﻿976 
  

  

  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  can 
  measure 
  the 
  distance 
  from 
  tbe 
  centreof 
  the 
  concentration 
  

   image 
  to 
  the 
  most 
  distant 
  appendage 
  3pectrum 
  which 
  onght 
  

   to 
  be 
  retained. 
  The 
  distance 
  so 
  measured 
  he 
  is 
  to 
  employ 
  as 
  

   his 
  T 
  in 
  constructing 
  diagrams 
  like 
  those 
  of 
  figs. 
  2, 
  ',\, 
  and 
  4, 
  

   which 
  will 
  then 
  reveal 
  to 
  him 
  the 
  kind 
  of 
  vision 
  of 
  the 
  sup- 
  

   posed 
  object 
  upon 
  the 
  planet 
  which 
  his 
  astronomical 
  telescope 
  

   would 
  be 
  competent 
  to 
  supply. 
  

  

  91. 
  Many 
  experiments 
  with 
  the 
  microscope 
  throw 
  

   additional 
  light 
  upon 
  this 
  interesting 
  subject. 
  One 
  of 
  these 
  

   1 
  may 
  mention. 
  The 
  object 
  was 
  the 
  proboscis 
  of 
  the 
  Blow- 
  

   fly, 
  and 
  the 
  j 
  art 
  of 
  the 
  specimen 
  which 
  was 
  made 
  use 
  of 
  was 
  

  

  a 
  -mall 
  triangular 
  patch 
  of 
  bright 
  light 
  which 
  happened 
  to 
  

   be 
  -hut 
  in 
  between 
  three 
  of 
  the 
  hairs 
  or 
  bristles 
  which 
  grow 
  

   near 
  the 
  l<a<o 
  of 
  the 
  proboscis. 
  These 
  hairs 
  had 
  been 
  

   pressed 
  when 
  mounting 
  the 
  specimen, 
  so 
  that 
  they 
  lay 
  near 
  

   to 
  one 
  another 
  and 
  nearly 
  in 
  a 
  plane 
  perpendicular 
  to 
  the 
  

   optic 
  axis. 
  Another 
  thinner 
  hair 
  which 
  we 
  may 
  call 
  the 
  

   canal 
  crossed 
  the 
  triangle 
  and 
  lay 
  parallel 
  to 
  one 
  side. 
  This 
  

   canal 
  divided 
  the 
  triangle 
  of 
  light 
  into 
  a 
  smaller 
  triangle 
  

   below, 
  and 
  a 
  quadrilateral 
  space 
  above 
  the 
  canal. 
  Within 
  

   tin- 
  quadrilateral 
  was 
  seen 
  about 
  half 
  of 
  the 
  base 
  of 
  another 
  

   small 
  hair, 
  presenting 
  the 
  semicircular 
  appearance 
  in 
  fig. 
  5, 
  

   and 
  being 
  somewhat 
  darker 
  on 
  the 
  right-hand 
  part 
  of 
  the 
  

   semicircle 
  than 
  on 
  the 
  left. 
  The 
  rest 
  of 
  this 
  hair 
  lay 
  out- 
  

   side 
  the 
  trianale 
  of 
  light 
  to 
  which 
  attention 
  is 
  being 
  called. 
  

  

  This 
  object 
  was 
  examined 
  through 
  one 
  of 
  Zeiss's 
  24 
  mm. 
  

   apochromatics, 
  over 
  which 
  an 
  iris 
  diaphragm 
  had 
  been 
  fitted 
  

   to 
  enable 
  the 
  observer 
  to 
  diminish 
  its 
  aperture 
  to 
  any 
  desired 
  

   extent. 
  The 
  succession 
  of 
  appearances 
  while 
  the 
  aperture 
  

   was 
  being 
  diminished 
  was 
  most 
  suggestive. 
  The 
  object 
  was 
  

   fairly 
  well 
  seen 
  until 
  the 
  aperture 
  became 
  rather 
  small, 
  but 
  

   then 
  on 
  still 
  further 
  contracting 
  the 
  aperture 
  a 
  succession 
  of 
  

   new 
  phenomena 
  sprang 
  into 
  existence, 
  until 
  at 
  a 
  certain 
  stage 
  

  

  