260 On the Effect of the Motion of a Body on Light. 
sufficient to produce a change in the rotation of the plane of 
polarization equal to ;;)5th part of the magnitude of that 
rotation. 
In order to test this result by experiment, a series of glass 
plates were interposed in the path of a polarized beam of parallel 
rays of light. The primitive plane of polarization was determined 
by a divided circle, and the rotation which this plane underwent 
by the action of the plates was measured by means of a second 
graduated circle fixed to aconvenient analyser. The instrument 
could, moreover, be fixed in any direction so as to study the in- 
fluence of all terrestrial motions upon the phenomena. 
In order to make the two necessary observations conveniently 
and rapidly, two mirrors were previously fixed on the east and on 
the west of the instrument, and upon each, alternately, a beam 
of solar light was thrown by means of a heliostat, and thence re- 
flected towards the instrument. 
The greatest difficulties were encountered in the annealing of 
the glass plates of the series; and as perfectly homogeneous 
plates could not be obtained, it was necessary to employ various 
compensating expedients, all which will be found described in 
the memoir itself. 
The conclusions to which M. Fizeau was led by means of more 
than 2000 observations are thus stated :— * 
1. The rotation of the plane of polarization produced by a 
series of inclined glass plates is always greater when the light 
which traverses them comes from the west than when it comes 
from the east; the observation being made about noon. 
2. This excess of rotation is decidedly at a maximum at or 
about noon during the solstices. Before and after this hour it 
is less, and at about 4 o’clock is scarcely perceptible. 
3. The numerical values deduced from the numerous series of 
observations present notable differences, the cause of which may 
be guessed, though it cannot yet be determined with certitude. 
4. The influence of the earth’s annual motion, as determined 
by calculation on the hypothesis of Fresnel, leads to values of 
the above excess of rotation which agree tolerably well with the 
majority of the values deduced from observation. 
5. Theory, as well as experiment, therefore, lead us to con- 
clude that the azimuth of the plane of polarization of a refracted 
ray is really influenced by the motion of the refracting medium, 
and that the motion of the earth in space exerts an influence of 
this kind upon the rotation of the plane of polarization produced 
by a series of inclined glass plates. 
