160 ESSAY ON THE VELOCITY OF LIGUT. 
For equal lengths of the two media the ratio of the two deviations should be, after one or the 
other of the theories of light, either $ or 3. “s 
“But, instead of taking equal lengths, we can take them so that they will be for the 
water and for the air in the ratio of 4 to 3. According to the theory of emission, these 
lengths are equivalent, or are run over in equal times, and the deviations should be equal. 
According to the theory of undulations, on the contrary, these lengths should be run over in 
very different times, which are for water and for air in the ratio of 16 to 9, and the deviations 
should be in the same ratio - ; 
“We have, therefore, adopted for water a length of 3 metres, (9 feet 10.11 inches, ) and 
for air a length of 2.25 metres, (7 feet 4.53 inches. ) 
‘« The experiment made simultaneously on the two media becomes a very delicate differen- 
tial experiment, in which it is not necessary to know exactly the velocity of rotation of the 
mirror. We have only to compare the simultaneous deviations of the two images. 
‘The apparatus is entirely constructed, but the condition of the atmosphere has not yet 
allowed us to make the observation, and these experiments require so intense a light that it 
is not possible to substitute for the sun’s rays artificial light. If the sky had been cloudless 
yesterday or to-day, we would have been able to have presented to-day the result to the 
Academy. If our experiments are not yet accomplished, it is because we waited until M. 
Arago should authorize us to engage in researches which belong to him.” 
Six weeks later, as we have already said, MM. Fizeau and Bréguet announced 
to the Academy (meeting of 17th June, 1850) the complete success of their 
experiment. ‘They obtained very neat results; the phenomena observed were 
altogether in accord with the theory of undulation, and evidently opposed to 
the theory of emission. 
Thus the beautiful experiment of Arago was doubly realized. That which 
had specially facilitated the success of this experiment, and given a great pre- 
cision to the observation of the phenomena produced, was the modification of 
the original arrangement as indicated by Arago, a modification which had for 
its object to replace the fugitive image observed only by chance in a given 
direction, by a permanent image produced in a determinate place, where we 
can observe it with precision and at our ease. M. Foucault rightly attached a 
great importance to that modification, and in the communication to the Academy, 
the Gth May, 1850, he claimed its invention in these terms: 
‘This memoir has also for its ebject to fix the date of a series of applications of the new 
method, which consists essentially a the observation of the fixed image of a moving unage.” 
But we may see in the communication of M. Fizeau, which we are about to 
reproduce, that, in the apparatus which he had devised with M. Bréguet for 
the execution of the experiment, the same modification is found introduced. 
We there read in fact : 
‘“'The optical arrangement ~* if ‘i ti iy *  -* is founded on the return 
of the rays on themselnes, produced by means of a perpendicular reficction.” 
And further on : 
‘The phenomenon produced by the rotation consists in the deviation of the image in its 
return, which is a@ permanent image resulting from the very rapid succession of instantaneous 
superposed images.” 
M. Fizeau even refers this modification to the arrangement he kad adopted in 
his beautiful experiment of Suresnes, for in speaking of the return of the rays 
on themselves, produced by means of a perpendicular reflection, he adds: “ This 
is the arrangement which I have described in a preceding work.’* ‘The ecom- 
munication of the notices of MM. Foucault and Fizeau, at the same academic 
meeting, constitutes a simultaneous publication which does not allow the slightest 
priority to be established between them on this subject; but M. Moucault took 
care to assure that priority in publishing two days in advance, in a political 
journal, the detailed description of his apparatus.* That which belongs essen- 
* See, at the end of the essay, the article published by M. Foucault, in the Journal des 
Débats of May 4, 1850, 
