Mathematical tracts on the lunar and planetary theories, the figure of the earth, precession and nutation, the calculus of variations, and the undulatory theory of optics.

HYPOTHESES ON THE NATURE OF LIGHT. 257
points in the direction in which the wave is going, but of
none in the direction from which it care. In figure 4 for
instance, if a single wave is going from AB towards CD,
and if EF be the front of the wave at any time, then we
know that the displacement in EF is the cause of future
displacement in GH, because in consequence of the existence
of this wave there will hereafter be a wave at GH: but
we know that the displacement in EF causes no future displacement between EF and AB, because, though the displacement in EF exists, there will hereafter be no wave between
AB and EF. If then we divide EF into a great number
of parts, we must consider the displacement in each as causing a hemispherical or nearly hemispherical wave, which diverges only before the front of the great wave and not
behind it.
APPLICATION OF THIS THEORY TO THE EXPLANATION OF
THE PHIENOMENA OF LIGHT WrHICH DO NOT DEPEND
ON POLARIZATION.
23. We shall suppose that light is the undulation of
a medium called ether whicli pervades all transparent bodies.
Respecting the direction of vibration of each particle we
shall make no supposition till we treat of polarized light,
as the results of this section are independent of the direction
of vibration: to fix the ideas, however, the reader may conceive it to be of the kind represented in fig. 2. We shall
suppose that a great number of similar waves follow without
interruption, and that the function which expresses the displacement of a particle is
a. sin (vt - ) + A}.
When in our final results we have found the expression
c sin vt + C- - }
for the displacement of the particles touching a screen or
touching the eye, we shall assume the intensity of the
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