The  interferometer  they  used  was  similar  to  that  employed  by
Michelson  for  obtaining  his  "  visibility  "  curves  (see  page  314),
the  only  difference  being  that  very  thin  quartz  plates  were
used  in  place  of  the  glass  ones  used  by  Michelson,  in  order  to
reduce  the  absorption  of  the  rays  to  a  minimum.  In  one  case
a  plane  parallel  pair  was  used,  each  being  o"6  mm.  thick,  and
in  the  other  case  a  wedge-shaped  pair  tapering  from  o"8  to
o"4mm.  thick  was  used.  The  pitch  of  the  micrometer  screw
was  0*5228  mm.,  and  as  the  drum  actuating  this  screw  was
divided  into  100  parts,  a  rotation  of  one  division  corresponds
to  a  displacement  of  5'2  28;«..
