1) by any spherical body--by a comet, for example; then,
under the excitation of the waves that it is traversing, and through its
velocity, the comet will itself enter into action, and produce
mechanical waves in its turn. As the trace produced in the solar waves
consists of an agitation of the ether on such trace, it will become
apparent, if we admit that every luminous effect is produced by an
excitation--a setting of the ether in vibration. The mechanical waves
engender of themselves, then, an emission of optical waves that render
perceptible the alteration which they create in each other.
Let a be the position of the comet. The altered wave, a, will carry
along the mark of such alteration in the direction a b, while at the
same time extending transversely the waves emitted by the comet. During
this time the comet will advance to a', and the wave will be altered in
its turn, and carry such alteration in the direction, a' b'.
The succession of all these alterations will be found, then, upon a
curve a'' d' d, whose first elements, on coming from the comet, will be
upon the resultant of the comet's velocity, and of the propagation of
the solar waves. Consequently, the slower the motion of the comet, with
respect to the velocity of the solar waves, the closer will such
resultant approach the line of centers, and the more rectilinear will
appear the trace or tail of the comet.
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