Velocity of white and of coloured light. 
28 i 
On February 25, 1881, 4 P.M., B was greater than A. A was bluish compared 
with B when the toothed wheel was removed. Three observations were made, but 
they show no decided difference between Sx and Sy. The indiarubber was used as 
before. 
No. 1. 12th equality of lights.Blue-red=2‘5 seconds. 
No. 2. 13th „ „ . „ ,, =4*0 „ 
No. 3. (at 5*30 p.m.) 12 th equality of light . ,, ,, = 6*0 ,, 
Now the loss of speed caused by the indiarubber in one second is O'49 revolution 
per second. The 12th equality is at a speed of about 420 revolutions a second; and 
the 13th equality about 460 revolutions a second. From these we get the difference 
between the velocity for red and for blue light, in percentages of the whole, as 
follows :— 
From No. 1 we have 0'29 per cent. 
,, No. 2 ,, 0'43 „ 
„ No. 3 „ 0*70 
On February 27, between noon and 1 p.m., measurements were made in the usual 
way with the indiarubber, which was a most convenient method of working at different 
speeds:— 
No. 1. 12th equality.Blue-red =11 seconds. 
No. 2. 13th „ . „ „ =111 „ 
(In this observation the indiarubber did not seem to act well after the blue equality). 
The observations were continued at 4 p.m. at various speeds and slower than those 
we had previously experimented upon :— 
No. 3. 9th equality .... 
No. 4. 10th ,, .... 
No. 5. 9th „ .... 
No. 6. 10th „ .... 
No. 7. 10th „ .... 
No. 8. 11th ,, .... 
The following remarks are appended :— 
“ Observations from 3 to 8, lights splendidly steady and equal. The difference in 
velocity for red and blue most striking. A and B are precisely the same colour. Both 
A and B looked like clearly defined circles with no diffraction phenomena. This is 
very uncommon, and the observations are therefore valuable.” 
On the same day some more chronographic measurements were made to determine 
the reduction in speed per second of time produced by the indiarubber, at the speeds 
MDCCCLXXXII. 2 O 
Blue-red = 9 , 0 seconds. 
-15*5 „ 
= 16*5 
= 11*0 
= 14*0 
= 9*5 
5? 
