ESSAY ON THE VELOCITY OF LIGHT. 157 
‘I can only, in the present condition of my sight, accompany with my good wishes the 
experimenters who desire to follow my ideas, and to add a new proof in favor of the wave 
system to that which I have deduced from a phenomenon of interierence teo well known to 
physicists to need recalling here.” 
The communication of Arago, of which we have here given the principal parts, 
was hardly printed in the Compte Rendu, of the meeting of April 29, 1850, 
when the Academy received, at the following meeting, (May 6.) two important 
communications on this subject, one of M. Foucault, the other of MM. Fizeau 
and Bréguet. M. Foucault announced that he had realized with entire success 
the beautiful experiment projected by Arago ;* he made known at the same 
time the modifications he had given to the mode of arrangement indicated by 
the illustrious perpetual secretary of the Academy, a modification which had 
allowed him to arrive at this important result, which gave entire evidence of 
the truth of the theory of undulation as opposed to that of emission. MM. 
Fizeau and Bréguet had not progressed so far; they announced to the Academy 
that their apparatus was adjusted and ready to work, and also showed in what 
this apparatus differed from the original arrangement of Arago. Finally, a 
few weeks later, June 17, they returned to announce to the Academy that 
they likewise on their part had made this remarkable experiment; the result, 
agreeing with that of M. Foucault, was in favor of the theory: of undulation. 
Two important modifications had been made by M. Foucault in the arrange- 
ment of Arago’s apparatus. ‘The first of these changes had for its object to 
render the realization of the experiment incomparably easier. It is remem- 
bered that, in the experiment of Arago, the light had to set out, so to say, 
instantaneously from two luminous points, or rather from a luminous line 
shining only during an excessively short time; that one beam of that light, 
after having travelled in the air, and the other beam in a liquid, were to fall 
upon a mirror endowed with an excessively rapid movement of rotation; that, 
finally, after being reflected from this turning mirror, they should arrive at the 
eye of the observer, furnished for that purpose with a good telescope. The 
direction of the ray could very easily be determined in advance, since it was 
that of a line going from the source of light to the mirror; but it is not the 
same with the direction of the reflected rays which depend essentially upon 
the position occupied by the mirror at the instant the reflection takes place, and 
as the motion of the mirror and the reflection of the luminous rays from its 
surface are independent of cach other, it is only by chance that the mirror 
would be found in such and such a position; the observer, therefore, cannot 
know in what direction he should place his telescope in order to receive the 
luminous rays after their reflection. To obviate that difficulty Arago supposed 
thatthe observer, being stationed anywhere in the space the reflected rays 
could reach, and having directed his telescope towards the revolving mirror, 
would successively repeat a great number of times the instantaneous emission 
of light towards the rotating mirror, so that the rays from each of these emis- 
sions being reflected from the mirror when in different positions, theré necessa- 
rily would be some which would fall upon the objective of the telescope and 
thus reach the eye of the observer. It was therefore a fugitive phonomenon 
which the revolving mirror projected at each instant into the surrounding 
space, sometimes in one direction, sometimes in another, and which by chance, 
from time to time, would strike the telescope of the observer. It was to mul- 
tiply the chances of observation of this fugitive phenomenon that Arago, at 
the end of his communication of 1838, spoke of substituting for the single 
mirror a vertical prism of eight or ten facets, and to employ at the same time 
several observers placed in different positions, and each provided with a 
*See, at the end of this essay, the article published by M. Foucault, in the Journal des 
Débats, of May 4, 1850. 
