﻿1875.] 
  

  

  Power 
  of 
  the 
  Eye 
  and 
  the 
  Microscope. 
  

  

  525 
  

  

  5th 
  observation. 
  — 
  Four 
  dark-brown 
  hairs 
  having 
  been 
  arranged 
  on 
  paper 
  

   at 
  equal 
  intervals 
  by 
  means 
  of 
  modeller's 
  wax 
  attached 
  to 
  the 
  ends, 
  and 
  then 
  

   fixed 
  at 
  the 
  ends 
  with 
  gum-arabic, 
  the 
  paper 
  was 
  cut 
  away 
  between 
  the 
  

   ends, 
  and 
  the 
  two 
  slips 
  of 
  paper 
  to 
  which 
  the 
  ends 
  were 
  gummed 
  were 
  

   fixed 
  to 
  the 
  window-pane 
  : 
  the 
  diameter 
  of 
  the 
  hairs 
  was 
  inch 
  

   (0-00267) 
  very 
  nearly, 
  the 
  lengths 
  were 
  nearly 
  1 
  inch, 
  and 
  the 
  intervals 
  

   were 
  very 
  nearly 
  the 
  same 
  as 
  the 
  diameter 
  of 
  the 
  hair, 
  the 
  whole- 
  width 
  

   being 
  0-019 
  inch. 
  The 
  hairs 
  could 
  be 
  seen 
  to 
  be 
  more 
  than 
  one 
  at 
  28 
  

   inches 
  distance, 
  and 
  they 
  could 
  be 
  counted 
  at 
  21 
  inches 
  ; 
  the 
  angles 
  sub- 
  

   tended 
  by 
  the 
  intervals 
  at 
  these 
  distances 
  were 
  20" 
  and 
  26"*5 
  respectively. 
  

  

  It 
  thus 
  appears 
  that 
  at 
  a 
  distance 
  greater 
  than 
  28 
  inches 
  the 
  four 
  

   hairs 
  appeared 
  as 
  one, 
  when 
  each 
  hair 
  subtended 
  an 
  angle 
  sixteen 
  

   times 
  greater 
  than 
  it 
  could 
  be 
  observed 
  at 
  when 
  seen 
  alone 
  (see 
  the 
  3rd 
  

   observation). 
  This 
  curious 
  fact 
  was 
  pointed 
  out 
  by 
  Dr. 
  Jurin 
  in 
  the 
  

   essay 
  already 
  cited. 
  He 
  found 
  when 
  two 
  pins 
  were 
  placed 
  near 
  to 
  each 
  

   other 
  on 
  a 
  window, 
  that 
  the 
  interval 
  between 
  them 
  could 
  not 
  be 
  perceived 
  

   when 
  it 
  subtended 
  an 
  angle, 
  of 
  40", 
  whereas 
  a 
  single 
  pin 
  could 
  be 
  

   seen 
  at 
  an 
  angle 
  of 
  from 
  2 
  to 
  3 
  seconds. 
  Mayer, 
  who 
  also 
  made 
  ob- 
  

   servations 
  on 
  parallel 
  lines 
  nearly 
  twenty 
  years 
  after 
  Jurin, 
  does 
  not 
  

   seem 
  to 
  have 
  remarked 
  this 
  fact 
  *. 
  

  

  The 
  5th 
  observation 
  was 
  repeated 
  with 
  four 
  white 
  hairs 
  from 
  a 
  horse's 
  

   tail 
  ; 
  these 
  were 
  arranged 
  at 
  equal 
  intervals, 
  the 
  mean 
  diameter 
  of 
  the 
  

   hair 
  being 
  0*0105 
  and 
  the 
  mean 
  interval 
  0*0110 
  inch. 
  The 
  hairs 
  could 
  

   be 
  seen 
  to 
  be 
  more 
  than 
  one 
  9 
  feet 
  distant, 
  when 
  the 
  angle 
  subtended 
  

   by 
  each 
  hair 
  was 
  23"*1 
  ; 
  and 
  the 
  hairs 
  could 
  be 
  counted 
  at 
  6 
  feet 
  dis- 
  

   tant, 
  when 
  the 
  angle 
  was 
  30"*7. 
  These 
  angles 
  are 
  about 
  one 
  sixth 
  greater 
  

   than 
  for 
  the 
  human 
  hairs, 
  the 
  difference 
  being 
  probably 
  due 
  to 
  the 
  differ- 
  

   ence 
  of 
  light, 
  and 
  perhaps 
  partly 
  to 
  the 
  different 
  length, 
  which 
  was 
  not 
  

   noted 
  t. 
  

  

  6th 
  observation. 
  — 
  A 
  series 
  of 
  lines 
  0*7 
  inch 
  long 
  were 
  drawn 
  on 
  sepa- 
  

   rate 
  slips 
  of 
  paper 
  with 
  different 
  widths 
  and 
  intervals. 
  The 
  papers 
  were 
  

   fixed 
  successively 
  to 
  the 
  wall 
  of 
  a 
  room 
  lighted 
  by 
  a 
  window 
  to 
  N.W., 
  

   the 
  light 
  falling 
  at 
  an 
  angle 
  of 
  about 
  45° 
  on 
  the 
  paper. 
  The 
  following 
  

   Table 
  contains 
  the 
  results 
  of 
  the 
  observations, 
  first, 
  when 
  the 
  intervals 
  

   and 
  lines 
  were 
  of 
  equal 
  width, 
  and, 
  second, 
  when 
  the 
  intervals 
  were 
  1, 
  2, 
  

   3, 
  . 
  . 
  . 
  times 
  the 
  width 
  of 
  the 
  lines. 
  

  

  * 
  Mayer's 
  observations 
  are 
  given 
  in 
  Pezenas's 
  translation 
  of 
  Smith's 
  ' 
  Optics/ 
  t. 
  ii. 
  

   p. 
  409. 
  I 
  am 
  not 
  acquainted 
  with 
  the 
  original 
  memoir.. 
  As 
  already 
  stated, 
  Jurin's 
  

   and 
  Mayer's 
  observations 
  were 
  known 
  to 
  me 
  only 
  after 
  the 
  above 
  observations 
  had 
  been 
  

   made. 
  

  

  t 
  Dr. 
  Jurin 
  has 
  given, 
  as 
  an 
  example 
  of 
  the 
  difficulty 
  of 
  counting 
  parallel 
  fines, 
  the 
  

   following 
  series: 
  — 
  

  

  and 
  has 
  shown 
  the 
  advantage 
  of 
  employing 
  commas 
  in 
  such 
  numbers 
  as 
  the 
  following, 
  

   100000000000 
  and 
  100,000,000,000. 
  

  

  