164 ESSAY ON THE VELOCITY OF LIGHT. 
ARTICLE PUBLISHED BY M. FOUCAULT, IN THE ‘‘ JOURNAL DES DEBATS,” 
ON THE REALIZATION OF THE EXPERIMENT OF ARAGO. 
[Number of Tuesday, April 30, 1850. ] 
To THE Eprror—Sir: I will not wait for the expiration of the fortnight 
to give you an account of what most occupied the Academy of Sciences during 
their meeting of yesterday. All knew that M. Arago was to continue the 
account of his beautiful researches of polarization and of photometry. The 
attendance was large, and the Academy recorded at its session a foreign asso- 
ciate and two corresponding members—Mr. David Brewster, Lord Brougham, 
and M. de la Rive, of Geneva. But what was not expected was, that M. Arago 
recalled attention to one of the most beautiful projected experiments that the 
genius of a savant has ever produced, and he declared that, after having con- 
ceived it, he had left to the young generation the care and the honor of per- 
forming it. ‘T'his experiment has more than once occupied the attention of the 
Academy; it proposes to decide, by means of a revolving mirror, whether 
light moves faster in air than in water, and to seek, in the probable result of 
this experiment, the confirmation of the theory at present adopted to explain 
all optical phenomena. You may judge, sir, of the emotion with which I 
heard this generous declaration; I, who for several days had in my hands the 
experimental solution of this great problem! Nevertheless I thought it proper 
to postpone to the next meeting the reading of the paper in which I have 
recorded my results. In the mean time permit me, sir, to announce, in a few 
words, the results which I have observed. 
Light employs more time to run over the same path in water than in air, and 
the time which it takes to traverse these two different media is shown by the 
deviation of the ray which is reflected at a given moment from a mirror re- 
volving with a great velocity. All things remaining equal, the deviations were 
found to be proportional to the indices of refraction of air and of water. It is 
not possible to entertain the least doubt as to the reality of these results; they 
have been obtained by two different methods. The two deviations were first 
observed successively and found unequal for the same velocity of the mirror. 
They were then observed simultaneously, which cendered the observation still 
more certain. ; 
Permit me to limit myself to the rather technical expression of these new 
results. When the columns of the Journal are unoccupied I shall enter into 
such developments as will render these propositions more intelligible to your 
readers. 
Receive, sir, &e., &e. 
LEON FOUCAULT. 
[Number of Saturday, May 4, 1850.] 
We published last Tuesday a letter of M. Foucault announcing the success 
of an optical experiment originally devised by M. Arago, and which, in giving 
the relative velocities of light in air and in water, accomplished the overthrow 
of the emission theory in favor of the theory of undulation. The sun haying 
appeared during the few days past, they have been able to repeat several times 
the experiment in presence of a certain number of French and foreign savants, 
and already the methods which have insured success are generally known to 
the public. In waiting for the communication which will be given at the 
meeting of the Academy next Monday, we will concisely indicate the funda- 
mental parts of the experiment. , 
A beam of sunlight reflected from a heliostat in a fixed direction penctrates 
horizontally a dark room; it first passes through a small opening of 2 milli- 
metres (.0797 of an inch) square, then a,reticule extended behind this opening, 
