686  Lord  Rayleigh  :  Some  Measurements  of 
skillfully  worked  out  by  Fabry  and  Perot.  In  using  an 
accepted  phrase  it  may  be  well  to  say  definitely  that  these 
methods  have  no  more  claim  to  the  title  than  has  the  method 
which  employs  the  grating.  The  difference  between  the 
grating  and  the  parallel  plates  of  Fabry  and  Perot  is  not 
that  the  latter  depends  more  upon  interference  than  the 
former,  but  that  in  virtue  of  simplicity  the  parallel  plates 
allow  of  a  more  accurate  construction.  In  Fabry  and 
Perot's  work  the  wave-lengths  are  directly  compared  with 
the  green  and  red  of  cadmium  ;  and  they  have  obtained 
numbers,  apparently  of  great  accuracy,  for  artificial  lights 
from  vacuum-tubes  containing  various  substances,  e.  g. 
mercury,  for  numerous  lines  from  an  iron  arc,  and  also  for 
various  rays  of  the  solar  spectrum.  While,  so  far  as  I  can 
judge,  there  has  been  every  disposition  to  receive  with 
favour  work  which  not  only  bears  the  marks  of  care  but  is 
explained  with  great  discrimination,  it  must  still  be  felt 
that,  in  accordance  with  an  almost  universal  rule,  confirmation 
by  other  hands  is  necessary  to  complete  satisfaction.  It  was 
with  this  feeling  that  about  a  year  ago  I  commenced  some 
observations  of  which  I  now  present  a  preliminary  account. 
I  was  not  without  hope  that  I  might  be  able  to  introduce 
some  variations  which  would  turn  out  to  be  improvements, 
and  which  would,  at  any  rate,  promote  the  independence  of 
my  results. 
In  this  method  the  interference  rings  utilized  are  of  the 
kind  first  observed  by  Haidinger,  dependent  upon  obliquity. 
Their  theory  is  contained  in  the  usual  formulae  for  the 
reflexion  and  transmission  of  parallel  light  by  a  "thin  plate." 
Thus,  if  A,  be  the  wave-length  of  monochromatic  light, 
k=27t/\,  8  the  retardation,  e  the  reflecting  power  of  the 
surface,  we  have,  in  the  usual  notation  for  the  intensity  of 
reflected  light  *, 
"R  =      4 12  sin2  (%k8)  m 
l-2*2cos*:8  +  <24;     •      •     •     •     W 
and 
B  =  2/jut  cos  a!, (2) 
where  t  denotes  the  thickness  of  the  plate,  p  the  refractive 
index,  and  a!  the  obliquity  of  the  rays  within  the  plate. 
Another  form  of  (1)  is 
R~    +4*2sin2(l*S)'     "     ■     '     *     *     {6) 
*  See,  for  example,  Wave  Theory,  Enc.  Brit. ;  Scientific  Papers,  iii. 
pp.  64,  65. 
