﻿522 
  

  

  Mr. 
  J. 
  A. 
  Broun 
  on 
  the 
  

  

  [June 
  17 
  } 
  

  

  X. 
  "On 
  the 
  Power 
  of 
  the 
  Eye 
  and 
  the 
  Microscope 
  to 
  see 
  

   Parallel 
  Lines/" 
  By 
  J. 
  A. 
  Broun, 
  E.R.S. 
  Received 
  June 
  

   16, 
  1875. 
  

  

  Since 
  Robert's 
  test-lines 
  have 
  been 
  employed 
  for 
  the 
  purpose 
  of 
  deter- 
  

   mining 
  the 
  comparative 
  powers 
  of 
  microscopes, 
  several 
  curious 
  specula- 
  

   tions 
  have 
  been 
  made 
  and 
  conclusions 
  arrived 
  at 
  by 
  different 
  well-known 
  

   microscopists 
  as 
  to 
  the 
  ultimate 
  capability 
  of 
  that 
  instrument 
  and 
  

   even 
  as 
  to 
  the 
  ultimate 
  atoms 
  of 
  matter. 
  The 
  lines 
  in 
  Robert's 
  

   test-bands 
  were 
  believed 
  to 
  approach 
  to 
  each 
  other 
  in 
  a 
  regularly 
  dimi- 
  

   nishing 
  series 
  of 
  distances, 
  such 
  that 
  when 
  the 
  intervals 
  left 
  by 
  the 
  

   graving-point 
  were 
  about 
  80> 
  q 
  Q 
  q 
  - 
  of 
  an 
  -English 
  inch 
  wide, 
  no 
  micro- 
  

   scope, 
  however 
  high 
  the 
  theoretical 
  power, 
  could 
  show 
  the 
  separation 
  of 
  

   the 
  lines. 
  

  

  The 
  Jury 
  of 
  the 
  Great 
  Exhibition 
  of 
  1851, 
  in 
  the 
  Eeport 
  (p. 
  268), 
  said 
  

   that 
  to 
  see 
  the 
  lines 
  in 
  the 
  first 
  and 
  second 
  bands 
  of 
  a 
  Robert's 
  test-plate 
  

   of 
  10 
  bands 
  a 
  power 
  of 
  100 
  was 
  sufficient, 
  whereas 
  to 
  distinguish 
  those 
  

   of 
  the 
  tenth 
  band 
  a 
  magnifying-power 
  of 
  2000 
  was 
  required. 
  Dr. 
  

   Lardner 
  (in 
  his 
  Museum 
  of 
  Science 
  and 
  Art) 
  considered 
  this 
  assertion 
  

   erroneous, 
  and 
  stated 
  that 
  if 
  a 
  power 
  of 
  100 
  could 
  show 
  the 
  lines 
  when 
  

   there 
  were 
  11,000 
  to 
  the 
  inch 
  (as 
  in 
  the 
  first 
  band), 
  a 
  power 
  of 
  450 
  

   should 
  show 
  the 
  lines 
  when 
  there 
  were 
  50,000 
  to 
  the 
  inch 
  (as 
  in 
  the 
  

   tenth 
  band). 
  As 
  there 
  can 
  be 
  no 
  doubt 
  of 
  the 
  accuracy 
  of 
  the 
  Jury's 
  

   statement, 
  there 
  has 
  been 
  considerable 
  difficulty 
  in 
  ascertaining 
  the 
  cause 
  

   of 
  the 
  great 
  difference 
  betwixt 
  fact 
  and 
  theory 
  ; 
  and 
  the 
  most 
  absurd 
  hy- 
  

   potheses 
  have 
  been 
  suggested 
  for 
  this 
  end. 
  This 
  and 
  the 
  speculations 
  of 
  

   different 
  microscopists 
  induced 
  me 
  several 
  years 
  ago 
  to 
  begin 
  the 
  exami- 
  

   nation 
  of 
  this 
  question. 
  

  

  The 
  first 
  thing 
  requiring 
  to 
  be 
  clearly 
  understood 
  was, 
  what 
  the 
  micro- 
  

   scopes 
  were 
  employed 
  to 
  examine, 
  what 
  Robert's 
  test-bands 
  really 
  were. 
  

   Some 
  microscopists 
  stated 
  the 
  number 
  of 
  lines 
  to 
  the 
  inch 
  when 
  they 
  

   could 
  not 
  be 
  seen 
  to 
  be 
  about 
  80,000 
  ; 
  whereas 
  Dr. 
  Carpenter 
  (in 
  his 
  

   work 
  on 
  the 
  Microscope 
  and 
  its 
  Revelations) 
  said 
  " 
  it 
  was 
  a 
  matter 
  of 
  

   faith 
  whether 
  lines 
  existed 
  at 
  a 
  narrower 
  interval 
  than 
  84 
  ,q 
  00 
  of 
  an 
  

   inch." 
  In 
  this 
  case 
  if 
  the 
  lines 
  were 
  as 
  wide 
  as 
  the 
  intervals, 
  the 
  num- 
  

   ber 
  to 
  the 
  inch 
  would 
  be 
  42,000. 
  It 
  was 
  then 
  essential 
  to 
  know 
  what 
  

   were 
  the 
  widths 
  of 
  the 
  lines 
  and 
  intervals 
  in 
  the 
  different 
  bands, 
  whether 
  

   as 
  the 
  intervals 
  would 
  appear 
  equally 
  light, 
  the 
  lines 
  were 
  equally 
  dark 
  ; 
  

   and, 
  finally, 
  with 
  reference 
  to 
  the 
  bands 
  in 
  which 
  the 
  lines 
  were 
  so 
  close 
  

   that 
  they 
  could 
  not 
  be 
  seen, 
  whether 
  there 
  really 
  were 
  lines 
  at 
  all, 
  or 
  lines 
  

   which 
  really 
  ought 
  to 
  be 
  seen 
  were 
  the 
  microscope 
  practically 
  equal 
  to 
  its 
  

   theoretical 
  power 
  — 
  whether, 
  in 
  fact, 
  the 
  failure 
  was 
  not 
  rather 
  in 
  the 
  en- 
  

   graving-machine 
  than 
  in 
  the 
  microscope. 
  

  

  It 
  was 
  only 
  in 
  1869 
  that 
  I 
  was 
  able, 
  through 
  the 
  kindness 
  of 
  Mr. 
  

   Eulenstein, 
  of 
  Dresden 
  (previously 
  of 
  Cannstadt), 
  to 
  examine 
  the 
  magni- 
  

  

  