Distances  from  the  intersection  of  the  rails  proportional  to  3
and  4  are  accurately  marked  out  on  the  two  beams  respectively,
and  the  beams  adjusted  until  the  distance  between  the  two
points  thus  found  is  proportional  to  5.  Next,  the  axes  at  the
ends  of  the  girder  must  be  made  parallel,  while  the  girder  is
under  stress  ;  for  this  purpose  the  girder  is  supported  at  its  two
ends,  and  the  axis  adjusted  by  the  control  screws  until  the  axes
are'  vertical.  The  camera,  grating,  and  slit  are  now  put  into
position  at  the  proper  height,  care  being  taken  that  the  grating
is  so  placed  that  the  brightest  spectra  are  observed,  for,  as  was
pointed  out  before,  the  spectra  obtained  with  a  grating  are
generally  brighter  on  one  side  than  on  the  other,  A  candle
1  Phil.  Mag.  (s),  27.  369  (1889).
