﻿1875.] 
  

  

  Power 
  of 
  the 
  Eye 
  and 
  the 
  Microscope. 
  

  

  523 
  

  

  fied 
  photographs 
  of 
  Nobert's 
  test-bands 
  by 
  Dr. 
  E. 
  Carter, 
  Surgeon 
  of 
  the 
  

   U.S. 
  Army. 
  The 
  results 
  of 
  this 
  examination 
  will 
  be 
  given 
  at 
  the 
  end 
  of 
  

   this 
  note. 
  

  

  I 
  was 
  induced 
  meanwhile 
  to 
  examine 
  the 
  power 
  of 
  the 
  eye, 
  in 
  order 
  to 
  

   compare 
  it 
  with 
  the 
  power 
  of 
  the 
  microscope, 
  and 
  to 
  determine 
  what 
  a 
  

   microscope 
  of 
  given 
  power 
  should 
  be 
  able 
  to 
  show. 
  The 
  following 
  obser- 
  

   vations 
  had 
  been 
  made 
  early 
  in 
  1869, 
  before 
  I 
  was 
  acquainted 
  with 
  the 
  

   observations 
  of 
  Dr. 
  Jurin 
  and 
  of 
  Tobias 
  Mayer, 
  to 
  which 
  I 
  shall 
  allude. 
  

  

  The 
  first 
  question 
  which 
  presented 
  itself 
  was 
  as 
  to 
  the 
  power 
  of 
  the 
  

   eye 
  to 
  see 
  single 
  lines 
  under 
  the 
  ordinary 
  illumination 
  of 
  a 
  northern 
  sky. 
  

  

  1st 
  observation. 
  — 
  A 
  black 
  line 
  0*042 
  inch 
  wide, 
  1'75 
  inch 
  long, 
  drawn 
  

   with 
  common 
  writing-ink 
  on 
  white 
  paper, 
  and 
  a 
  white 
  line 
  of 
  the 
  same 
  

   width 
  and 
  length 
  between 
  two 
  black 
  lines, 
  each 
  0'20 
  inch 
  wide, 
  were 
  seen 
  

   equally 
  well 
  within 
  a 
  room 
  lighted 
  by 
  a 
  window 
  to 
  N.W. 
  at 
  a 
  distance 
  of 
  

   30 
  feet, 
  the 
  angle 
  subtended 
  by 
  the 
  width 
  of 
  the 
  lines 
  being 
  24" 
  nearly. 
  

  

  2nd 
  observation. 
  — 
  A'dark-brown 
  hair, 
  0-0026 
  inch 
  wide, 
  2*5 
  inches 
  long, 
  

   was 
  fixed 
  by 
  dots 
  of 
  transparent 
  gum-arabic 
  to 
  the 
  window-pane, 
  and 
  

   was 
  seen 
  against 
  the 
  N.W. 
  sky 
  by 
  a 
  young 
  eye 
  at 
  36 
  feet 
  (I 
  could 
  

   not 
  see 
  it 
  myself 
  at 
  a 
  greater 
  distance 
  than 
  30 
  feet) 
  : 
  the 
  diameter 
  of 
  

   the 
  hair 
  subtended 
  an 
  angle 
  of 
  1"'24 
  at 
  the 
  eye. 
  The 
  same 
  eye 
  exa- 
  

   mined 
  fine 
  lines 
  divided 
  on 
  glass 
  at 
  a 
  distance 
  of 
  6 
  inches, 
  and, 
  other 
  

   things 
  equal, 
  should 
  have 
  been 
  able 
  to 
  see 
  a 
  line 
  21 
  \ 
  Q( 
  ^ 
  inc^h 
  wide 
  at 
  

   that 
  distance. 
  [June 
  5, 
  1875. 
  I 
  find 
  that 
  a 
  young 
  eye 
  can 
  see 
  lines 
  on 
  

   glass 
  i 
  - 
  Q 
  1 
  0Qo 
  inch 
  wide, 
  inch 
  long, 
  angle 
  3"*5 
  nearly.] 
  

  

  Dr. 
  Jurin 
  could 
  see 
  a 
  silver 
  wire 
  T 
  J- 
  5 
  - 
  inch 
  diameter 
  placed 
  on 
  white 
  

   paper 
  when 
  the 
  diameter 
  subtended 
  an 
  angle 
  of 
  3"*5, 
  and 
  a 
  silk 
  fibre 
  one 
  

   fourth 
  the 
  diameter 
  of 
  the 
  wire 
  when 
  it 
  subtended 
  an 
  angle 
  of 
  3"*35*. 
  

  

  3rd 
  observation. 
  — 
  Whether 
  the 
  length 
  of 
  the 
  line 
  affects 
  its 
  visibility. 
  

   The 
  hair 
  just 
  observed 
  was 
  cut 
  into 
  pieces 
  of 
  different 
  lengths 
  and 
  fixed, 
  

   as 
  before, 
  to 
  the 
  window-pane; 
  they 
  could 
  be 
  seen 
  at 
  the 
  following 
  

   distances 
  : 
  — 
  

  

  Length 
  

  

  Distance 
  

  

  Angle 
  subtended 
  by 
  

  

  of 
  hair. 
  

  

  seen. 
  

  

  Diameter. 
  

  

  Length. 
  

  

  in. 
  

   090 
  

  

  feet. 
  

   37 
  

  

  l"-21 
  

  

  413 
  

  

  0-25 
  

  

  32 
  

  

  1-39 
  

  

  134 
  

  

  0-133 
  

  

  22 
  

  

  2-03 
  

  

  104 
  

  

  0-020 
  

  

  10 
  

  

  4-46 
  

  

  86 
  

  

  * 
  See 
  Jurin's 
  essay 
  on 
  distinct 
  and 
  indistinct 
  vision 
  in 
  Smith's 
  ' 
  Complete 
  System 
  of 
  

   Opticks,' 
  1738. 
  I 
  am 
  acquainted 
  with 
  Jurin's 
  observations 
  from 
  the 
  Rev. 
  Father 
  

   Pejzenas's 
  translation 
  of 
  Smith's 
  work, 
  ' 
  Cours 
  complet 
  d'Optique,' 
  1767, 
  p. 
  282. 
  

  

  