﻿808 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  plotted 
  down 
  upon 
  one 
  diagram, 
  and 
  then 
  we 
  can 
  read 
  off 
  on 
  

   it 
  any 
  required 
  intermediate 
  values 
  of 
  D, 
  a, 
  or 
  yu,*. 
  

  

  48. 
  Quite 
  a 
  small 
  telescope 
  is 
  sufficient 
  for 
  use 
  in 
  the 
  

   control 
  experiments. 
  The 
  author's 
  was 
  a 
  small 
  pocket 
  tele- 
  

   scope 
  with 
  an 
  objective 
  of 
  2 
  cm. 
  aperture 
  and 
  ten-inch 
  focal 
  

   length. 
  The 
  erecting 
  apparatus 
  and 
  eyepiece 
  were 
  removed, 
  

   and 
  simple 
  plano-convex 
  ilenses 
  took 
  their 
  place 
  as 
  eyepieces. 
  

   These, 
  when 
  the 
  convex 
  side 
  is 
  turned 
  towards 
  the 
  eye, 
  are 
  

   sufficiently 
  good 
  eyepieces 
  of 
  the 
  low 
  powers 
  indicated 
  in 
  

   column 
  5 
  of 
  the 
  table. 
  Each 
  of 
  these 
  lenses 
  should 
  be 
  

   mounted 
  in 
  a 
  tube 
  with 
  an 
  eyestop, 
  to 
  keep 
  the 
  eye 
  in 
  the 
  

   right 
  position, 
  that 
  is, 
  where 
  the 
  plano-convex 
  lens 
  can 
  form 
  

   its 
  image 
  of 
  the 
  objective. 
  To 
  limit 
  the 
  aperture 
  of 
  the 
  

   telescope, 
  a 
  hole 
  a 
  few 
  mm.'s 
  in 
  diameter 
  may 
  be 
  made 
  in 
  

   the 
  cover 
  which 
  is 
  usually 
  sold 
  with 
  such 
  telescopes 
  for 
  

   keeping 
  dust 
  off 
  the 
  objective, 
  and 
  thin 
  disks 
  of 
  brass 
  may 
  

   be 
  provided 
  like 
  those 
  used 
  for 
  microscope 
  stops, 
  with 
  central 
  

   holes 
  of 
  the 
  sizes 
  entered 
  in 
  column 
  4 
  of 
  the 
  table. 
  Any 
  one 
  

   of 
  these 
  when 
  fitted 
  inside 
  the 
  dust-cover 
  will, 
  when 
  the 
  

   cover 
  is 
  slipped 
  on 
  to 
  the 
  objective, 
  limit 
  its 
  aperture 
  in 
  one 
  

   of 
  the 
  required 
  degrees. 
  This 
  formed 
  the 
  entire 
  outfit 
  : 
  and 
  

   it 
  may 
  for 
  convenience 
  be 
  strapped 
  on 
  to 
  an 
  equatoreal 
  stand. 
  

   The 
  whole 
  was 
  adapted 
  for 
  comparisons 
  with 
  the 
  performance 
  

   of 
  an 
  astronomical 
  telescope 
  of 
  12-inch 
  aperture. 
  To 
  fit 
  it 
  

   for 
  the 
  minority 
  of 
  occasions 
  when 
  the 
  state 
  of 
  our 
  atmo- 
  

   sphere 
  allows 
  an 
  18-inch 
  or 
  a 
  24-inch 
  aperture 
  to 
  be 
  used 
  

   with 
  more 
  advantage 
  upon 
  Mars, 
  the 
  numbers 
  in 
  columns 
  4 
  

   and 
  5 
  will 
  need 
  to 
  be 
  increased 
  in 
  the 
  same 
  ratio 
  as 
  the 
  

   astronomical 
  aperture 
  is 
  increased. 
  This 
  will 
  necessitate 
  a 
  

   small 
  addition 
  to 
  the 
  stock 
  of 
  eyepieces 
  and 
  of 
  disks 
  of 
  brass 
  

   with 
  holes 
  in 
  them, 
  which 
  will 
  have 
  to 
  be 
  provided 
  ; 
  and 
  it 
  

   will 
  be 
  convenient 
  to 
  add 
  to 
  the 
  diagram 
  described 
  in 
  § 
  47 
  

   the 
  straight 
  lines 
  which 
  will 
  indicate 
  which 
  of 
  these 
  is 
  to 
  be 
  

   used 
  on 
  each 
  occasion. 
  It 
  will 
  of 
  course 
  be 
  understood 
  that 
  

   the 
  observer 
  will 
  when 
  observing 
  carefully 
  make 
  allowance 
  

  

  * 
  D, 
  the 
  distance 
  of 
  Mars, 
  has 
  been 
  computed 
  from 
  the 
  assumption 
  

   that 
  when 
  its 
  distance 
  from 
  the 
  earth 
  is 
  7 
  metro-tens 
  (10 
  10 
  metres) 
  its 
  

   apparent 
  semi-diameter 
  is 
  ]0", 
  which 
  is 
  a 
  sufficiently 
  close 
  approxi- 
  

   mation. 
  L 
  was 
  deduced 
  from 
  the 
  assumption 
  that 
  the 
  mean 
  distance 
  of 
  

   the 
  Moon 
  is 
  884,000 
  kilometres, 
  by 
  applying 
  two 
  corrections 
  to 
  this 
  

   value 
  — 
  1°, 
  by 
  subtracting 
  from 
  it 
  1/180 
  part 
  to 
  allow 
  for 
  the 
  effect 
  of 
  the 
  

   perturbation 
  known 
  as 
  Variation 
  upon 
  the 
  Moon 
  when 
  in 
  syzjgy 
  ; 
  and 
  

   2°, 
  by 
  further 
  subtracting 
  8/4 
  of 
  the 
  length 
  of 
  the 
  earth's 
  radius 
  to 
  allow 
  

   roughly 
  for 
  the 
  observations 
  being 
  made 
  from 
  the 
  surface 
  of 
  the 
  earth 
  

   and 
  not 
  from 
  its 
  centre. 
  Of 
  course, 
  if 
  tnese 
  approximations 
  are 
  not 
  

   thought 
  sufficient, 
  the 
  observer 
  can 
  make 
  a 
  separate 
  calculation 
  for 
  each 
  

   day 
  of 
  observation, 
  using 
  the 
  exact 
  values 
  for 
  that 
  day 
  : 
  but 
  the 
  author 
  

   did 
  not 
  think 
  it 
  necessary 
  to 
  take 
  this 
  trouble. 
  

  

  