Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/436

 stars in the line of sight, which has proved of great service in astrophysical research, was suggested by Fizeau in 1848.

Passing now from the astronomical observatory, we must examine the information which has been gained in the physical laboratory regarding the effect of the earth's motion on optical phenomena. We have already referred to the investigations by which the truth of Fresnel's formula was tested. An experiment of a different type was suggested in 1852 by Fizeau, who remarked that, unless the aether is carried along by the earth, the radiation emitted by a terrestrial source should have different intensities in different directions. It was, however, shown long afterwards by Lorentz that such an experiment would not be expected on theoretical grounds to yield a positive result; the amount of radiant energy imparted to an absorbing body is independent of the earth's motion. A few years later Fizeau investigated another possible effect. If a beam of polarized light is sent obliquely through a glass plate, the azimuth of polarization is altered to an extent which depends, amongst other things, on the refractive index of the glass. Fizeau performed this experiment with sunlight, the light being sent through the glass in the direction of the terrestrial motion, and in the opposite direction; the readings seemed to differ in the two cases, but on account of experimental difficulties the result was indecisive.

Some years later, the effect of the earth's motion on the rotation of the plane of polarization of light propagated along the axis of a quartz crystal was investigated by Mascart. The result was negative, Mascart stating that the rotation could not have been altered by more than the (1/40,000)th part when the orientation of the apparatus was reversed from that of