690 
Lord  Rayleigh  :  Some  Measurements  of 
regard  to  temperature  afterwards  found  to  be  necessary. 
The  formula  at  the  basis  of  the  calculation  assumes  that 
e,  the  thickness  of  the  plate,  is  constant,  but  in  fact  it  changes 
Cd 
red. 
Cd 
green. 
Cd 
blue. 
Hg 
green. 
1 
Hg 
yellow. 
14788 
9 
14790 
1 
o 
3 
4 
5 
6 
7 
8 
9 
14800 
1 
2 
3 
4 
5 
6 
7 
8 
9 
1   14810 
1 
2 
3 
4 
5 
18721-03 
19836-04 
19840-07 
19844-09 
19848-12 
19850-80 
19852-14 
19856-16 
19860-19 
19862-87 
19866-90 
19870-92 
17435-24 
17459-04 
17462-36 
17465-90 
16518-68 
16530-96 
18726-09 

18729-89 
18731-15 
18734-95 
18740-01 

18745-08 
18748-88 
18750-14 
18753-94 
! 
with  temperature.  On  this  account  alone  erroneous  results 
will  be  obtained  unless  the  observations  are  well  alternated, 
so  as  to  eliminate  such  effects.  The  numbers  finally  arrived 
at,  in  substitution  for  the  row  in  the  table,  are 
14814,  18753-95,  19870-95,  17465-97,  16531*00. 
The  deviations  from  integers  still  outstanding  have  their 
origin  in  a  complication  which  must  be  admitted  to  be  a 
drawback  to  the  method  and  might  at  first  sight  be  estimated 
even  more  seriously.  The  optical  thickness  e  of  the  plate,  on 
which  everything  depends,  is  not  really  constant,  as  has  been 
assumed,  when  we  pass  from  one  part  of  the  spectrum  to 
another  somewhat  distant  from  the  first.  The  question  is 
discussed  by  Fabry  and  Perot.     If,  to  take  account  of  this 
