﻿136 
  

  

  D. 
  A. 
  Kr 
  eider 
  — 
  Detection 
  and 
  

  

  2. 
  

  

  mounted 
  in 
  cork 
  discs 
  which 
  permit 
  of 
  either 
  one 
  being 
  easily 
  

  

  brought 
  in 
  or 
  out 
  of 
  tlie 
  field 
  ; 
  y 
  is 
  

   a 
  mirror 
  which 
  reflects 
  the 
  light 
  from 
  

   X 
  vertically 
  into 
  the 
  polarizer. 
  By 
  

   employing 
  a 
  long 
  rubber 
  tube 
  to 
  

   supply 
  a?, 
  the 
  apparatus 
  is 
  made 
  port- 
  

   able 
  and 
  hence 
  specifically 
  adapted 
  

   p 
  to 
  observations 
  of 
  crystals 
  during 
  

   growth, 
  by 
  simply 
  moving 
  it 
  so 
  as 
  

   to 
  bring 
  the 
  flat-bottomed, 
  shallow 
  

   beaker 
  containing 
  the 
  crystallizing 
  

   solution 
  between 
  o 
  and 
  n. 
  The 
  

   beakers 
  were 
  held 
  between 
  two 
  

   long 
  supports 
  which 
  enabled 
  one 
  to 
  

   see 
  through 
  the 
  solution 
  vertically 
  ; 
  

   whence 
  by 
  simply 
  moving 
  the 
  appa- 
  

   ratus 
  so 
  as 
  to 
  include 
  one 
  after 
  the 
  

   other, 
  observations 
  could 
  be 
  taken 
  without 
  any 
  disturbance 
  

   of 
  the 
  crystallizing 
  solutions. 
  

  

  The 
  most 
  satisfactory 
  arrangement, 
  however, 
  is 
  that 
  which 
  I 
  

   have 
  employed 
  to 
  enlarge 
  the 
  field 
  of 
  view, 
  so 
  that 
  a 
  whole 
  

   crop 
  of 
  crystals 
  could 
  be 
  examined 
  at 
  once; 
  

   the 
  number 
  of 
  each 
  counted 
  or 
  separated 
  by 
  

   pushing 
  to 
  one 
  side 
  or 
  picking 
  out, 
  as 
  desired. 
  

   Here, 
  as 
  in 
  the 
  first 
  form, 
  the 
  line 
  of 
  sight 
  was 
  

   vertical. 
  Fig. 
  2 
  shows 
  the 
  arrangement 
  of 
  the 
  

   parts, 
  which 
  were 
  simply 
  held 
  by 
  universal 
  

   clamps 
  to 
  an 
  upright 
  rod. 
  The 
  source 
  of 
  light 
  

   was 
  an 
  Argand 
  or 
  Welsbach 
  burner, 
  sending 
  

   light 
  in 
  the 
  direction 
  from 
  a 
  to 
  the 
  mirror, 
  &, 
  

   which 
  reflects 
  the 
  light 
  vertically 
  into 
  the 
  

   polarizer, 
  g. 
  Upon 
  the 
  Nicol 
  was 
  placed 
  a 
  

   double 
  convex 
  lens, 
  d^ 
  of 
  short 
  focal 
  length, 
  

   and 
  in 
  its 
  focus 
  was 
  fastened 
  the 
  field 
  crystal, 
  

   ^, 
  of 
  sodium 
  chlorate 
  mounted 
  in 
  a 
  disc 
  of 
  

   cork. 
  Thus 
  all 
  of 
  the 
  light 
  was 
  subject 
  to 
  

   rotatory 
  polarization 
  and 
  formed 
  a 
  strongly 
  

   diverging 
  beam. 
  Where 
  the 
  cone 
  of 
  light 
  

   had 
  a 
  section 
  of 
  about 
  7 
  or 
  8"^'" 
  in 
  diameter, 
  a 
  

   2)lano-convex 
  lens, 
  f^ 
  of 
  longer 
  focal 
  length, 
  

   ^ 
  ^ 
  / 
  b 
  was 
  placed, 
  with 
  the 
  plane 
  side 
  up, 
  and 
  upon 
  

  

  this 
  was 
  laid 
  a 
  sheet 
  of 
  glass, 
  which 
  acted 
  as 
  a 
  

   table 
  for 
  the 
  crystals 
  to 
  be 
  investigated. 
  Thus 
  the 
  light 
  was 
  

   spread 
  and 
  again 
  converged, 
  the 
  crystals 
  occupying 
  the 
  

   broadest 
  part 
  of 
  the 
  illuminated 
  path. 
  At 
  the 
  focus 
  of 
  the 
  

   light 
  from 
  the 
  second 
  lens 
  was 
  placed 
  the 
  analyzing 
  Nicol, 
  

   ^, 
  through 
  which 
  the 
  crystals 
  were 
  viewed 
  directly, 
  by 
  bring- 
  

   ing 
  the 
  eye 
  close 
  to 
  g. 
  Upon 
  turning 
  the 
  analyzer, 
  the 
  

  

  51 
  

  

  ^ 
  

  

  