VELOCITY OP WHITE AND OF COLOURED LIGHT. 
273 
mator. If, then, the object-glass be not accurately achromatised the one reflector may 
be focussed accurately for blue rays and the other for red rays. Thus we should have 
one star intrinsically redder than the other. In consequence of these considerations, 
we used, whenever we noticed a difference in colour, to mention the fact, in order that 
this cause of inaccuracy in the observation might be taken account of. We did not 
take particular notice which star was red and which blue, though we sometimes noted 
the fact. We had no idea that any information might be gained by always noticing 
which star was red and which one blue. 
On different days the distinctness and steadiness of the stars varied enormously. 
The days when the stars were steady and distinct were the days on which we got 
the best observations, and felt most certainty about the exact speeds which produced 
equality in the two stars, except that it was often on these days that the difference in 
colour troubled us most. 
On the 11th February, 1881, we were making the regular observations for determin¬ 
ing the speed of revolution of the toothed wheel required to give the 12th, 13th, and 
14th equalities, corresponding to speeds of about 410, 450, and 490 revolutions a 
second. These speeds were obtained by using three, four, or five weights to drive the 
mechanism. The observations in the observing book are numbered from 1 upwards. 
The following observations, 1 to 5, were taken between 9.30 a.m. and 9.50 a.m. The 
remarks are extracted from the observing book, and were written at the time, an entry 
being made after each observation. 
“ February 11, 1881.-—Splendid morning. A and B very bright and steady. If 
anything, A is greater than B. 
1 . —Three weights. 
B 
increasing 
with 
increase 
of speed. 
B reddish, A bluish 
2.—Four weights. 
A 
increasing 
33 
>> 
33 
A 
33 
B 
33 
3.—Five weights. 
B 
increasing 
33 
>> 
33 
B 
33 
A 
33 
4. — Four weights. 
A 
increasing 
33 
>> 
33 
A 
33 
B 
33 
5.—Three weights. 
B 
increasing 
33 
>> 
33 
B 
33 
A 
33 
(Not a very good observation.)” 
I he mark (?) expressing doubt about the colour in the first observation was inserted 
after the second observation had been made, and was so inserted because they seemed 
to be antagonistic. 
When the observations numbered 6 and 7 respectively were being taken at 
10.55 a.m., the following remark is entered in the observing book: “The same 
phenomenon as above.” This refers to the colours. 
A number of trials were then made at different speeds (in which no use was made 
of the chronograph) to examine still further this remarkable phenomenon, and the 
following statement is made in the observing book:—“ Always the light which is 
MDCCCLXX XII, 2 N 
