Iri  exactly  the  same  way  may  the  formula  be  applied  to
the  measurements  of  the  deviations  given  by  a  prism.  For
example,  we  may  take  the  wave-length  curve  reproduced  on
page  147,  and- calculate  its  constants.
Three  readings  on  this  curve  are —

X  =  7800  D  =  47°    o'  =  2820  minutes.
X  =  5400  D  =  48°  20'  =  2900       ,,
X  =  4200  D  =  50°  20'  =  3020        „

For  the  sake  of  simplicity  we  may  subtract  2820  from  these
readings  of  minutes,  and  then  we  have  o,  80,  200,  or  more
simply  o,  8,  20,  if  we  adopt  ten  minutes  as  the  unit.  From
these  equations  we  readily  find  the  constant  to  be  Xo  =  2400,
c  =  54000,  and  Do  =  —  10.  These  will  then  be  found  to  fit
the  curve  with  a  very  fair  accuracy.
These  examples  will  doubtless  suffice  to  show  how  the
formula  is  applied  and  how  serviceable  it  can  prove.
An  interesting  application  has  been  made  of  this  formula
by  Fowler  and  Eagle.^  If  a  prismatic  photograph  of  a  specf  rum
be  copied  by  means  of  a  camera  lens,  then  it  is  possible  to
obtain  the  lines  in  the  copy  distributed  according  to  their
'  wave-lengths,  i.e.  it  is  possible  to  obtain  a  normal  spectrum.
This  is  done  by  setting  the  spectrum  plate  and  the  plate  on
which  the  image  is  received  at  certain  angles  to  the  axis  of  the
lens  of  the  copying  apparatus.  Fowler  and  Eagle  show  how
these  angles  may  be  calculated  if  the  focal  length  of  the  lens

'  Astrofhys.  Journ.,  28.  284  (1908).
THE   PRISM  SPECTROSCOPE   IN  PRACTICE     163

be  known,  and  also  the  constants  of  Hartmann's  formula  for
the  spectrum  in  question.
