VELOCITY OE WHITE AKD OF COLOURED LIGHT. 
279 
or about 2‘82 per cent, of the speed producing equality of white light (410 revolutions 
per second). 
It appears then from these experiments that the difference in velocity of red and 
blue lights is about 2‘82 per cent, of the total velocity of light. 
The eight separate observations give the following values: 3*36, 1*14, 2*40, 2*88, 
5*40, 2*46, 2*40, 2*52. 
On February 23 a red solution was substituted for the ruby red glass, as it allowed 
fewer of the more refrangible rays to pass. In these experiments the chronograph 
was used directly; the velocity of rotation of the toothed wheel being reduced as 
before by means of the indiarubber tubing. 
The observation book contains the following entry :— 
“Lights very bright. A greater than B decidedly. Observations perfectly satis¬ 
factory. Interval between first and second signals [those for blue and red lights] 
about 20 seconds. Whole set [four observations] completed in 10 minutes. Atmosphere 
very clear all the time. 
“ No. 1. A increasing with increase of speed. Blue light. (When blue lights were 
equal, on changing to red light, A was far too bright.) 
“ No. 2. A increasing with increase of speed. Bed light. 
“ No. 3. B „ „ „ „ Blue light. 
“ No. 4. B „ „ ,, „ Bed light.” 
The chronograph was used this day in a different manner to that usually adopted. 
Contrary to our expectation, the result was not satisfactory, and we do not think that 
the speeds obtained can be relied upon. We can only say therefore to-day that the 
general effect was the same as before. The defect we allude to is probably due to a 
small mistake in counting the number of pen marks in a certain space upon the 
chronograph cylinder. The readings of the chronograph and the reductions are, 
however, in the hands of the Boyal Society. 
Exceptional observation . 
On February 24, 10 p.m., the following entry was made:—“Tried two speeds. 
(1) A increasing with increase of speed, (2) B increasing. Used red and blue lights. 
There seemed to be a decided tendency for red to require a greater velocity than blue, 
to produce equality. This is contrary to all our previous experience. A was 
decidedly brighter than B, but both were tolerably steady and. bright.” 
A few minutes later the following observation was made :— 
“ With white light. 
“ A blue, B red. Increase of speed increases brightness of A. [This is contrary to 
previous experience.] This was with speed about 440. 
