ESSAY ON THE VELOCITY OF LIGHT. 159° 
mirror which made only twenty-five to thirty turns per second, the length of the double path: 
being four metres. 
‘* In order to make the experiment with water, we have only to place between the revolving 
mirror and the concave mirror a column of this liquid, held between two parallel plates of 
giass in a conical metallic tube, varnished inside with copal, so that the water would remain. 
clear, to take the necessary precautions that the terminal plates are not strained in their frames, 
and to obviate the inconvenience of the change of focus by the interposition of a liquid layer 
of 3 metres (9.84 feet) thickness, having paral.el surfaces. Jn the end we succeeded in easily 
obtaining, with the feeble and green ray which has traversed the water, an image as distinct 
as that which is formed withou: the interposition of the liquid. Therefore it is required but 
to turn the mirror and to measure with precision its velocity of rotation if we desire to deduce: 
the absolute velocities in air and in water, or to operate simultaneously on these two media 
if we wish to know only the character and difference of these velocities.” 
The second modification introduced by M. Foucault, in the arrangement of 
the apparatus Arago proposed to employ, was in reference to the means of giving: 
to the revolving mirror an extremely rapid movement of rotation and of keep- 
ing up this motion a sufficient length of time. In the apparatus of Arago, M. 
Bréguet produced this movement of rotation by means of wheels with oblique 
teeth revolving under the action of a weight. M. Foucault substituted the 
direct action of steam, evolved at a certain pressure from a steam-boiler, on a. 
little turbine fitted to the axis carrying the mirror. The much greater simpli- 
city of this rotating mechanism offered very notable advantages in the execution: 
of the experiments. 
In his communication to the Academy, M. Foucault thus announced the re- 
sults he had already obtained : 
‘In confining myself to the determinations of the velocity (of the mirror) by the sound, 
(produced by the action of the steam on the little turbine, ) as I have already proved by two 
successive observations that the deviation of the image after the passage of the light through 
the uir is less than after its passage through the water, I have also made another confirmatory 
experiment, which consists in observing the image formed in part by the light which has 
traversed the air, and in part by the light which has traversed the water. During low 
velocities the stripes of the compound image were sensibly the continuations of cach other, 
and, by the acceleration of the movement of rotation, the image is carried to one side, and the 
stripes are broken at the boundary line, at the junction of the air image with the water image, 
the stripes of the latter being in advance in the direction of the common deviation. Moreover. 
in taking into account the lengths of air and water traversed, the deviations were seen to be- 
proportional to the indices of refraction. ‘These results demonstrate a velocity of light less in 
water than in air, and fully confirm, according to the views of Arago, the indications of the 
theory of undulations.” 
M. Fizeau, on his part, made to the Academy of Sciences, in the same meet- 
ing of the 6th of May, the point to which he and M. Bréguet had arrived in 
their attempts to attain the same result. We reproduce here his communication 
in full, taken from the Compte rendu of the meeting: 
‘*We have undertaken to realize the important experiment of which M. Arago spoke to: 
the Academy at its last meeting, and which he is not able to undertake at this moment on 
account of the feeble condition of his sight. 
“The rotary apparatus of M. Bréguet carries a small mirror of 12 millimetres (.472 of an 
inch) in diameter, to which can be given a velocity of nearly 2,000 turns per second, and 
which easily makes 1,200 or 1,500 revolutions. 
‘**The optical arrangement that we employ is founded on the return of the rays on them- 
selyes, produced by means of a perpendicular reflection: this is the arrangement which I 
described in a preceding work. 
‘*The light emanating from a luminous image, formed at the focus of the telescope, 
traverses the objective, meets the revolving mirror, and is reflected normally trom a fixed. 
mirror; then comes back to the revolving mirror, traverses the object glass, and returns 
finally to the focus. 
‘Tbe phenomenon produced by the rotation consists in the deviation of the return-image, 
which is a permanent image resulting from the very rapid succession af instantaneous super- 
posed images; the deviation results from the angular movement made by the revolving 
mirror, whilst the light runs over double the distance which separates it from the fixed 
mirror. After having observed the deviations in air for distances which were varied, so as 
to obtain the greatest intensity of light and the best definition possible, we arranged the- 
experiment so as to observe simultaneously the corresponding deviations of water and of air.. 
