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  A. 
  H. 
  Phillips— 
  Symmetry 
  of 
  Crystals. 
  

  

  Art. 
  IV. 
  — 
  A 
  Simple 
  Model 
  for 
  Illustrating 
  the 
  Symmetry 
  

   of 
  Crystals 
  ; 
  by 
  Alexander 
  H. 
  Phillips. 
  

  

  For 
  several 
  years 
  past 
  I 
  have 
  been 
  using 
  in 
  my 
  class 
  work 
  a 
  

   model, 
  of 
  very 
  simple 
  construction, 
  to 
  illustrate. 
  the 
  symmetry 
  

   of 
  the 
  various 
  types 
  of 
  crystals. 
  It 
  has 
  been 
  very 
  helpful 
  and 
  

  

  Fig. 
  1. 
  

  

  most 
  effective 
  in 
  simplifying 
  some 
  of 
  the 
  points 
  in 
  crystallog- 
  

   raphy, 
  which 
  have 
  always 
  seemed 
  most 
  difficult 
  for 
  the 
  ordi- 
  

   nary 
  student 
  to 
  grasp 
  without 
  a 
  demonstration. 
  

  

  The 
  model 
  as 
  shown 
  in 
  the 
  figure 
  is 
  constructed 
  of 
  a 
  horizon- 
  

   tal 
  disk 
  of 
  tin, 
  representing 
  the 
  plane 
  of 
  the 
  lateral 
  axes 
  or 
  the 
  

   equator, 
  in 
  the 
  equatorial 
  types. 
  At 
  right 
  angles 
  to 
  this 
  disk 
  

   and 
  at 
  right 
  angles 
  to 
  each 
  other 
  are 
  two 
  semicircular 
  disks 
  ; 
  

   these 
  three 
  planes 
  divide 
  space 
  in 
  the 
  northern 
  hemisphere 
  

   into 
  the 
  usual 
  four 
  quadrants, 
  as 
  in 
  those 
  systems 
  in 
  which 
  the 
  

   axes 
  are 
  at 
  right 
  angles 
  and 
  represent 
  the 
  axial 
  or 
  diametral 
  

   planes, 
  the 
  intersections 
  of 
  which 
  will 
  represent 
  the 
  axes 
  

   a 
  : 
  b 
  : 
  c. 
  At 
  the 
  center 
  of 
  the 
  model 
  a 
  socket 
  is 
  cut 
  in 
  the 
  

  

  