﻿inclining 
  a 
  Preparation 
  in 
  the 
  Microscope. 
  131 
  

  

  The 
  chief 
  uses 
  of 
  this 
  instrument 
  in 
  petrography 
  are 
  the 
  

   rapid 
  determination 
  of 
  maximal 
  and 
  minimal 
  extinction 
  and 
  

   absorption 
  values, 
  the 
  orientation 
  of 
  feldspar 
  twins 
  for 
  deter- 
  

  

  Fig. 
  2. 
  New 
  model 
  of 
  instrument, 
  showing 
  revised 
  form 
  of 
  foot-plate, 
  wilh 
  sliding 
  posts 
  

   adjustable 
  to 
  clip-holes 
  of 
  any 
  stage 
  ; 
  an 
  extra 
  pair 
  of 
  these 
  posts 
  shown 
  at 
  the 
  left. 
  Key 
  is 
  in 
  

   position 
  The 
  clips 
  may 
  be 
  inserted 
  in 
  all 
  four 
  corners 
  of 
  the 
  elevated 
  clip-plate, 
  and 
  thus 
  

   turned 
  backward 
  or 
  cross-wise, 
  

  

  mination 
  by 
  Michel-Levy's 
  method, 
  and 
  the 
  inclination 
  of 
  the 
  

   section 
  to 
  give 
  well-defined 
  interference 
  figures 
  in 
  convergent 
  

   light. 
  This 
  latter 
  use 
  has 
  been 
  tested 
  by 
  the 
  writer 
  with 
  a 
  

   section 
  of 
  calcite 
  cut 
  parallel 
  to 
  the 
  face 
  of 
  the 
  cleavage 
  rhom- 
  

   bohedron 
  K, 
  (1011). 
  The 
  interference 
  figure 
  given 
  by 
  this 
  

   section, 
  when 
  placed 
  horizontally 
  in. 
  the 
  microscope 
  in 
  conver- 
  

   gent 
  polarized 
  light, 
  is 
  a 
  black 
  bar, 
  representing 
  one 
  arm 
  of 
  the 
  

   basal 
  cross, 
  and 
  a 
  small 
  arc 
  of 
  two 
  of 
  three 
  faintly 
  defined 
  

   chromatic 
  rings 
  on 
  the 
  extreme 
  border 
  of 
  the 
  field. 
  If 
  we 
  

   now 
  incline 
  the 
  section 
  in 
  the 
  ball-and-socket 
  clip, 
  the 
  complete 
  

   cross 
  and 
  rings 
  may 
  be 
  brought 
  into 
  the 
  field 
  and 
  accurately 
  

   centred, 
  thus 
  bringing 
  the 
  c 
  axis 
  into 
  coincidence 
  with 
  the 
  axis 
  

   of 
  the 
  microscope. 
  This 
  implies 
  rotation 
  of 
  the 
  section 
  through 
  

   an 
  angle 
  of 
  over 
  44°, 
  which 
  is 
  about 
  the 
  working 
  limit 
  of 
  the 
  

   instrument, 
  and 
  is 
  of 
  course 
  far 
  greater 
  than 
  would 
  usually 
  be 
  

   required. 
  

  

  For 
  this 
  work 
  a 
  where 
  very 
  high 
  powers 
  are 
  used, 
  a 
  tube 
  must 
  

   be 
  provided 
  for 
  raising 
  the 
  conoscopic 
  condenser 
  15 
  mm 
  to 
  the 
  

   level 
  of 
  the 
  elevated 
  object-glass. 
  In 
  addition 
  it 
  is 
  desirable 
  

   that 
  this 
  condenser 
  be 
  mounted 
  in 
  a 
  steeply 
  conical 
  cell, 
  in 
  case 
  

   any 
  considerable 
  tipping 
  of 
  the 
  section 
  about 
  its 
  apex 
  is 
  

   required 
  ; 
  the 
  high-power 
  objectives 
  offer 
  no 
  obstruction 
  in 
  

   this 
  respect, 
  as 
  they 
  are 
  usually 
  mounted 
  in 
  cells 
  sufficiently 
  

   tapering. 
  For 
  interference 
  figures 
  from 
  mineral 
  sections 
  where 
  

   such 
  high 
  magnification 
  is 
  unnecessary, 
  a 
  special 
  long-focus 
  

   conoscopic 
  combination 
  of 
  objective 
  and 
  condenser 
  is 
  recom- 
  

   mended, 
  as 
  these 
  allow 
  ample 
  space 
  for 
  free 
  inclination 
  of 
  the 
  

   object-glass.* 
  

  

  * 
  R. 
  Fuess 
  in 
  Berlin 
  makes 
  such 
  a 
  combination 
  for 
  use 
  with 
  the 
  Fedorow 
  Uni- 
  

   versal 
  Stage: 
  v. 
  Erganzungen 
  zu 
  den 
  Preis-Verzeichnissen 
  1891 
  und 
  1894, 
  R. 
  

   Fuess, 
  1895, 
  p. 
  18, 
  No. 
  14, 
  last 
  paragraph. 
  

  

  