_Undulations are excited in this Ether whenever a Body becomes
luminous._
_Scholium_. I use the word undulation, in preference to vibration,
because vibration is generally understood as implying a motion which is
continued alternately backwards and forwards, by a combination of the
momentum of the body with an accelerating force, and which is naturally
more or less permanent; but an undulation is supposed to consist in a
vibratory motion, transmitted successively through different parts of a
medium, without any tendency in each particle to continue its motion,
except in consequence of the transmission of succeeding undulations,
from a distinct vibrating body; as, in the air, the vibrations of a
chord produce the undulations constituting sound.
_Passages from_ NEWTON.
“Were I to assume an hypothesis, it should be this, if propounded more
generally, so as not to determine what light is, further than that it
is something or other capable of exciting vibrations in the ether; for
thus it will become so general and comprehensive of other hypotheses,
as to leave little room for new ones to be invented.” (BIRCH. Vol. III.
p. 249. Dec. 1675.)
“In the second place, it is to be supposed, that the ether is a
vibrating medium like air, only the vibrations far more swift and
minute; those of air, made by a man’s ordinary voice, succeeding one
another at more than half a foot, or a foot distance; but those of
ether at a less distance than the hundred thousandth part of an inch.
And, as in air the vibrations are some larger than others, but yet all
equally swift, (for in a ring of bells the sound of every tone is heard
at two or three miles distance, in the same order that the bells are
struck,) so, I suppose, the ethereal vibrations differ in bigness, but
not in swiftness. Now, these vibrations, beside their use in reflection
and refraction, may be supposed the chief means by which the parts of
fermenting or putrifying substances, fluid liquors, or melted, burning,
or other hot bodies, continue in motion.” (BIRCH Vol. III. p. 251. Dec.
1675.)
“When a ray of light falls upon the surface of any pellucid body,
and is there refracted or reflected, may not waves of vibrations, or
tremors, be thereby excited in the refracting or reflecting medium?—And
are not these vibrations propagated from the point of incidence to
great distances? And do they not overtake the rays of light, and by
overtaking them successively, do not they put them into the fits of
easy reflection and easy transmission described above?” (Optics, Qu.
17.)
“Light is in fits of easy reflection and easy transmission, before its
incidence on transparent bodies. And probably it is put into such fits
at its first emission from luminous bodies and continues in them during
all its progress.” (Optics. Second Book. Part III. Prop. 13.)