152 ESSAY ON THE VELOCITY OF LIGHT. 
measure of the velocity of light was first accomplished on the surface of the 
earth. 
MEASURE OF THE VELOCITY OF LIGHT, BY M. FIZEAU,. 
If we refer to the first pages of this essay we will there see indicated a very 
simple means that was first mentioned as suitable for the determination of the 
velocity of light, a method which would certainly have long ago furnished the 
value of that velocity if the transmission of light in space was not effected 
with such amazing velocity. Itis this method, barely modified, that M. Fizeau 
has put into practice, and which he has rendered completely successful by 
employing a process as simple as ingenious for measuring the excessively short 
time that the light takes in running over a distance of several kilometres, or a 
few miles. 
We have supposed that two lamps, A and B, were placed at a distance of 
several kilometres from each other, and that we shut off suddenly the light of 
A by means of a screen, then the very instant that the disappearance of the 
light of the lamp A was perceived at B, we screened B likewise, the interval 
of time which elapsed between the instant that the lamp A was screened and 
that when the disappearance of the light of lamp B was perceived at A is 
evidently the time employed by the light to run over twice the distance which 
separates the lamps. M. Fizeau replaced the lamp B by a mirror destined to 
receive and to reflect perpendicularly to its surface the light of the other lamp. 
In this manner the observer stationed near the lamp <A, and looking at the 
mirror just spoken of, ought to see the image of the lamp A; if this lamp is 
suddenly screened he should see its image disappear in the mirror, not imme- 
diately, but after a certain interval of time—that which the light takes to run 
over double the distance of the lamp from the mirror. 
To measure the interval of time spoken of M. Fizeau made use of a disk 
toothed at its circumference like a cog-wheel, so that this disk presented on its 
periphery a succession of equal intervals alternately open and closed. If we 
rapidly turn such a disk in its plane and around its centre, the closed and 
open spaces formed by the cogs and the intervening openings will pass suc- 
cessively before the same point in space, and the time employed by each tooth 
or by each opening to come to the place previously occupied by a tooth or by 
an opening can easily be reduced to a very small fraction of a second. In 
placing the apparatus so that the teeth of the turning disk pass in succession 
before the light of the lamp near the observer, these teeth will act like screens — 
which periodically intercept the light. If the time employed by each opening 
to come to the place occupied by the next preceding cog or tooth is precisely 
that required by the light to go from the lamp to the mirror and from the mir- 
ror back again to the lamp, the observer, stationed quite near to this lamp and 
looking through the openings of the turning disk as they pass before his eye, 
ought to see through each epening the extinction of light caused by the 
passage of the preceding tooth before the lamp. This eclipse of the lamp, 
produced during the whole time of the passage of the tooth before its light, is 
perceived by the observer during the whole time of the transit of the following 
opening ; and as the light of the lamp wheu it passes through an opening of 
the disk, after its reflection from the mirror, meets the tooth which is now in 
the place of this opening, it follows that the observer does not perceive at all 
the image of the lamp in the mirror during all the time that the disk revolves 
with the special velocity with which we have supposed it endowed. 
After what has just been said it is easily understood what will happen when, 
the apparatus adjusted, we commence by giving to the disk a gradually in- 
creasing velocity of rotation. The disk first turning with an extreme slowness, 
the observer, who has his eye quite near the side of the lamp and who looks 
in the direction of the mirror, sees the teeth and the openings of the disk sue, 
