In  all  doubly  refracting  crystals  there  is  one,  and  sometimes
two,  directions  along  which  double  refraction  does  not  take
place.  These  directions  are  called  the  optic  axes  of  the
crystal]  quartz  and  calcite  are  both  uniaxial,  that  is  to  say,
there  is  one  direction  through  these  crystals  along  which  there
is  no  double  refraction.  When  prisms,  therefore,  are  made
out  of  quartz  or  calcite,  it  is  necessary  that  they  be  so  cut  that
the  rays  inside  the  prism  pass  along  the  optic  axis  of  the
crystal,  because,  if  not,  the  'double  refraction  will  cause  the
spectrum  lines  to  be  doubled.  It  has  been  pointed  out
(p.  9)  that  the  path  of  the  rays  in  a  prism  set  at  minimum
deviation  is  parallel  to  the  base,  and  thus  it  follows  that  prism
of  quartz  or  calcite  should  be  so  cut  that  the  optic  axis
T,  P.  c.  ■"     ~


98  SPECTROSCOPY

lies  in  a  principal  plane  and  parallel  to  the  base,  in  order  that
the  rays  at  minimum  deviation  should  not  undergo  double
refraction.  It  so  happens  that  in  the  case  of  calcite  this  is
a  very  wasteful  way  of  cutting  the  prism,  as  can  be  seen  by

reference  to  Fig.  35,  which  repre-
sents a  calcite  crystal.  The  crystal
is  a  rhombohedron,  bounded  by
six  parallelograms,  each  of  which
has  two  angles  of  78°  5',  and  two
of  101°  55';  it  will  be  seen  on
reference  to  the  diagram  that  two
jTjQ  2j_  of  the  corners  of  the  crystal,  and

only  two,  are  formed  by  the  inter-
section of  three  wide  angles.  The  optic  axis  lies  along  the  line
joining  these  two  angles,  and  is  shown  by  the  dotted  diagonal
in  the  figure.  The  prism  must  be  so  cut  that  the  dotted
diagonal  lies  in  a  principal  plane  and  parallel  to  the  base ;  this
is  a  difficult  operation,  because  the  prism  faces  are  cut  at  an
angle  with  the  lines  of  cleavage,  and,  therefore,  the  crystal  is
very  readily  split.  The  finished  prism  must  also  be  carefully
handled,  as  it  is  very  easily  chipped.  In  cutting  the  native
crystal  it  is  usual  to  use  a  fine  copper  wire,  which  has  been
dipped  in  a  mixture  of  emery  and  oil,  the  wire  being  drawn
backwards  and  forwards  like  a  saw.
