BGP? ESSAY ON THE VELOCITY OF LIGHT. 
If, on the contrary, the mirror makes alittle move than 400 turns per second, the 
teeth of the disk at the moment they are illuminated appear more and more 
behind a fixed position, and the disk seems to turn slowly in a direction the 
reverse of its real motion. When we have so adjusted the velocity of rotation 
of the mirror that the appearance of the border of the toothed disk which we 
see in the field of microscopé appears immovable, we are certain that the mir- 
ror makes exactly one turn while the circumference of the disk progresses one 
division, and consequently whilst the mirror makes exactly 400 turns per second. 
It was by adding this improvement or complement to his apparatus of 1850, 
by substituting compressed air (evolved from the blowing-machine of constant 
pressure of M. Cavaillé-Coll) for steam as the motive power of the little turbine 
attached to the axis of the mirror, by increasing the length of the path of the 
light between the two reflections from the revolving mirror from 4 to 20 metres 
(from 13 feet 1.48 inches to 65 feet 7.4 inches) by means of successive reflec- 
tions from intermediate fixed mirrors, by taking every possible precaution to 
measure the displacement of a few tenths of a millimetre given by the revolving 
mirror to the position of the image in its return, that M. Foucault, in September 
1862, succeeded in determining with a certain precision the velocity of light in 
air. He thus founda velocity of 298,000 kilometres (185,177 miles) per second, 
a velocity a little below that of 308,000 kilometres, (191,391 miles,) which re- 
sult from the value of aberration (20.45 seconds) deduced by M. Struve, from 
very exact astronomical observations, combined with the value (8.57 seconds) 
adopted up to this period for the parallax of the sun. 
Let us recapitulate the series of labors which has led to so remarkable a 
result : 
Mr. Wheatstone (1834-’36) devised the use of a rapidly revolving mirror, to 
render appreciable excessively small intervals of time. 
Arago showed (1838) how, with the aid of such a revolving mirror, we would 
be able to determine the difference of velocities with which light traverses air 
and a liquid. 
In the mean while M. Fizeau succeeded (1849) in being the first to render 
evident the progressive transmission of light and to measure its velocity, by 
means of an experiment made on the surface of the earth, in the space of a few 
kilometres, and adopted an altogether different method from that proposed by 
Arago. 
An important modificatien, introduced (May, 1850) in the arrangement de- 
vised by Arago, rendered incomparably easier the observation of the effect due 
to the difference of velocity of light in air and in a liquid. 
Owing to this modification the experiment of Arago was executed nearly at 
the same time (May and June, 1850,) first by M. Foucault, and thea by MM. 
FPizeau and Bréguet. 
Finally, M. Foucault, perfecting his apparatus, measured (in 1862) the velocity 
of light in operating entirely in the interior of a laboratory of moderate dimen- 
sions. 
An admirable experiment in physics, in which, by the power of intellect and 
manual skill, we have succeeded not only in rendering sensible, but even 
measurable, the time employed by light to rua overa path of 20 metres, (65 feet 
7.4 inches ) although this time barely equals the psga/ogqgth of a second! and 
which, if we repeat it so as to vary its elements, and thus make evident the 
constant causes of error wh.ch affect the result, appears capable of giving a 
determination of the velocity of light altogether as precise as that which is de- 
duced from astronomical phenomena! 
We would be unjust if we did not mention that the success of the experi- 
ment of M. Foucault was greatly indebted to M. Froment, the able constructor 
of the different parts of the apparatus. The clock-work which gave to the 
