﻿532 
  F. 
  E. 
  Wright 
  — 
  Methods 
  in 
  Microscopical 
  Petrography. 
  

  

  Table 
  III. 
  

  

  V 
  

  

  tan 
  2 
  V 
  

  

  V 
  

  

  tan 
  2 
  V 
  

  

  V 
  

  

  tan 
  2 
  V 
  

  

  0° 
  

  

  O'OOO 
  00 
  

  

  15° 
  

  

  0-071 
  80 
  

  

  30° 
  

  

  0-333 
  33 
  

  

  1 
  

  

  •000 
  30 
  

  

  6 
  

  

  •082 
  22 
  

  

  1 
  

  

  •361 
  03 
  

  

  2 
  

  

  •001 
  22 
  

  

  7 
  

  

  •093 
  47 
  

  

  9 
  

  

  •390 
  46 
  

  

  3 
  

  

  •002 
  75 
  

  

  8 
  

  

  •105 
  57 
  

  

  3 
  

  

  •421 
  73 
  

  

  4 
  

  

  •004 
  89 
  

  

  9 
  

  

  •118 
  56 
  

  

  4 
  

  

  •454 
  96- 
  

  

  5 
  

  

  0*007 
  65 
  

  

  20 
  

  

  0-132 
  47 
  

  

  35 
  

  

  0-490 
  29 
  

  

  6 
  

  

  •Oil 
  05 
  

  

  1 
  

  

  •147 
  35 
  

  

  6 
  

  

  •527 
  86 
  

  

  7 
  

  

  •015 
  08 
  

  

  2 
  

  

  •163 
  24 
  

  

  7 
  

  

  •567 
  84 
  

  

  8 
  

  

  •019 
  75 
  

  

  3 
  

  

  •180 
  18 
  

  

  8 
  

  

  •610 
  41 
  

  

  9 
  

  

  •025 
  09 
  

  

  4 
  

  

  •198 
  23 
  

  

  8 
  

  

  •655 
  75 
  

  

  10 
  

  

  0-031 
  09 
  

  

  25 
  

  

  0-217 
  44 
  

  

  40 
  

  

  0-704 
  09 
  

  

  1 
  

  

  •037 
  78 
  

  

  6 
  

  

  •237 
  88 
  

  

  1 
  

  

  •755 
  66 
  

  

  2 
  

  

  •045 
  18 
  

  

  7 
  

  

  •259 
  62 
  

  

  2 
  

  

  •810 
  73 
  

  

  3 
  

  

  •053 
  30 
  

  

  8 
  

  

  •282 
  71 
  

  

  3 
  

  

  •869 
  58 
  

  

  4 
  

  

  •062 
  16 
  

  

  9 
  

  

  •307 
  26 
  

  

  4 
  

  

  •932 
  56 
  

  

  15 
  

  

  0-071 
  80 
  

  

  30 
  

  

  0-333 
  33 
  

  

  45 
  

  

  1-000 
  00 
  

  

  From 
  Plate 
  YI 
  we 
  find 
  with 
  these 
  values 
  2 
  Va 
  = 
  52°*2. 
  

  

  rpu 
  • 
  * 
  * 
  i 
  ', 
  + 
  2T 
  r 
  1-874-1-864 
  0-010 
  

  

  Ine 
  approximate 
  formula 
  reads 
  tan 
  Va 
  = 
  -— 
  = 
  . 
  

  

  ^ 
  1-864 
  — 
  1-824 
  0-040 
  

  

  Hence 
  from 
  Plate 
  YI, 
  2 
  Va 
  = 
  53°*2, 
  a 
  value 
  which 
  is 
  1° 
  too 
  

   high. 
  

  

  (2) 
  Compute 
  the 
  optic 
  axial 
  angle 
  of 
  petalite 
  from 
  its 
  refrac- 
  

   tive 
  indices, 
  a 
  = 
  1-504, 
  = 
  1*510, 
  y 
  = 
  1-516. 
  From 
  Table 
  II 
  

  

  1 
  1 
  

  

  we 
  have 
  -5 
  = 
  0*442084, 
  - 
  

  

  _0'438577, 
  ^ 
  = 
  0-435113; 
  

   „_ 
  003464 
  

  

  1 
  

  

  = 
  0-003507, 
  j 
  t 
  - 
  ^ 
  =0-003464 
  ; 
  tan 
  8 
  Va= 
  ~" 
  3 
  ^ 
  , 
  from 
  which 
  

  

  we 
  find 
  2 
  Va 
  = 
  89°*7. 
  The 
  mineral 
  is 
  therefore 
  optically 
  

   negative, 
  if 
  the 
  refractive 
  indices 
  be 
  given 
  correctly. 
  From 
  

  

  the 
  approximate 
  formula, 
  tan 
  2 
  V— 
  | 
  - 
  = 
  n 
  — 
  77^ 
  = 
  1, 
  we 
  are 
  

  

  1 
  L 
  p 
  — 
  a 
  0-006 
  

  

  unable 
  to 
  determine 
  the 
  optical 
  character, 
  for 
  it 
  indicates 
  a 
  

   value 
  of 
  2 
  V= 
  90°. 
  In 
  the 
  text 
  books 
  the 
  optical 
  character 
  

   of 
  petalite 
  is 
  stated 
  to 
  be 
  optically 
  -f- 
  and 
  the 
  optic 
  axial 
  angle, 
  

   2 
  Vy 
  == 
  83°. 
  There 
  is, 
  therefore, 
  a 
  discrepancy 
  between 
  these 
  

   data 
  which 
  should 
  be 
  investigated. 
  

  

  