Wave-Lengths  with  a  Modified  Apparatus.  701 
cadmium  red  have  been  verified  by  my  observations  to  about 
one  part  in  a  million  : — 
I"  6438-4722  ) 
Cadmium  J  5085-8240  I  Michelson. 
[  4799-911  J 
f  5790-659  ^| 
Mercury  \  ™^%  I  Fabry  &  Perot. 
(  4358-343  J 
f  6362-345  j 
„.      J  4810-535  L-,  ,   ,  -p   , 
Zinc  <  4722-164  f    y 
[  4680-138  j 
a   ,     f      5895-932      1  -,  ,       s  v      , 
hoda{      5889-965      }  Fabry  &  Perot. 
I  have  spoken  of  an  agreement  to  about  1  part  in  a  million. 
In  several  cases  the  confirmation  was  decidedly  closer.  In 
one  only,  that  of  zinc  red,  did  there  appear  an  indication  of 
a  disagreement  rather  outside  the  limits  of  error.  My  ob- 
servations would  point  to  a  wave-length  about  1  millionth  part 
greater  than  that  of  Fabry  and  Perot  ;  but  in  view  of  the 
difficulty  of  observations  with  the  trembler,  I  am  not  dis- 
posed to  insist  upon  it.  The  soda  observations  were  on  light 
from  a  cadmium  vacuum-tube  in  which  soda  accidentally 
presented  itself.  The  numbers  quoted  from  Fabry  and  Perot 
relate  to  a  soda-flame. 
As  an  example  in  which  the  ratios  of  wave-lengths  were 
less  accurately  known  beforehand,  I  will  give  some  details 
relative  to  helium,  beginning  with  observations  of  Feb.  9 
by  the  1  mm.  gauge,  already  referred  to.  The  Table  I. 
annexed  gives,  in  the  second  column  the  wave-lengths  of 
the  various  helium  lines  recorded  by  Runge*,  in  the  fifth 
the  same  reduced  to  Michelson\s  scale  as  employed  by  Fabry 
and  Perot.  The  third  column  gives  the  corrections  for 
obliquity  as  calculated  from  the  observations  with  the  1  mm. 
apparatus  already  recorded,  the  fourth  the  differences  from 
the  corresponding  quantity  for  cadmium  red.  Taking,  for 
example,  the  helium  ray  of  longest  wave-length  in  com- 
parison with  cadmium  red,  we  get  by  (5) 
P'  =  33286^A3f'4J?  ( 1  +  -000084)  =  3033-03. 
70b5'22 
These  numbers  should  be  integers,  were  the  wave-lengths 
*  A.strophysical  Journal,  January  1896. 
