﻿530 
  

  

  Mr. 
  J. 
  A. 
  Broun 
  on 
  the 
  

  

  [June 
  17, 
  

  

  nishes 
  ; 
  on 
  the 
  other 
  hand, 
  /3, 
  the 
  angle 
  subtended 
  by 
  the 
  length 
  of 
  the 
  

   lines, 
  decreases 
  much 
  more 
  rapidly. 
  In 
  this 
  case, 
  as 
  in 
  that 
  of 
  the 
  4th 
  

   observation, 
  we 
  find 
  a 
  and 
  (3 
  are 
  connected 
  nearly 
  by 
  the 
  following 
  

   formula 
  : 
  — 
  

  

  a 
  ±= 
  constant 
  (9) 
  

  

  10th 
  observation. 
  — 
  The 
  long 
  parallel 
  lines 
  were 
  seen 
  by 
  different 
  per- 
  

   sons 
  at 
  a 
  greater 
  distance 
  when 
  inclined 
  to 
  the 
  horizontal 
  by 
  an 
  angle 
  of 
  

   about 
  25° 
  below 
  it 
  to 
  the 
  right 
  and 
  above 
  it 
  to 
  the 
  left, 
  but 
  the 
  visi- 
  

   bility 
  varies 
  at 
  different 
  angles 
  for 
  different 
  persons. 
  

  

  11th 
  observation. 
  — 
  Jurin's 
  observations 
  of 
  the 
  difference 
  of 
  visibility 
  of 
  

   parallel 
  lines 
  and 
  a 
  single 
  line 
  had 
  reference 
  to 
  the 
  case 
  of 
  'only 
  two 
  black 
  

   lines 
  with 
  a 
  white 
  line 
  between. 
  On 
  comparing 
  the 
  distances 
  at 
  which 
  

   parallel 
  lines 
  could 
  be 
  seen 
  0-4 
  inch 
  long 
  and 
  of 
  the 
  same 
  width 
  and 
  

   interval 
  as 
  before 
  (0*048 
  inch), 
  it 
  was 
  found 
  that 
  it 
  was 
  most 
  difficult 
  to 
  

   separate 
  the 
  lines 
  when 
  there 
  were 
  only 
  two. 
  The 
  following 
  are 
  the 
  ob- 
  

   servations 
  made 
  : 
  — 
  

  

  Number 
  of 
  lines. 
  D. 
  a. 
  

  

  ft. 
  in. 
  

  

  1 
  50 
  16 
  

  

  2 
  13 
  4 
  62 
  

  

  3 
  15 
  55 
  

  

  4 
  17 
  2 
  48 
  

  

  As 
  my 
  object 
  at 
  present 
  has 
  been 
  simply 
  to 
  state 
  the 
  facts 
  observed, 
  I 
  

   shall 
  now 
  proceed 
  to 
  my 
  examination 
  of 
  the 
  photographs 
  of 
  Robert's 
  test- 
  

   bands. 
  The 
  use 
  of 
  these 
  I 
  owed 
  to 
  the 
  kindness 
  of 
  Mr. 
  Eulenstein 
  (a 
  

   histologist 
  well 
  acquainted 
  with 
  the 
  microscope 
  and 
  its 
  tests), 
  who 
  in- 
  

   formed 
  me 
  that 
  these 
  lines 
  were 
  made 
  with 
  the 
  same 
  diamond-point, 
  but 
  

   that 
  the 
  pressure 
  on 
  the 
  point 
  is 
  made 
  to 
  increase 
  as 
  the 
  number 
  of 
  lines 
  

   to 
  the 
  inch 
  diminishes. 
  The 
  widths 
  of 
  the 
  lines 
  and 
  of 
  their 
  interspaces 
  

   were 
  measured 
  by 
  me 
  by 
  means 
  of 
  a 
  glass 
  scale 
  accurately 
  divided 
  with 
  

   one 
  thousand 
  divisions 
  to 
  the 
  inch, 
  the 
  dividing 
  lines 
  being 
  about 
  one 
  tenth 
  

   the 
  width 
  of 
  the 
  interspaces. 
  This 
  scale 
  was 
  placed 
  one 
  half 
  on 
  thin 
  white 
  

   paper, 
  the 
  other 
  covering 
  the 
  photographed 
  lines. 
  The 
  readings 
  were 
  

   made 
  with 
  the 
  aid 
  of 
  a 
  pocket-lens 
  of 
  1*1 
  inch 
  focal 
  distance. 
  The 
  width 
  

   of 
  every 
  line 
  and 
  interspace 
  was 
  measured 
  for 
  the 
  first 
  six 
  bands 
  ; 
  from 
  

   eight 
  to 
  ten 
  lines 
  and 
  spaces 
  were 
  measured 
  from 
  the 
  YH.th 
  to 
  the 
  Xlll.th 
  

   bands 
  inclusive 
  ; 
  in 
  each 
  case 
  the 
  means 
  of 
  these 
  measures 
  are 
  given. 
  

   For 
  the 
  remaining 
  bands 
  the 
  number 
  of 
  lines 
  and 
  their 
  spaces 
  were 
  counted 
  

   and 
  their 
  whole 
  width 
  observed 
  to 
  the 
  XVII.th 
  band. 
  In 
  the 
  XVJII.th 
  

   and 
  XlX.th 
  bands 
  the 
  lines 
  run 
  into 
  each 
  other 
  in 
  different 
  places 
  (in 
  some 
  

   of 
  the 
  previous 
  bands 
  the 
  lines 
  occasionally 
  fail). 
  It 
  was 
  evident 
  that 
  for 
  

   the 
  highest 
  bands 
  the 
  machine 
  failed 
  to 
  make 
  the 
  number 
  of 
  separate 
  

   lines 
  which 
  were 
  drawn. 
  In 
  several 
  cases 
  diffraction-fringes 
  interfered 
  

   with 
  the 
  accuracy 
  of 
  the 
  measures 
  ; 
  but 
  as 
  these 
  were 
  generally 
  made 
  

  

  