(3)  The    Visual  or  Optical  Method.— This   divides  itself
into  two  branches,  first,  the  use    of    the  eye  only,  which
under    favourable    conditions    of    experience    and    uniform
lighting    of     the    hardening    room     produces     surprisingly
uniform  results,  and  is,  of  course,  by  far  the  most  easy  of
application  ;   and  the  optical  instrument  method,  with  which
may  for  the  moment    be    classed   the  radiation  instrument
method.     There  can  be  no  doubt  that  the  optical  or  colour
instrument  puts  into  the  hands  of  the  hardener  means  of
accuracy  impossible  by  any  observation  with  the  unaided
eye,    and   this   method   appears   therefore   to   have   a   large
field  of  usefulness  before  it.     There  is  one  point,  however,
touching   any   colour-detection   method    which    should   not
be    lost    sight    of.     This    is    the    desirability    of    a    colour-
blindness   test    of    the    operator's    sight.     Given    a    perfect
colour   vision   on   the   hardener's  part,   the  colour   method
appears  very  suitable  to  the  average  hardening  shop.     The
radiation    method   yields    a    graphic    representation    of   the
rising  heat.
