﻿F. 
  E. 
  Wright 
  — 
  Methods 
  in 
  Microscopical 
  Petrography. 
  513 
  

  

  quently 
  than 
  has 
  heretofore 
  been 
  the 
  case. 
  It 
  is 
  a 
  well-known 
  

   principle 
  in 
  physics, 
  but 
  even 
  there 
  is 
  not 
  commonly 
  employed. 
  

   Logarithmic 
  paper 
  is 
  the 
  result 
  of 
  this 
  principle 
  and 
  is 
  of 
  

   value 
  wherever 
  logarithmic 
  functions 
  are 
  to 
  be 
  plotted. 
  

  

  Recently 
  Dr. 
  A. 
  Hutchinson* 
  has 
  described 
  a 
  graphical 
  

   plot 
  — 
  for 
  use 
  in 
  connection 
  with 
  his 
  total 
  refractometer 
  — 
  in 
  

   which 
  this 
  principle 
  of 
  plotting 
  the 
  functions 
  directly 
  has 
  

   been 
  employed. 
  Such 
  instances, 
  however, 
  are 
  rare 
  in 
  petro- 
  

  

  Fig. 
  3. 
  

  

  graphic 
  literature 
  and 
  this 
  is 
  the 
  more 
  surprising 
  when 
  we 
  

   consider 
  the 
  importance 
  and 
  wide 
  range 
  of 
  application 
  of 
  the 
  

   principle. 
  

  

  Still 
  another 
  form 
  of 
  graphical 
  plot 
  might 
  be 
  used 
  which 
  is 
  

   simpler 
  to 
  construct 
  than 
  Plates 
  II 
  to 
  IX. 
  It 
  was 
  not 
  adopted, 
  

   however, 
  because 
  of 
  the 
  pronounced 
  distortion 
  of 
  the 
  values 
  

   represented. 
  The 
  construction 
  follows 
  from 
  the 
  fact 
  that 
  the 
  

   relation 
  

  

  A 
  = 
  BC 
  

  

  can 
  be 
  written 
  

  

  log 
  A 
  = 
  log 
  B 
  + 
  log 
  C 
  

  

  (20) 
  

  

  If 
  in 
  this 
  equation 
  log 
  B 
  be 
  considered 
  constant, 
  the 
  general 
  

   equation 
  represents 
  a 
  set 
  of 
  straight 
  lines 
  passing 
  through 
  the 
  

   point, 
  log 
  B 
  ; 
  similarly 
  if 
  log 
  C 
  be 
  a 
  constant 
  we 
  have 
  a 
  

   second 
  set 
  of 
  straight 
  lines 
  passing 
  through 
  the 
  point, 
  log 
  0. 
  

   If 
  now 
  we 
  draw 
  two 
  straight 
  parallel 
  lines 
  and 
  plot 
  log 
  B 
  to 
  

   scale 
  on 
  the 
  one 
  line 
  and 
  log 
  G 
  to 
  scale 
  on 
  the 
  second, 
  then, 
  

   because 
  log 
  A 
  is 
  equal 
  to 
  the 
  sum, 
  log 
  B 
  4- 
  log 
  6 
  r 
  v 
  we 
  can 
  

   determine 
  the 
  value 
  of 
  log 
  A 
  by 
  means 
  of 
  a 
  third 
  straight 
  line 
  

   parallel 
  to 
  the 
  first 
  two 
  (fig. 
  "6). 
  

  

  * 
  Mineralogical 
  Magazine, 
  xvi, 
  236-238. 
  1912. 
  

  

  