136 ESSAY ON THE VELOCITY OF LIGHT. 
According to the system of undulation light is produced by a succession of 
vibratory motions of the ether which emanate from the luminous body and are 
propagated indefinitely around it into space. We can arrive at a clear idea of 
such propagation by observing the circular waves which dilate from a centre 
of agitation on a smooth liquid surface, such, for example, as are produced when 
we cast a stone into a calm lake. ‘The luminous waves are spherical, and have 
for common centre the source of the light; they expand as they move forward, 
like the circular waves formed on the surface of the water. If we imagine a 
straight line drawn from the centre of vibration and extended indefinitely into 
space, each luminous wave will reach the different points of that line along 
which it uniformly progresses ; the velocity of the light is the length the wave 
runs over on that line in the unit of time. But we can also have a clear idea 
of the velocity of light without adopting any hypothesis in reference to the 
manner in which it is produced. If a source of light, a candle for example, is 
lighted or extinguished suddenly, this instantaneous phenomenon will not be 
perceived at the same instant at all points of the space from which we can 
observe it. If we are quite near to the light we will perceive it immediately ; 
if we are far off there will be a certain amount of time between the production 
of the phenomenon and its perception by the eye, and this time will be so much 
the longer as the distance between the source of light and the eye is greater. 
The phenomenon of which we have just spoken takes, therefore, a certain time 
to run over the distance which separates the place of its production from the 
eye of the observer, and it is natural to admit that it takes equal times to run 
over equal portions of that distance; that is to say, that it progresses in any direc- 
tion whatever, like a moving body animated with a uniform velocity. ‘The 
velocity of light is the velocity with which a phenomenon of light is thus 
propagated to a distance; or, in other terms, it is the distance at which the 
eye ought to be placed trom the point where the phenomenon is produced, in 
order that a unit of time may elapse between the instant of its production and 
the instant of its perception by the eye. 
After what has been said, nothing can be easier than to invent the means to 
determine the velocity of light. It will suflice to proceed as we always do 
when we desire to measure the velocity of a mass moving uniformly. If we 
wish to determine, for example, the velocity of a train on a railroad, we place 
ourselves in one of the ears, and, furnished with a second-watch, we observe 
how many seconds elapse between the instant of passing one mile-post and the 
instant of passing the next post following; if we thus find 2 minutes or 120 
seconds, we divide the distance of 1 mile or 5,280 feet by 120, and the quotient 
44 shows that the train runs over 44 feet in 1 second. In general, we observe 
the time employed by the body to move over a known distance, and then we 
divide the distance by the time expressed in seconds, and the quotient is the 
velocity in one second of time. 
In order to apply this method to the measure of the velocity of light, let us 
imagine two sources of light, A and B, two lamps with reflectors, for example, 
placed several thousand yards apart. If we suddenly place just before the 
lamp A a screen, the observer at B will not at that instant see the light at A 
disappear; he will see it disappear only after the time employed by the light 
to run over the distance between the two lamps. If at the very instant that 
the observer at B sees the light A disappear he screens, in his turn, the light B, 
the observer at A will not see the light B disappear until a short time after it 
-has been screened, on account of the time the light occupies to run over the 
distance between the two lamps. The interval of time comprised between the 
instant the lamp A was screened and the instant when the observer stationed ~ 
at that lamp perceived the disappearance of the lamp B, is, therefore, the time 
employed by the light to run over twice the distance A B; if we measure this 
