﻿512 
  F. 
  E. 
  Wright— 
  Methods 
  in 
  Microscopical 
  Petrography. 
  

  

  tan 
  <£ 
  

   cos 
  p 
  

   cot 
  <j> 
  

   cos 
  p 
  

  

  tan 
  X 
  1 
  

   sin 
  p, 
  x 
  

   tan 
  A 
  2 
  

   sin 
  ^ 
  2 
  

  

  cot 
  A 
  2 
  

   sin 
  fi 
  a 
  

   cot 
  A, 
  

   sin 
  p 
  

  

  sin 
  A, 
  

   cos 
  A 
  x 
  

   sin 
  A 
  2 
  

   cos 
  A 
  2 
  

  

  COS 
  <p 
  

   COS 
  <f> 
  

  

  sin 
  <p 
  

   sin 
  <j> 
  

  

  cos 
  A 
  x 
  

   sin 
  A 
  x 
  

   cos 
  A 
  2 
  

   sin 
  A 
  2 
  

  

  cot 
  /x 
  t 
  

   cos 
  /a 
  1 
  

  

  cot 
  fJL 
  2 
  

  

  cos 
  /x 
  2 
  

  

  tan 
  p 
  

   sin 
  p 
  

   tan 
  p 
  

   sin 
  p 
  

  

  cot 
  /x 
  x 
  

   cos 
  /a 
  2 
  

  

  COt 
  fJL 
  3 
  

  

  cos/x 
  2 
  

  

  (V) 
  

  

  (8) 
  

  

  w 
  

  

  (10) 
  

  

  (ii) 
  

  

  (12) 
  

   (13) 
  

   (14) 
  

  

  (15) 
  

   (16) 
  

  

  (1?) 
  

   (18) 
  

  

  Fig. 
  2. 
  

  

  All 
  of 
  the 
  above 
  equations, 
  1 
  to 
  18, 
  can 
  be 
  put 
  into 
  the 
  

   general 
  form 
  

  

  A 
  _ 
  C 
  

  

  B 
  ~ 
  ~I 
  (19) 
  

  

  If 
  the 
  values 
  of 
  A, 
  i?, 
  and 
  C 
  be 
  plotted 
  directly 
  the 
  equa- 
  

   tion 
  can 
  be 
  represented 
  by 
  similar 
  triangles 
  of 
  the 
  form 
  indi- 
  

   cated 
  in 
  figure 
  2, 
  in 
  which 
  OJST 
  = 
  1 
  and 
  the 
  two 
  triangles 
  KOL 
  

   and 
  MON 
  are 
  similar. 
  By 
  taking 
  advantage 
  of 
  this 
  principle 
  

   of 
  plotting 
  the 
  different 
  functions 
  directly 
  as 
  abscissae 
  and 
  ordi- 
  

   nates, 
  we 
  are 
  able 
  to 
  represent 
  the 
  variations 
  by 
  straight 
  lines 
  

   entirely, 
  whereas 
  if 
  we 
  plot 
  the 
  values 
  under 
  the 
  function 
  

   (thus 
  x 
  instead 
  of 
  sin 
  x) 
  we 
  obtain 
  a 
  series 
  of 
  curves 
  inter- 
  

   polated 
  between 
  points 
  which 
  have 
  been 
  separately 
  computed. 
  

   Such 
  curves 
  are 
  more 
  difficult 
  to 
  draw 
  than 
  straight 
  lines 
  and 
  

   are 
  less 
  accurate. 
  The 
  principle 
  of 
  plotting 
  the 
  functions 
  

   directly 
  is, 
  therefore, 
  of 
  fundamental 
  importance 
  in 
  micro- 
  

   scopical 
  petrography 
  and 
  might 
  well 
  be 
  applied 
  more 
  fre- 
  

  

  

  