698  Lord  Rayleigh  :  Some  Measurements  of 
Cadmium  (5  mm.  gauge). 
Red. 
Green. 
Blue. 
Eight. 
Left. 
Right. 
Left. 
Right. 
Left. 
•398 
•221 
•401 
•217 
•406 
•213 
•399 
•221 
•402 
•217 
•404 
•211 
•399 
•220 
•402 
•216 
•406 
■210 
•3987 
•2207 
•4017 
•2167 
•4053 
•2113 
Diff 
.  =  -1780 
Diff 
.  =  -1850 
Diff 
.  =  -1940 
The  numbers  entered  are  the  actual  readings  o£  the 
microscope  in  inches  for  settings  on  the  right  and  left  sides 
of  the  rings.  Each  horizontal  row  constitutes  really  a 
complete  set.  In  order  to  eliminate  temperature  effects  as 
far  as  possible,  the  readings  are  taken  in  a  certain  sequence. 
Thus  in  the  first  row  the  sequence  was  Red  (R),  Green  (R), 
Blue  (R),  Blue  (L),  Green  (L),  Red  (L).  The  differences, 
representing  the  diameters  of  the  rings,  are  thus  appropriate 
to  the  middle  of  the  time  occupied.  If,  as  happened  here  and 
usually,  the  temperature  was  rising,  so  that  the  rings  dilated, 
the  first  reading  (*398)  on  the  red  is  too  small,  but  the  error  is 
compensated  in  the  last  reading  (*221),  which  is  equally  too 
small.  As  a  matter  of  convenience  the  next  row  would  be 
taken  in  the  reverse  order,  beginning  with  a  repetition  of 
Red  (L),  and  so  on. 
Since  the  radius  of  the  circle  described  by  the  point  of 
observation  is  10  inches,  the  angular  diameters  (#)  of  the 
rings  are  as  follows  : — 
Red. 
G-EEKN. 
Blue. 
X   
•01780 
10~4X3-168 
10~4x-3960 
•01850 
10_4x3-422 
10_4x-4277 
10_4x-0317 
•01940 
10~4x  3-764 
10~4x-4705 
10_4x-0745 
a?2       
1  X1 
Diff 
1 
The  calculation  now  proceeds  by  means  of  (5).     If  P  refer 
to  cadmium  red  and  P'  to  green,  we  have  with  Michelson's 
