Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/141

 tion of the apparatus relative to the direction of the earth's motion. E. Mascart in 1872 discussed experimentally the question of the effect of notion of the source or recipient of light in all its bearings, and showed that the light of the sun and that derived from artificial sources are alike incapable of revealing by diffraction-phenomena the translatory motion of the earth.

The greatest problem now confronting the investigators of light was to reconcile the facts of polarization with the principles of the wave-theory. Young had long been pondering over this, but had hitherto been baffled by it. In 1816 he received a visit from Arago, who told him of a new experimental result which he and Fresnel had lately obtained —namely, that two pencils of light, polarized in planes at right angles, do not interfere with each other under circumstances in which ordinary light shows interference-phenomena, but always give by their reunion the same intensity of light, whatever be their difference of path.

Arago had not long left him when Young, reflecting on the new experiment, discovered the long-sought key to the mystery: it consisted in the very alternative which Bernoulli had rejected eighty year's before, of supposing that the vibrations of light are executed at right angles to the direction of propagation,

Young's ideas were first embodied in a letter to Arago, dated Jan. 12, 1817. "I have been reflecting," he wrote, "on the possibility of giving an imperfect explanation of the affection of light which constitutes polarization, without departing from the genuine doctrine of undulations. It is a principle in this theory, that all undulations are simply propagated through homogeneous mediums in concentric spherical surfaces like the