﻿526 
  

  

  Mr. 
  J. 
  A. 
  Broun 
  on 
  the 
  

  

  [June 
  17, 
  

  

  Lines 
  and 
  intervals 
  equal. 
  

  

  Lines 
  and. 
  intervals 
  unequal. 
  

  

  Mean 
  

  

  

  

  Width 
  of 
  

  

  

  

  ■width. 
  

  

  

  A 
  1 
  

   lg 
  e. 
  

  

  

  

  

  .Angle. 
  

  

  

  

  Lines. 
  

  

  Intervals. 
  

  

  

  

  in. 
  

  

  ieet. 
  

  

  

  

  

  feet. 
  

  

  37-0 
  

  

  0-021 
  

  

  8-3 
  

  

  44 
  

  

  oW 
  

  

  (K)21 
  

  

  95 
  

  

  0041 
  

  

  15*5 
  

  

  45 
  

  

  0-045 
  

  

  0-079 
  

  

  21-7 
  

  

  38-0 
  

  

  0-081 
  

  

  25-0 
  

  

  56 
  

  

  0-043 
  

  

  0121 
  

  

  28-5 
  

  

  260 
  

  

  0-118 
  

  

  36-7 
  

  

  55 
  

  

  0-044 
  

  

  0-164 
  

  

  340 
  

  

  22-0 
  

  

  0-164 
  

  

  47*5 
  

  

  59 
  

  

  0-044 
  

  

  0-203 
  

  

  390 
  

  

  19-4 
  

  

  

  

  

  0-045 
  

  

  0-241 
  

  

  40-5 
  

  

  19-0 
  

  

  

  

  

  0-044 
  

  

  0-477 
  

  

  55-5 
  

  

  13-5 
  

  

  "When 
  the 
  lines 
  and 
  intervals 
  were 
  equal, 
  the 
  angle 
  increased 
  from 
  the 
  

   smallest 
  (0*021 
  inch) 
  to 
  the 
  largest 
  (0*164 
  inch) 
  ; 
  the 
  greater 
  angle 
  for 
  

   the 
  middle 
  width 
  (0-081) 
  is 
  probably 
  connected 
  with 
  some 
  irregularity 
  in 
  

   the 
  lines 
  ; 
  the 
  increase 
  of 
  the 
  angle 
  at 
  the 
  greater 
  distance 
  is 
  probably 
  

   chiefly 
  due 
  to 
  the 
  constancy 
  of 
  the 
  length 
  of 
  the 
  lines 
  (see 
  8th 
  obser- 
  

   vation) 
  . 
  

  

  When 
  the 
  lines 
  and 
  intervals 
  are 
  unequal, 
  the 
  angle 
  subtended 
  by 
  the 
  

   width 
  of 
  the 
  line 
  diminishes 
  as 
  the 
  interval 
  increases 
  ; 
  the 
  limiting 
  angle 
  

   would 
  be 
  that 
  at 
  which 
  a 
  single 
  line 
  0*70 
  inch 
  long 
  and 
  0*044 
  inch 
  wide 
  

   would 
  disappear. 
  

  

  7tli 
  observation. 
  — 
  It 
  was 
  now 
  sought 
  to 
  determine 
  in 
  what 
  degree 
  the 
  

   angle 
  of 
  visibility 
  for 
  parallel 
  lines 
  varied 
  with 
  the 
  relative 
  darkness 
  of 
  

   the 
  lines, 
  the 
  intervals 
  between 
  the 
  Hues 
  remaining 
  equally 
  bright. 
  One 
  

   drop 
  of 
  a 
  weak 
  writing-ink 
  having 
  been 
  mixed 
  with 
  thirty 
  drops 
  of 
  water, 
  

   four 
  lines, 
  0*70 
  inch 
  long, 
  0-081 
  inch 
  wide, 
  were 
  made 
  (at 
  intervals 
  of 
  

   0-081 
  inch) 
  with 
  a 
  camel-hair 
  pencil 
  ; 
  the 
  first 
  set 
  (IJ 
  received 
  only 
  one 
  

   coat, 
  the 
  second 
  (I 
  2 
  ) 
  two 
  coats, 
  and 
  I. 
  five 
  coats. 
  The 
  slips 
  of 
  paper 
  having 
  

   been 
  pinned 
  successively 
  to 
  the 
  wall 
  of 
  the 
  room 
  as 
  before, 
  the 
  following 
  

   are 
  the 
  results 
  of 
  the 
  observations 
  : 
  — 
  

  

  Distance. 
  Angle. 
  Calculated. 
  

  

  Tint. 
  feet. 
  

  

  I, 
  15-5 
  90*0 
  91*0 
  

  

  1 
  2 
  19*5 
  71*5 
  69*7 
  

  

  1 
  3 
  21*5 
  64*7 
  63-1 
  

  

  1 
  4 
  23-0 
  60*5 
  60-0 
  

  

  I. 
  23-6 
  59*0 
  58*4 
  

  

  l' 
  n 
  25-0 
  55-7 
  56 
  : 
  

  

  The 
  last 
  (1,0 
  is 
  the 
  observation 
  already 
  given 
  (6th 
  observation) 
  for 
  the 
  

   same 
  lines 
  made 
  with 
  a 
  drawing-pen 
  and 
  as 
  black 
  as 
  they 
  could 
  be 
  made. 
  

   It 
  was 
  found 
  that 
  one 
  hundred 
  coats 
  of 
  watered 
  ink 
  did 
  not 
  make 
  lines 
  so 
  

   dark 
  as 
  those 
  of 
  I„ 
  ; 
  and 
  it 
  is 
  obvious 
  that 
  increase 
  of 
  darkness 
  after 
  the 
  

   fifth 
  coat 
  (a 
  very 
  faint 
  shade) 
  made 
  little 
  difference 
  in 
  the 
  visibility 
  of 
  the 
  

  

  