ESSAY ON THE VELOCITY OF LIGHT. 161 
tially to M. Foucault is the substitution of the steam turbine in place of the 
wheel-work employed by M. Bréguet to rapidly turn the mirrow, and that modi- 
fication has also a great importance on account of the facility it gives to vary 
the velocity of the mirror, to regulate it, and to maintain it at a uniform velocity 
during as long a time as we wish. 
MEASURE OF THE VELOCITY OF LIGHT, BY M. FOUCAULT. 
The apparatus which served M. Foucault to determine that light moves faster 
in air than in water was not devised solely for that comparative experiment; 
its principal object was to furnish the absolute value of the velocity of light. 
It was in this point of view that M. Foucault brought it forward in 1850, in 
indicating in a precise manner the means which he proposed to employ in order 
to arrive at a certain precision in that measure. 
We have seen that in this apparatus the return of the rays upon themselves 
gives place to the formation of a permanent image which is displaced trans- 
versely by a quantity so much the greater as the revolving mirror turns more 
rapidly. The measure of this displacement of the image can make known the 
quantity the mirror has turned during the interval of two successive reflections 
of light from its surface in going and in returning—that is to say, while the light 
had run over twice the distance of the revolving mirror from the fixed mirror; 
it is therefore sufficient to know exactly the velocity of rotation of the mirror 
in order to deduce the time elapsed between these two successive reflections— 
that is to say, the time employed by the light to make double the journey from 
the turning to the fixed mirror, and consequently the value of the velocity of 
light. 
"The following is the very ingenious method by which M. Foucault was 
enavied to determine exactly this velocity of rotation, or rather so to adjust the 
motion that the mirror can be made to turn with a velocity determined before- 
hand. 
A wheel-work mechanism gives a uniform movement of rotation to a disk, 
toothed like a circular saw. ‘his disk makes exactly one turn in one second. 
The teeth-which arm its contour, and which are accurately cut equidistant, are 
to the number of 400; so that the time employed by one of these teeth to take 
the place of the one which has preceded it is exactly the ;35th part of a second. 
We so place the wheel that its border cuts the plane of the field of view of the 
microscope with which we observe the return-image from the mirror. If this 
field were continuously illuminated, the teeth of the disk would appear to pass 
before the eye with the velocity of their motion; but itis not thus. The light 
only comes to the field of the microscope at the instant a reflection takes place 
from the revolving mirror ; this field, and consequently the border of the toothed 
disk, are only illuminated by successive flashes of light, and those: flashes are 
governed by the rotation of the mirror, which at each revolution sends rays 
into the interior of the microscopes. If the mirror makes exactly 400 turns 
per second, then the interval between two successive iluminations of the field 
of the microscope is exactly equal to the time employed by each tooth to take 
the place of that which preceded it; so that at the moment of the successive 
illuntinations we always see a tooth of the disk in the same point of the field 
of view: the disk appears absolutely immovable. Suppose now that the mir- 
ror makes a little dess than 400 turns per second; whilst it makes a revolution, 
each tooth of the disk goes a little further than it ought in order to take exactly 
the place of the preceding tooth; at the moment of the successive illumina- 
tions the teeth which replace cach other do not appear any longer exactly at 
the same point in the field of view of the microscope; they appear little by little 
in advance in the direction of the motion of the rotating disk, so that the disk 
appears to have a slow movement of rotation in the direction of its real motion. 
ll s 
