158 ESSAY ON THE VELOCITY OF LIGHT. 
telescope. Instead of that, M. Foucault modified the instrument so that the 
reflected rays which should reach the eye of the observer quitted the revolving 
mirror in one direction, determined beforehand, so that the observer could sta- 
tion himself so as to receive conveniently all the reflected rays without leaving 
anything to chance. To accomplish this the light from the luminous object 
fell upon the revolving mirror, not in an intermittent manner, but continuously, 
so that it was reflected in all directions around its axis; in one of these direc- 
tions the reflected light met a fixed mirror on which it fell perpendicularly, 
and which caused it to return over its path in sending it again to the rotating 
mirror; there it experienced a new reflection which sent it to the luminous 
object whence it set out. It was, therefore, near that object that the observer 
placed his eye to receive the reflected rays, rays which, for convenience, M. 
Foucault turned to one side by means of a transparent plate of glass inclined 
45 degrees, similar to that which M. Fizeau had employed in his experiment 
of Suresnes. The following are the words of M. Foucault in describing his 
apparatus : 
‘*A direct ray of light, penetrating a square opening, meets, very near the aperture, a reti- 
cule of eleven vertical wires of platinum to the millimetre, (.03937 of an inch ; ) thence it 
passes towards an excellent achromatic lens of long focus, placed at a distance from the 
reticule less than double the principal focal distance. The image of the reticule of greater 
or less dimensions would be formed on the other side, but, after having traversed the lens, 
the pencil, before its convergence at the focus, falls upon the surface of the revolving mirror, 
and, animated with an angular motion double that of the mirror, it forms in space an image 
of the vertical wires, which is disp'aced with great rapidity. During a small portion of its 
revolution this image meets the sutace of a concave mirror, whose centre of curvature coin- 
cides with the centre of figure and the axis of rotation of the revolving mirror, and, during 
all the time it passes over its surface, the light which has concurred to form it retraces its 
path and falls upon the reticule itself, producing there its image, equal to it insize. In order 
to observe this image without shuiting out the original beam, we place obliquely to the beam 
of light, near the reticule, between it and the object glass, a glass plate, and we observe with 
a powerful ocular the image thrown to one side. * * * The mirror, in revolving, causes 
this image to reappear at each revolution, and, if the velocity of the motion of rotation is 
uniform, it remains immovable in space. For velocities which do not surpass thirty turns 
per second, its successive apparitions are more or less distinct, but over thirty turns give a 
persistence to the impressions on the eye, and the image appears absolutely fixed. 
‘*It is easy to demonstrate that the mirror, in revolving with greater or less rapidity, will 
displace this image in the direction of the motion of rotation. In fact, the light which passes 
between the wires of the reticule does not return to the wires until it has received from the 
revolving mirror two reflections, separated by the time it takes to run over double the path 
from the revolving mirror to the concave mirror. But, if he mirror revolves very fast, this 
time taken by the liglit to go and come back, even over the small length of 4 metres, (13.12 
feet, ) cannot be regarded as inappreciable, and the mirror has had the time to change sen- 
sibly its position, which is shown by the change of position in the image formed by the re- 
turning beam. Rigorously speaking, this effect takes place as soon as the mirror turns, even 
slowly ; but it cannot be observed until the mirror has acquired a certain velocity, and only 
when we employ certain precautions in the experiment. All my efforts have tended to render 
this deviation as apparent as possible. 
**The principal obstacle to surmount is that, in so complicated a path, the light cannot 
converge to the focus in a neat, clear image. ‘The deadening which the pencil experiences, 
in being reflected twice from a turning mirror of small surface, necessarily destroys the nicety 
of the image, and produces in its contour an unavoidable mistiness. It is for this reason that 
we have chosen for source of light the equi-distant linear spaces between the wires of a very 
fine net. Although the image obtained is never clear, yet it is presented under the form of a 
system of white and black stripes, similar to colorless diffraction bands, each having a well- 
defined maximum and a minimum of light. Like the wires of the net, these luminous or 
obscure spaces are distant from each other one-cleventh of a millimetre, (a millimetre equals 
-03937 of an inch, ) and if, to observe them, we place in the occular a micrometer divided into. 
tenths of a millimetre, the two systems ef lines will operate, by their relative displacements, 
as a vernier, and will permit us to measure in the image, with certainty, a displacement of 
the one hundredth of a millimetre. : 
‘After the known velocity of light, with an objective of 2 metres (6.56 feet) focus, and in 
using the double path of 4 metres, (13.12 feet,) we find that we need not give to the mirror 
an extreme velocity (six or eight hundred revolutions per second) in order to obtain displace- 
ments of two and three-tenths of a millimetre. Fé = - S * ~ 
‘Such is the construction of the optical apparatus which has permitted me to show the 
successive propagation of luminous rays. My first attempts succeeded in the air with a 
