VELOCITY OF WHITE AND OE COLOURED LIGHT. 
241 
the rays proceed onwards through the eye-piece to the eye of the observer, who sees 
an illuminated image of the distant object-glass, which from its small size looks like 
a star of light. If now the toothed wheel revolves very slowly he sees the star 
eclipsed at intervals by the teeth.; but if it be revolving so that at least ten teeth pass 
in a second, then, owing to the persistence of visual impressions he sees the star as a 
continuous light upon a brightish field produced by the illuminated rotating teeth. 
If the speed be further increased the brightness of the star diminishes owing to the 
light which passed between two teeth in leaving the observing station being partially 
stopped on its return by the advance of a tooth towards that space. So with increasing 
speed he sees the star disappear, then re-appear, attain its full brightness, diminish, 
disappear, reappear, &c., passing through similar phases with perfect regularity. 
One thing more is required either in the method of M. Fizeau or our own, and that 
is to have the means of determining at any instant the velocity of rotation of the 
toothed wheel. Cornu was the first to attempt to do this with accuracy: he 
connected the mechanism of the toothed wheel electrically with a chronograph so as to 
make a mark every 100 revolutions of the toothed wheel. A clock at the same time 
marked seconds, and by means of a vibrating spring tenths of a second were marked; 
while a fourth marker was under the control of the observer, who signalled the 
instant when he wished the velocity to be determined. Cornu did not attempt 
to maintain a uniform speed of revolution in the toothed wheel, but was able by means 
of the chronographic record to tell the velocity and rate of change of velocity at 
definite times, and hence, by interpolation, the exact velocity at any instant. 
The plan of the present research was arranged in 1872, but it was not until 1875 
that the apparatus was made. Since then the apparatus has been partially modified 
in order to overcome the optical and mechanical difficulties which arose in the course 
of the work. The most important pieces were—(1) the telescope, (2) the reflectors, 
(3) the revolving toothed wheel, (4) the clock, (5) the chronograph, (6) the dynamo- 
electric machine, and (7) the lamp. 
1. The telescope .—This consists of a 5-inch achromatic object-glass of good quality, 
with a focal length of 7 feet. A Bohnenberger’s eye-piece is employed, consisting of 
an erecting eyepiece with a piece of plain glass in front of the field lens and inclined to 
the axis of the telescope at an angle of 45°. A lateral hole in the tube of the eye-piece 
allows the light from a lamp, &c., at the side, to be reflected by this diagonal mirror 
along the axis of the telescope, and thence to the distant reflectors. The adapter, 
which connects the eye-piece with the body of the telescope, is a tube whose lower half 
is cut away so as to allow the revolving mechanism to be placed below in such a way 
as to bring the upper part of the revolving toothed wheel into the axis of the 
telescope and exactly in its focus. The light from a lamp or from the sun can be con¬ 
centrated by a lens so as to throw its image upon the top of the toothed wheel. By 
looking along the axis of the telescope, from the object-glass, an observer can notice 
whether any of the light falls upon the inner sides of the telescope, and in this manner 
MDCCCLXXXII. 2 I 
