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But there is another subject connected with our firma—
ment, of a more subtle and recondite character than even
its color. I mean that “ mysterious and beautiful phenom-
enon,” 1 the polarization of the light of the sky. The po-
larity of a magnet consists in its two-enclodness, both ends,
or poles, acting in opposite ways. Polar forces, as most of
you know, are those in which the duality of attraction and
repulsion is manifested. And a kind of two-sidedness—
noticed by Huyghens, commented on by Newton, and dis-
covered by a French philosopher, named Malus, in a beam
of light which had been reflected from one of the windows
of the Luxembourg Palace in Paris—receives the name of
polarization. We must now, however, attach a distinct-
ness to the idea of a polarized beam, which its discoverers
were not able to attach to it. For in their day men’s
thoughts were not sufficiently ripe, nor optical theory suffi-
ciently advanced, to seize upon or express the physical
meaning of polarization. When a gun is fired, the explo-
sion is propagated as a wave through the air. The shells
of air, if I may use the term, surrounding the centre of con-
cussion, are successively thrown into motion, each shell
yielding up its motion to that in advance of it, and return-
ing to its position of equilibrium. Thus, while the wave
travels through long distances, each individual particle of
air concerned in its transmission performs merely a small
excursion to and fro.2 In the case of sound, the vibrations
of the air-particles are executed in the direction in which
the sound travels. They are, therefore, called longitudinal
Vibrations. In the case of light, on the contrary, the vibra-
tions are transversal; that is to say, the individual particles
of ether move to and fro across the direction in which the
light is propagated. In this respect waves of light resem-
ble ordinary water-waves, more than waves of sound. In

1 Herschel’s flIeteorology, Art. 233.
9 Lectures on Sound, p. 3. (Longmans.)

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