700  Lord  Rayleigh  :   Some  Measurements  of 
Helium. 
Red 
7065 
Red 
6678 
Yellow 
5876 
Green 
5016 
Green 
4922 
Blue 
4713 
Violet 
4472 
•637 
II 
•704 
•673 
•721 
•630 
•684 
•665 
The  first  question  is  as  to  the  ratio  P'/P  derived  by  (5) 
from  these  numbers  for  cadmium.  The  integral  value  of  P 
for  the  cadmium  red  ring  was  3328.     From  this  we  find 
P'  cad.  green  =  4213*946, 
P'  cad.  blue    =  4464-935, 
on  the  basis  of  Michelson's  wave-lengths.  For  the  green  the 
fractional  part  is  practically  identical  with  that  deduced  above 
from  one  set  of  observations  with  the  5  mm.  gauge.  In  the 
case  of  the  blue  the  fractional  part  is  now  distinctly  lower. 
The  above  are  the  results  of  work  on  single  evenings.  On 
the  mean  of  all  the  comparisons  with  the  two  intervals  there 
resulted  : — 
Cadmium. 
5  mm. 
1  mm. 
3328 
4213-95 
446494 
Red 
14814 
18753-95 
19870-95 
Blue 
As  already  explained  the  agreement  of  the  fractional  parts 
constitutes  a  complete  verification  of  Michelson's  ratios  of 
wave-lengths,  accurate  to  one  part  in  2  millions  in  the  case 
of  red  and  blue  and  to  a  still  closer  accuracy  in  the  case  of 
red  and  green.  And  it  appears  further  that  the  phase- 
changes,  upon  which  depend  the  deviations  from  integers,  are 
decidedly  greater  than  in  the  examples  recorded  by  Fabry 
and  Perot. 
The  above  results  for  cadmium  suffice  to  indicate  what 
deviations  from  integral  values  are  to  be  expected  when  any 
radiation  is  compared  with  cadmium  red  assumed  integral. 
In  so  far  as  the  expected  fractional  parts  appear  in  the  results, 
so  far  are  the  ratios  of  wave-lengths  assumed  in  the  calcula- 
tion verified.  The  following  are  the  wave-lengths,  reckoned 
in    air  at   15°  C.   and   760  mm.   pressure,  whose    ratios   to 
