Wave-Lengths  with  a  Modified  Apparatus.  687 
and  from  this  we  see  that  if  e=l  absolutely. 
1/R=R  =  1 
for  all  values  of  8.  If  6  =  1  very  nearly,  R  =  l  nearly  for  all 
values  of  8  for  which  sin  (J«8)  is  not  very  small.  In  the 
light  reflected  from  an  extended  source,  the  ground  will  be 
of  the  full  brightness  corresponding  to  the  source,  but  it  will 
be  traversed  by  narrow  dark  lines.  By  transmitted  light 
the  ground,  corresponding  to  general  values  of  the  obliquity, 
will  be  dark,  but  will  be  interrupted  by  narrow  bright  rings 
whose  position  is  determined  by  sin  (J/e8)  =  0.  In  permitting 
for  certain  directions  a  complete  transmission  in  spite  of  a 
high  reflecting  power  (e)  of  the  surfaces,  the  plate  acts  the 
part  o£  a  resonator. 
There  is  no  transparent  material  for  which,  unless  at  high 
obliquity,  e  approaches  unity.  In  Fabry  and  Perot's  appar- 
atus the  reflexions  at  nearly  perpendicular  incidence  are 
enhanced  by  lightly  silvering  the  surfaces.  In  this  way  the 
advantage  of  narrowing  the  bright  rings  is  attained  in  great 
measure  without  too  great  a  sacrifice  of  light.  The  plate  in 
the  optical  sense  is  one  of  air,  and  is  bounded  by  plates  of 
glass  whose  inner  silvered  surfaces  are  accurately  flat  and 
parallel  *.  The  outer  surfaces  need  only  ordinary  flatness, 
and  it  is  best  that  they  be  not  quite  parallel  to  the  inner  ones. 
It  will  be  seen  that  the  optical  parts  are  themselves  of 
extreme  simplicity;  but  they  require  accuracy  of  construction 
and  adjustment,  and  the  demand  in  these  respects  is  the  more 
severe  the  further  the  ideal  is  pursued  of  narrowing  the  rings 
by  increase  of  reflecting  power.  Two  forms  of  mounting  are 
employed.  In  one  instrument,  called  the  interferometer,  the 
distance  between  the  surfaces-— the  thickness  of  the  plate — 
is  adjustable  over  a  wide  range.  In  its  complete  development 
this  instrument  is  elaborate  and  costly.  The  actual  measure- 
ments of  wave-lengths  by  Fabry  and  Perot  were  for  the  most 
part  effected  by  another  form  of  instrument  called  an  etalon 
or  interference-gauge.  The  thickness  of  the  optical  plate  is 
here  fixed  ;  the  glasses  are  held  up  to  metal  knobs,  acting  as 
distance-pieces,  by  adjustable  springs,  and  the  final  adjustment 
to  parallelism  is  effected  by  regulating  the  pressure  exerted 
by  these  springs. 
The  theory  of  the  comparison  of  wave-lengths  by  means  of 
this  apparatus  is  very  simple,  and  it  may  be  well  to  give  it, 
following    closely    the     statement    of    Fabry    and    Perot  |. 
*  The  most  important  requirement  is  the  equidistance  of  the  surfaces, 
and  would  not  be  inconsistent  with  equal  and  opposite  finite  curvatures. 
t  Ann.  de  Chimie,  xxv.  p.  110  (1902).  A  good  account  is  given  in 
Baly's  '  Spectroscopy.' 
