154 ESSAY ON THE VELOCITY OF LIGHT. 
measure the velocity of rotation of the disk, we can easily calculate the time 
required by a tooth to take the same position as was just before occupied by. 
the next preceding opening ; and, as this time is precisely that employed by 
the light to run over twice the distance of the mirror from the lamp, it is easy 
thence to deduce the value of the velocity of light. Such is, in principle, the 
mode of arrangement of the apparatus of M. Fizeau. We will now give such 
an account of the details as will convey a clear idea of this apparatus. 
In the explanation which precedes, we have supposed that the cye of the 
observer is placed very near the lamp and looks in the direction in which the 
lamp sends its light to the mirror, so that the incident ray, setting out from the 
lamp, and the reflected ray, coming back from the mirror, to strike the eye, follow 
one and the same path. Such conditions seem very difficult to obtain; but all 
difficulty disappears by using a transparent glass plate, which, being inclined 
45 degrees on the direction of the luminous rays, reflects to a right angle a 
part of these rays from its surface, whilst the remaining portion traverses it: 
without change of direction. Such a glass plate being placed before the eye, 
with an inclination of 45 degrees on the line drawn from the eye to the mirror, 
the lamp can be placed laterally, so as to send its light on the glass perpen- 
dicularly to that line; the glass reflects towards the mirror a portion of the 
light it thus receives from the lamp; that light, after being reflected, returns 
to the glass plate, part of which passes through it to reach the eye. 
Rays of light which emanate from a luminous source diverge to all points 
around their place of origin; it follows that the further a given surface is from 
the source of light, the less it must receive. If we therefore do not adopt some 
particular means in the experiment we are engaged in, the mirror, placed at 
several kilometres from the lamp, will receive only an insignificant quantity of 
light ; moreover, only a very small part of that light will come back to strike 
on the eye, so that we would really perceive nothing. In order that the light 
may not thus be lost in surrounding space by the divergence of its rays, we 
employ converging lenses placed one near the source of light and the other 
near the mirror. ‘These lenses are so disposed that the rays radiating from 
the lamp, after having traversed the first lens, become parallel and form a 
beam which falls without loss upon the second lens; after having passed 
through the second lens and converged towards its focus, where is placed the 
mirror, they are reflected, come back to the lens, which renders them anew 
parallel for their return to the point of departure; then they pass through the 
first lens, which causes them again to converge, and finally they are viewed 
by the eye aided with an ocular. In fact, the two converging lenses of which 
we speak are objectives of two telescopes placed at the two extremities of the 
distance over which the light travels, and directed towards each other so that 
the image formed by the objective of the one is seen at the focus of the other. 
It is in the interior of the first telescope, between the focus and the oceular, 
that is placed the transparent glass plate inclined 45 degrees, of which we 
spoke above, to receive and to reflect along the axis of the telescope the light 
of a lamp placed on the side. As to the mirror which sends back the light 
to its point of departure, we place it exactly at the focus of the second tele- 
scope. M. Fizcau, in his notice read before the Academy of Sciences, (meeting 
of July 23, 1849,) says: 
“This arrangement succeeds very well, even when the telescopes are separated by con- 
siderable distances. With telescopes of 6 centimetres (234; inches) aperture, the distance can 
be 8 kilometres (nearly 5 miles) without the light becoming too feeble. We thus see a 
duminous point like a star, and formed by the light, which, setting out from this point, has 
traversed a distance of 16 kilometres, (nearly 10 miles,) then returned and passed exactly 
through the same point to reach the eye. ° 
“It is exactly at this point that the teeth of the revolving disk must pass to produce the 
effects spoken of. ‘The experiment succeeds very well, and we observe that, according to the 
greater or less velocity of rotation, the luminous point shines brilliantly or is totally eclipsed. 
