﻿330 
  Hutchins 
  — 
  Adjustment 
  of 
  the 
  Quartz 
  Spectrograph. 
  

  

  and 
  advance 
  the 
  plate 
  each 
  time, 
  so 
  as 
  to 
  secure 
  a 
  succession 
  of 
  

   images. 
  

  

  From 
  an 
  inspection 
  of 
  the 
  resulting 
  negative 
  the 
  point 
  of 
  

   sharpest 
  focus 
  at 
  the 
  middle 
  of 
  the 
  plate 
  may 
  be 
  found. 
  On 
  a 
  

   plate 
  2 
  inches 
  wide 
  6 
  or 
  8 
  exposures 
  may 
  be 
  made. 
  The 
  nega- 
  

   tive 
  obtained 
  as 
  above 
  will 
  probably 
  also 
  show 
  which 
  way 
  the 
  

   plate 
  must 
  be 
  inclined 
  in 
  order 
  to 
  bring 
  the 
  ends 
  into 
  focus, 
  

   which 
  is 
  accomplished 
  by 
  successive 
  small 
  changes 
  of 
  inclina- 
  

   tion 
  and 
  exposure 
  as 
  outlined 
  above. 
  When 
  the 
  proper 
  incli- 
  

   nation 
  is 
  found, 
  it 
  is 
  best 
  to 
  go 
  back 
  and 
  change 
  the 
  focus 
  by 
  

   fractions 
  of 
  a 
  millimeter 
  at 
  a 
  time 
  in 
  order 
  to 
  secure 
  the 
  finest 
  

   definition 
  possible. 
  The 
  operator 
  is 
  advised 
  to 
  use 
  short 
  expo- 
  

   sures, 
  as 
  over-exposed 
  lines 
  often 
  present 
  the 
  same 
  appearance 
  

   as 
  lines 
  out 
  of 
  focus. 
  

  

  An 
  improvement 
  in 
  the 
  definition 
  in 
  the 
  region 
  of 
  long 
  

   waves 
  may 
  often 
  be 
  effected 
  by 
  partial 
  screening 
  of 
  the 
  prism 
  

   by 
  placing 
  stops 
  over 
  the 
  lenses. 
  With 
  a 
  prism 
  4 
  cm 
  high 
  and 
  

   5 
  cm 
  on 
  the 
  face 
  the 
  writer 
  uses 
  stops 
  of 
  3*5 
  cin 
  opening. 
  This 
  

   opening 
  being 
  greater 
  than 
  the 
  curtate 
  face 
  of 
  the 
  prism, 
  there 
  

   is 
  no 
  loss 
  of 
  resolving 
  power, 
  and 
  the 
  loss 
  of 
  light 
  is, 
  in 
  all 
  ordi- 
  

   nary 
  cases, 
  immaterial. 
  

  

  With 
  a 
  good 
  optical 
  equipment 
  the 
  above 
  proceeding 
  should 
  

   result 
  in 
  excellent 
  definition 
  from 
  end 
  to 
  end 
  of 
  the 
  plate, 
  the 
  

   lines 
  showing 
  sharp 
  under 
  magnification 
  and 
  free 
  from 
  wings. 
  

  

  Bowdoin 
  College, 
  Brunswick, 
  Me., 
  July 
  17. 
  

  

  