276 
DR. J. YOUNG AND PROFESSOR G. FORBES ON THE 
of that colour."' The same thing will hold, though to a less extent, on comparing red 
and white light. Our only means at hand on February 11 was to determine the speed 
which produced an equality (l) in the ordinary way with the white light of the electric 
lamp, and (2) with the eye screened by a piece of ruby red glass. 
The observations made with this object on February 11 are named in the observing 
book No. 13 (white), No. 13 (red), No. 14 (white), and No. 14 (red). 
They were not very satisfactory, for the differences found between the velocities of 
red and white light were small. The observations No. 13 were at the speed producing 
the 13th equality. The observations No. 14 were at the speed producing the 14th 
equality. The speeds of rotation finally deduced from the chronograph records were 
as follows :—t 
Difference. 
Observation No. 13 (red) speed of rotation. 456''*84j 4^4 
,, No. 13 (white) ,, „ . 460 r *98 J 
„ No. 14 (red) speed of rotation. 494 r *85 "1 
„ No. 14 (white) ,, „ . 496''42 J 
Difference of velocity (red and white)_ ['90 per cent, from No. 13 
Absolute velocity (white) 1*32 „ „ No. 14 
These differences are small; but on the whole they indicate a greater speed for 
white than for red light. But these differences might be suspected to be due to 
irregularities in the action of the chronograph. The general result seemed to be that 
we must obtain a greater difference in speed by choosing two colours of light, differing 
considerably in wave-length, and that we might with advantage discard the chrono¬ 
graph as an absolute measurer of speeds, and adopt some more delicate means of 
measuring minute differences of speed. 
Great difficulty was found in obtaining a blue medium which would sufficiently stop 
out the red rays. The ordinary blue glass, coloured with cobalt, allows large quantities 
of red light to pass. We tried eight or nine solutions, which we put into glass cells 
with parallel sides and tested with a prism. We found that a nitrate of copper 
solution gave the least quantity of red. 
* § 4 of the mathematical theory gives its the value of n for the r th equality for red light, viz., 
_{2(l-D(l-/d+<l + D}N A N B _ N A N B 
Ur n b +/>n a -*-n b +^>n a 
If now vel ° Glt y QfvioUt h ght notice that for violet light N A and N B must be multiplied by <r, 
velocity of red light 
Thus we have 
N a N b _ 
ny— K ‘°’*N b -|-/oN a — 
«v_ 
ih~ a 
also 
n v — )i K difference of velocities for red and violet 
n R velocity of red light 
t The observations and reductions are in the hands of the Royal Society. 
Q. E. D. 
