﻿810 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  state 
  of 
  our 
  atmosphere 
  permit 
  ? 
  In 
  early 
  life 
  the 
  author 
  

   had 
  the 
  privilege 
  of 
  spending 
  some 
  years 
  as 
  Astronomical 
  

   Assistant 
  to 
  the 
  then 
  Earl 
  of 
  Eosse, 
  and 
  in 
  the 
  experience 
  

   he 
  had 
  while 
  in 
  charge 
  of 
  his 
  Observatory, 
  he 
  did 
  not 
  come 
  

   across 
  even 
  one 
  night 
  in 
  which 
  the 
  seeing 
  seemed 
  to 
  him 
  

   sufficiently 
  good 
  to 
  enable 
  a 
  planet 
  to 
  be 
  better 
  or 
  even 
  as 
  

   w 
  T 
  ell 
  seen 
  in 
  the 
  Great 
  Telescope 
  (except 
  perhaps 
  for 
  some 
  

   small 
  portion 
  of 
  a 
  second 
  at 
  a 
  time), 
  as 
  it 
  would 
  on 
  that 
  same 
  

   night 
  be 
  seen, 
  and 
  much 
  more 
  continuously, 
  in 
  smaller 
  in- 
  

   struments 
  if 
  optically 
  as 
  perfect. 
  He 
  had 
  opportunities 
  of 
  

   making 
  comparisons 
  between 
  apertures 
  of 
  6 
  feet, 
  3 
  feet, 
  and 
  

   20 
  inches. 
  And 
  on 
  almost 
  every 
  trial 
  the 
  state 
  of 
  the 
  air 
  

   was 
  such 
  that 
  the 
  best 
  vision 
  upon 
  a 
  planet 
  was 
  with 
  the 
  

   instrument 
  of 
  smaller 
  aperture. 
  

  

  52. 
  The 
  cause 
  of 
  this 
  was 
  made 
  impressively 
  evident 
  by 
  

   an 
  observation 
  he 
  had 
  occasion 
  to 
  make 
  in 
  1849 
  or 
  1850. 
  

   Lord 
  Rosse 
  had 
  two 
  specula 
  for 
  the 
  Great 
  Telescope, 
  with 
  

   one 
  of 
  which 
  he 
  continued 
  his 
  experiments 
  on 
  figuring 
  

   immense 
  mirrors, 
  while 
  the 
  other 
  was 
  in 
  use 
  in 
  the 
  telescope. 
  

   At 
  the 
  time 
  referred 
  to 
  the 
  mirrors 
  had 
  been 
  exchanged, 
  

   and 
  the 
  new 
  one 
  had 
  to 
  be 
  tested. 
  A 
  most 
  suitable 
  object 
  

   was 
  available 
  at 
  the 
  time 
  — 
  the 
  companion 
  of 
  <y 
  Andromedse, 
  

   itself 
  a 
  very 
  close 
  double 
  star 
  (0"*5 
  separation) 
  of 
  two 
  

   nearly 
  equal 
  constituents 
  and 
  not 
  too 
  bright 
  ; 
  and 
  it 
  was 
  

   besides 
  singularly 
  well 
  situated 
  as 
  it 
  crossed 
  the 
  meridian 
  

   not 
  far 
  from 
  the 
  zenith. 
  An 
  exceptionally 
  good 
  night 
  was 
  

   selected, 
  and 
  special 
  arrangements 
  were 
  made 
  by 
  which 
  the 
  

   mirrors 
  of 
  the 
  telescope 
  came 
  almost 
  accurately 
  into 
  adjust- 
  

   ment 
  when 
  the 
  telescope 
  was 
  pointed 
  towards 
  the 
  star. 
  The 
  

   constituents 
  of 
  this 
  excessively 
  close 
  double 
  were 
  then 
  (but 
  

   never 
  for 
  more 
  than 
  a 
  small 
  fraction 
  of 
  a 
  second) 
  exquisitely 
  

   seen 
  like 
  the 
  light 
  of 
  a 
  flame 
  seen 
  through 
  two 
  minute 
  holes 
  

   made 
  with 
  a 
  fine 
  needle 
  in 
  a 
  piece 
  of 
  card, 
  and 
  with 
  a 
  lane 
  

   of 
  sky 
  between 
  them 
  wider 
  I 
  think, 
  than 
  each 
  of 
  the 
  star 
  

   images. 
  This 
  appearance 
  every 
  now 
  and 
  again 
  presented 
  

   itself 
  like 
  a 
  flash, 
  but 
  w 
  r 
  as 
  almost 
  immediately 
  swallowed 
  up 
  

   by 
  the 
  two 
  images 
  boiling 
  together 
  into 
  a 
  single 
  larger 
  mass 
  

   of 
  light 
  which 
  seemed 
  in 
  a 
  state 
  of 
  intense 
  commotion. 
  This 
  

   was 
  the 
  usual 
  appearauce, 
  even 
  on 
  so 
  good 
  a 
  night 
  ; 
  and 
  the 
  

   moments 
  of 
  real 
  definition 
  were 
  only 
  occasional. 
  The 
  dis- 
  

   tance 
  between 
  the 
  two 
  stars 
  was 
  at 
  that 
  time 
  only 
  half 
  a 
  

   second 
  of 
  angle 
  according 
  to 
  Professor 
  See 
  who 
  has 
  studied 
  

   this 
  binary 
  system. 
  The 
  two 
  stars 
  are 
  sensibly 
  equal 
  and 
  

   were 
  of 
  a 
  remarkably 
  intense 
  blue 
  as 
  seen 
  in 
  the 
  Great 
  

   Telescope. 
  The 
  powers 
  employed 
  upon 
  the 
  telescope 
  were 
  

   750 
  and 
  1300, 
  and 
  the 
  object 
  was 
  exquisitely 
  seen 
  with 
  them 
  

   both, 
  perhaps 
  most 
  pleasingly 
  with 
  the 
  higher 
  power, 
  as 
  it 
  

  

  