﻿796 
  . 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  log/? 
  against 
  t 
  ; 
  this 
  is 
  accurate 
  for 
  5° 
  intervals. 
  Between 
  

   365° 
  and 
  450°, 
  formula 
  L 
  will 
  give 
  p 
  for 
  temperatures 
  not 
  

   given. 
  

  

  Density 
  and 
  Pressure. 
  

  

  It 
  should 
  be 
  remembered 
  that 
  the 
  vapour-pressure 
  of 
  

   mercury 
  has 
  not 
  been 
  observed 
  below 
  89°*4. 
  Pfaundler 
  and 
  

   Morley 
  observed 
  the 
  density 
  (for 
  they 
  measured 
  the 
  mass 
  

   per 
  unit 
  volume) 
  of 
  mercury-vapour 
  in 
  gases 
  saturated 
  with 
  

   it 
  at 
  different 
  temperatures, 
  and 
  calculated 
  the 
  pressure 
  

   assuming 
  that 
  the 
  vapour 
  obe}*s 
  the 
  gas 
  laws 
  and 
  has 
  a 
  

   density 
  of 
  100 
  (H 
  2 
  =l). 
  This 
  is 
  justified 
  at 
  higher 
  tempe- 
  

   ratures 
  ; 
  for 
  Gebhardt, 
  using 
  these 
  assumptions, 
  has 
  calculated 
  

   the 
  vapour-pressure 
  from 
  the 
  densities 
  of 
  mercury-vapour 
  as 
  

   observed 
  by 
  Jewett 
  *. 
  The 
  agreement 
  of 
  his 
  results 
  with 
  the 
  

   actual 
  pressures 
  (see 
  Table 
  Y.) 
  is 
  interesting, 
  for 
  it 
  shows 
  

   that 
  the 
  saturated 
  vapour 
  of 
  mercury 
  is 
  wholly 
  monatomic 
  at 
  

   nearly 
  half-an-atmosphere 
  pressure. 
  

  

  Table 
  Y. 
  

  

  Temperature... 
  140°. 
  160°. 
  180°. 
  200°. 
  220°. 
  240°. 
  260°. 
  280°. 
  300°. 
  310°. 
  

  

  ^from'L^ity.} 
  1 
  ' 
  84 
  4 
  " 
  23 
  91 
  24 
  ' 
  2 
  32 
  ' 
  2 
  62 
  ' 
  3 
  100 
  ' 
  6 
  155 
  25 
  ° 
  311mm. 
  

  

  Pr 
  TaWe 
  e 
  iV° 
  m 
  ^ 
  1<89 
  4 
  * 
  30 
  °" 
  05 
  17 
  ' 
  8 
  33 
  *° 
  58 
  ' 
  2 
  97 
  " 
  8 
  158 
  249 
  308 
  mm 
  - 
  

  

  It 
  does 
  not 
  appear 
  that 
  the 
  critical 
  point 
  of 
  mercury 
  has 
  

   been 
  reached 
  : 
  it 
  must 
  be 
  at 
  an 
  exceptionally 
  high 
  tempe- 
  

   rature, 
  apparently 
  above 
  850°. 
  

  

  LXYII. 
  Telescopic 
  Vision. 
  

   y 
  By 
  G. 
  Johnstone 
  Stoney, 
  M.A., 
  Sc.D., 
  F.R.S.t 
  

  

  Part 
  II. 
  — 
  Determination 
  of 
  the 
  kind 
  of 
  Image 
  which 
  

  

  CAN 
  BE 
  OBTAINED. 
  

  

  [Plate 
  XII] 
  

  

  CHAPTER 
  3. 
  

  

  The 
  elementary 
  parts 
  of 
  which 
  an 
  image 
  consists. 
  

  

  '62. 
  TN 
  the 
  First 
  Part 
  of 
  this 
  Memoir, 
  in 
  the 
  August 
  

  

  JL 
  number 
  of 
  the 
  Phil. 
  Mag., 
  wq 
  have 
  described 
  an 
  

  

  apparatus 
  (represented 
  in 
  Plate 
  XIL, 
  which 
  is 
  reproduced 
  

  

  in 
  the 
  present 
  number) 
  with 
  which 
  the 
  astronomer 
  can 
  make 
  

  

  experiments 
  that 
  will 
  disclose 
  to 
  him 
  the 
  imperfections 
  

  

  * 
  Phil. 
  Mag. 
  [6] 
  vol. 
  iv. 
  1902, 
  p. 
  546. 
  

   f 
  Communicated 
  by 
  the 
  Author. 
  

  

  