  is 
  easy 
  to 
  measure 
  the 
  changes 
  in 
  mag- 
  

   nitude 
  and 
  in 
  appearance 
  which 
  small 
  objects 
  seem 
  to 
  undergo 
  

   when 
  examined 
  through 
  telescopes 
  of 
  different 
  apertures 
  — 
  

   a 
  very 
  important 
  matter 
  for 
  the 
  astronomer 
  to 
  become 
  

   acquainted 
  with. 
  This 
  completed 
  the 
  arrangements 
  in 
  the 
  

   apparatus 
  which 
  the 
  author 
  put 
  together. 
  

  

  Our 
  main 
  task 
  in 
  the 
  second 
  part 
  of 
  this 
  paper 
  will 
  be 
  to 
  

   find 
  out 
  by 
  the 
  use 
  of 
  this 
  apparatus 
  the 
  actual 
  process 
  by 
  

   which 
  images 
  in 
  the 
  astronomical 
  telescope 
  or 
  in 
  the 
  pro- 
  

   telescope 
  are 
  brought 
  into 
  existence 
  by 
  nature 
  ; 
  and 
  inci- 
  

   dentally 
  we 
  shall 
  learn 
  what 
  that 
  familiar 
  but 
  very 
  remarkable 
  

   optical 
  phenomenon, 
  a 
  beam 
  of 
  light, 
  really 
  is. 
  

  

  Appendix 
  to 
  Chapter 
  2, 
  § 
  23. 
  

  

  29. 
  In 
  our 
  experimental 
  apparatus 
  light 
  from 
  the 
  sun 
  has 
  

   been 
  admitted 
  through 
  the 
  pro-star 
  s', 
  a 
  hole 
  about 
  -\ 
  mm. 
  in 
  

   diameter, 
  and 
  allowed 
  to 
  fall 
  upon 
  small 
  objects 
  z\ 
  usually 
  

   less 
  than 
  1| 
  mm. 
  in 
  diameter, 
  which 
  are 
  placed 
  at 
  a 
  distance 
  

   of 
  1J 
  metre 
  from 
  sf. 
  The 
  part 
  of 
  this 
  light 
  which 
  is 
  of 
  wave- 
  

   length 
  X 
  will 
  be 
  treated 
  as 
  a 
  single 
  u 
  f 
  W 
  (undulation 
  of 
  flat 
  

   w 
  T 
  aves) 
  where 
  it 
  falls 
  upon 
  those 
  small 
  objects, 
  and 
  we 
  have 
  

   to 
  show 
  that 
  it 
  is 
  legitimate 
  so 
  to 
  regard 
  it. 
  

  

  