VELOCITY OF WHITE AND OF COLOURED LIGHT. 
251 
(1.) The distances of the reflectors and toothed wheel form the points used in the 
triangulation. These are 
at A—9T8 feet 
at B —0*97 foot 
at C + 8‘83 feet 
giving a correction to 
CA=-0*35 foot 
and to 
CB=+7*86 feet. 
(2.) The thickness of the object-glasses, giving to CA and to CB a correction of 
+ 0T feet to each. 
These two corrections make CA= 18,210*3 feet and CB= 16,833*3 feet. 
(3.) The reduction to a sphere whose radius is the distance of Kelly from the earth’s 
centre instead of the sea level. The height of Kelly above the sea level is 100 feet. 
This correction is quite insensible. 
(4.) The reduction of the circular measure of the angle subtended by CA or CB at 
the earth’s centre to the chord of that arc. The angle is about S', and the reduction 
is quite insensible. 
(5.) The station B is 414 feet higher than Kelly, and A is also in the same line. 
Owing to this cause there is a correction of +1*7 feet to CB and + 1*9 feet to CA. 
(6.) Lastly, the rays of light do not go in a straight line from C to A or B because 
they are bent into a curve by refraction. The curvature is so small that we may 
consider it as a part of a circle of very large radius. Supposing that the refraction 
amounted to half as much as the horizontal refraction of a heavenly body, i.e., to 15', 
then the angle subtended by this part of a circle would be 30'. But the difference 
between the circular measure of 30'and the chord of the arc is insensible. Hence this 
correction is insensible. 
The final result is that the distance CA= 18,212*2 feet and CB = 16,835*0 feet. Or 
D a = CA= 3*44,928 miles. D b =CB= 3T8,845 miles. 
#=— = 1*08181 
B 
r+1 
r 
=^=1-08333 
l.Z 
r + 2 
r+1 
—=1-07697 
lo 
8 = s - f :=+°- 00152 
& '=?|-lr=-0' 0 0 484 
lo D b 
2 k 2 
