Wave-Lengths  with  a  Modified  Apparatus.  699 
values  of  the  wave-lengths : 
P' 
=  1-2659650  (1--00000317)  =  1-2659610, 
which  with  P  =  11811  gives 
P'  =  11814  +  3939-915  =  18753-915. 
In  like  manner  for  the  blue  referred  to  the  reel. 
P' 
p-  =  1-3113733  (1--00000715)  =  1-3113633, 
whence 
P' =  11811  + 5056-955  =  19870-955. 
The  wave-lengths  of  the  various  radiations  from  a  single 
source  can  thus  be  compared  with  great  ease,  and  but  little 
fear  of  temperature  error.  A  set  of  observations  from  which 
this  error  is  practically  eliminated  can  be  made  in  a  short 
time  and  a  few  repetitions  give  all  the  security  necessary. 
But  the  situation  is  not  so  favourable  when  we  compare 
radiations  from  different  sources.  More  time  is  occupied  and 
there  is  corresponding  opportunity  for  temperature  change. 
It  is  necessary  to  alternate  the  observations,  taking  the  first 
source  twice  and  the  second  once,  or  preferably  the  first  three 
times  and  the  second  twice.  Even  with  this  precaution  I 
believe  that  temperature  change  was  the  principal  source  of 
error  in  the  results  of  a  single  evening's  work. 
In  the  observations  with  an  interval  of  one  millimetre 
between  the  silvered  surfaces,  the  influence  of  temperature  is 
of  course  much  less  perceptible.  For  a  similar  reason  the 
identification  of  the  rings  is  a  much  easier  matter.  I  will 
give  as  a  specimen  a  series  of  operations  (Feb.  9)  in  which 
helium  was  compared  with  cadmium.  The  first  and  third 
sets,  each  containing  a  repetition,  related  to  cadmium  ;  the 
second  set  (twice  repeated)  related  to  helium.  Only  the  mean 
diameter  for  each  set  is  here  recorded  : — 
Cadi 
Bed. 
•749 
•752 
Green. 
•636 
643 
•039 
Blue. 
•641 
•043 
•042 
I 
Ill 
Mean      . . 
•750 
2  Z  2 
