﻿1875.] 
  Povjer 
  of 
  the 
  Eye 
  and 
  the 
  Microscope. 
  531 
  

  

  from 
  bands 
  near 
  the 
  middle 
  of 
  the 
  photographs, 
  there 
  could 
  be 
  no 
  doubt 
  

   whether 
  the 
  line 
  was 
  a 
  fringe 
  or 
  not. 
  

  

  The 
  following 
  Table 
  contains 
  the 
  results 
  of 
  these 
  observations 
  : 
  — 
  - 
  

  

  Measures 
  of 
  Robert's 
  Test-lines. 
  

  

  

  Number 
  

  

  

  Width 
  of 
  

  

  

  Number 
  to 
  the 
  

  

  inch. 
  

  

  

  Band. 
  

  

  of 
  

  

  

  

  

  

  

  

  Eatio. 
  

  

  lines. 
  

  

  Line. 
  

  

  Space. 
  

  

  Band. 
  

  

  Lines. 
  

  

  Spaces. 
  

  

  Both. 
  

  

  I. 
  

  

  7 
  

  

  28-57 
  

  

  

  

  58-67 
  

  

  553 
  

  

  35,000 
  

  

  17,040 
  

  

  11,460 
  

  

  1-00 
  

  

  II. 
  

  

  10 
  

  

  35-10 
  

  

  23 
  00 
  

  

  560 
  

  

  28,500 
  

  

  43,450 
  

  

  17,210 
  

   23,420 
  

  

  1-68 
  

  

  in! 
  

  

  13 
  

  

  27-85 
  

  

  14-85 
  

  

  555 
  

  

  35,910 
  

  

  67,340 
  

  

  211 
  

  

  iv! 
  

  

  15 
  

  

  i.v 
  lo 
  

  

  10 
  0/ 
  

  

  505 
  

  

  

  

  28,820 
  

  

  3*10 
  

  

  v. 
  

  

  17 
  

  

  1410 
  

  

  15-20 
  

  

  480 
  

  

  71,430 
  

  

  65,790 
  

  

  34,250 
  

  

  3-87 
  

  

  VT 
  

  

  V 
  _L. 
  

  

  20 
  

  

  11-55 
  

  

  lo 
  yu 
  

  

  495 
  

  

  oO,OoU 
  

  

  

  39,290 
  

  

  4-23 
  

  

  VII. 
  

  

  23 
  

  

  12-64 
  

  

  9-14 
  

  

  505 
  

  

  79,110 
  

  

  109,410 
  

  

  45^910 
  

  

  4-65 
  

  

  VIII. 
  

  

  25 
  

  

  8-71 
  

  

  10-71 
  

  

  475 
  

  

  114,810 
  

  

  93,370 
  

  

  51,390 
  

  

  549 
  

  

  ix". 
  

  

  28 
  

  

  7.4.7 
  

  

  993 
  

  

  478 
  

  

  133,870 
  

  

  100,700 
  

  

  57,470 
  

  

  5 
  - 
  92 
  

  

  X. 
  

  

  30 
  

  

  8-11 
  

  

  744 
  

  

  460 
  

  

  123.300 
  

  

  134,410 
  

  

  64,310 
  

  

  7-25 
  

  

  XL 
  

  

  34 
  

  

  8-10 
  

  

  6-90 
  

  

  500 
  

  

  123,300 
  

  

  144,930 
  

  

  66,670 
  

  

  7-25 
  

  

  XII. 
  

  

  37 
  

  

  6-81 
  

  

  7-28 
  

  

  503 
  

  

  146,840 
  

  

  137,360 
  

  

  70,970 
  

  

  8-08 
  

  

  XIII. 
  

  

  40 
  

  

  6-62 
  

  

  6-00 
  

  

  500 
  

  

  151,060 
  

  

  166,670 
  

  

  79,240 
  

  

  8-89 
  

  

  XIV. 
  

  

  43 
  

  

  6-00 
  

  

  6-00 
  

  

  510 
  

  

  166,670 
  

  

  166,670 
  

  

  83,330 
  

  

  9-81 
  

  

  XV. 
  

  

  45 
  

  

  5-56 
  

  

  5-56 
  

  

  495 
  

  

  179,860 
  

  

  179,860 
  

  

  89,930 
  

  

  10-50 
  

  

  XVI. 
  

   XVII. 
  

  

  (40) 
  

   (40) 
  

  

  

  

  522 
  

  

  77,280 
  

   77,880 
  

  

  

  

  

  1 
  

  

  r503l 
  

   1512) 
  

  

  

  

  

  

  

  1 
  

  

  

  

  

  XVIII. 
  

  

  (40) 
  

  

  

  

  i 
  

  

  '489-1 
  

   511/ 
  

  

  

  

  79,240 
  

  

  

  

  

  

  

  

  

  XIX. 
  

  

  (40) 
  

  

  

  

  

  '540 
  

  

  

  

  75,760? 
  

  

  

  

  

  

  

  

  

  Notes. 
  — 
  These 
  measures 
  are 
  frequently 
  mere 
  approximations 
  ; 
  and 
  in 
  several 
  bands 
  

   the 
  graving-point 
  has 
  made 
  a 
  wonderful 
  approach 
  to 
  an 
  equality 
  of 
  width 
  of 
  lines 
  and 
  

   spaces 
  ; 
  indeed 
  these 
  lines 
  are 
  marvels 
  of 
  mechanical 
  skill. 
  If, 
  in 
  the 
  case 
  of 
  each 
  band, 
  

   the 
  first 
  and 
  last 
  lines 
  had 
  been 
  drawn 
  longer 
  than 
  the 
  rest, 
  it 
  would 
  have 
  been 
  pos- 
  

   sible 
  to 
  measure 
  the 
  width 
  of 
  a 
  line 
  with 
  considerable 
  accuracy, 
  since, 
  as 
  has 
  been 
  shown, 
  

   the 
  visibility 
  of 
  a 
  single 
  line 
  is 
  nearly 
  twenty 
  times 
  that 
  for 
  the 
  series. 
  

  

  The 
  widths 
  of 
  the 
  lines 
  and 
  spaces 
  are 
  those 
  taken 
  from 
  the 
  photographs, 
  the 
  unit 
  

   being 
  j 
  inch. 
  The 
  photographs 
  are 
  magnified 
  to 
  1000 
  times. 
  In 
  bands 
  XVII. 
  and 
  

   1000 
  

  

  XVIII. 
  second 
  measures 
  are 
  given 
  from 
  photographs 
  magnifying 
  to 
  1600 
  times 
  (but 
  re- 
  

   duced 
  to 
  the 
  same 
  unit) 
  . 
  The 
  number 
  of 
  lines 
  in 
  ( 
  ) 
  are 
  the 
  numbers 
  counted 
  for 
  which 
  the 
  

   total 
  width 
  was 
  measured. 
  The 
  number 
  for 
  the 
  XlX.th 
  is 
  deduced 
  from 
  the 
  measure 
  of 
  a 
  

   few 
  where 
  the 
  lines 
  were 
  most 
  distinct. 
  The 
  numbers 
  of 
  lines 
  and 
  spaces 
  to 
  an 
  inch 
  

   are 
  the 
  numbers 
  which 
  could 
  be 
  put 
  in 
  an 
  inch 
  laid 
  side 
  by 
  side 
  (without 
  interval). 
  

   L 
  T 
  nder 
  " 
  Both 
  " 
  is 
  given 
  the 
  number 
  of 
  lines 
  to 
  the 
  inch 
  (with 
  interspaces), 
  as 
  in 
  the 
  bands. 
  

   The 
  "Eatio 
  " 
  is 
  that 
  of 
  the 
  number 
  for 
  the 
  widest 
  space 
  (17,000 
  to 
  the 
  inch) 
  to 
  the 
  number 
  

   for 
  the 
  widest 
  line 
  or 
  space 
  in 
  the 
  following 
  bands. 
  

  

  It 
  will 
  be 
  seen 
  that 
  the 
  least 
  width 
  of 
  the 
  lines 
  which 
  can 
  be 
  counted 
  and 
  

   measured 
  on 
  the 
  photographs 
  is 
  about 
  1(j0) 
  1 
  00 
  of 
  an 
  inch 
  (XIILth 
  band). 
  

   ~\Ve 
  have 
  seen 
  (5th 
  observation) 
  that 
  dark 
  parallel 
  lines 
  on 
  glass 
  can 
  be 
  

   seen 
  with 
  transmitted 
  light 
  when 
  their 
  width 
  subtends 
  an 
  angle 
  of 
  20" 
  to 
  

   26": 
  so 
  that 
  lines 
  stopping 
  the 
  light 
  moderately 
  (7th 
  observation) 
  of 
  lti0i 
  1 
  00 
  

  

  