the wonderful divisibility of matter is nowhere
more apparent than in the minuteness of tin
particles of light. Dr. Niewentüt computed—
from what data does not appear—that an inch
of candle, when converted into light, become
divided into a number of parts, which are
numerated by a row of forty-nine figures, which
I need not transcribe here; at which rate, then
must issue from a burning candle, during every
second of time that it burns, particles amounting
to a total represented by a line of forty-five
figures. The doctor seems to think that he
gives us a clear idea of this number, by stating
that it is vastly more than a thousand times a
thousand million times the number of grains of
sand which the whole earth could contain.

Newton's followers assumed that, if light
consisted in a mere pressure, vibration, or pulse, it
would be propagated to all distances in the
same instant of time. The example of sound
or the circular waves caused by throwing a
stone into still waters, ought to have taught
them the contrary. Instead of that, they took
the discovery that light does not move
instantaneously, but in time, as a proof that it is a
real body, a material substance; although that
circumstance does not really tell more in favour
of one hypothesis than of the other. They drew
an unwarranted inference from an accurate fact.
They were so delighted with their theory, that
they begged the question and took it for granted.
Newton, after showing that the light of the sun
is seven minutes [eight minutes eighteen seconds
is the time now allowed by astronomers] in reaching
the earth, and comparing its velocity with
that of a cannon-ball, evidently considered that
although a cannon-ball is a body which flies
very quickly, there is another body, light, whose
pace is ten million times quicker.

Moreover, they say, if light were not a body,
but consisted in a pulse or pressure, it would
not be propagated in straight lines only. The
force of gravity, they urge, tends downward;
but the pressure of water, arising from it, tends
every way with equal force. Waves on the
surface of water, meeting with an obstacle, are
deflected out of their course. The same is true,
though not to so great an extent, with the
waves or vibrations in the air, in which sound
consists. Sounds are propagated with equal
ease through curved tubes and through perfectly
straight tubes; but light was never known to
move in curves; no candle, unreflected, has
ever yet shone round a corner. Whence it was
concluded by Sir Isaac's disciples that the rays
of light are small corpuscules, emitted from the
luminous body and propelled with immense
velocity. Notwithstanding all which plausible
reasonings, and more, Mallebranche's theory of
light finds greater favour, at the present day,
than Newton's.

The admirable Annuaire for the year 1865,
published by the Bureau des Longitudes—an
annual which should lie on every library table,
seeing that it costs one franc only, and is full
of useful scientific information—contains a
Notice on the Speed of Light by M. Delaunay,
in which he justly boasts that the precise
determination of the rate at which light traverses
space (at first effected by astronomical observations,
and afterwards reduced to the proportions
of a simple experiment made in a laboratory of
small dimensions), is one of the marvels of
modern science. In what follows, I profit largely
by his masterly treatise.

A summary of Mallebranche's and Newton's
views will aid in arriving at a clear idea of what
is meant by the "speed of light."

To account for the different luminous phenomena,
philosophers, following those two leaders,
have imagined two systems as to the cause of
those phenomena.

According to one, every luminous body is
continually shooting into space, in all directions,
corpuscules of extreme tenuity, which,
penetrating our eye, produce in it the sensation of
light. This is called the "system of emission"—
sometimes, also, the Corpuscular, or Newtonian,
theory of light.

According to the other system, light is
nothing more than the vibratory movement of an
excessively rare fluid which pervades all space,
and which is designated by the name of Ether.*
A luminous body merely produces, and keeps up
around it, the vibratory movement of the ether,
which propagates itself to an indefinite distance.
This is the "system of undulations," or the
Undulatory theory of light.

* See Household Words, vol. xvii., page 558.

But, are we perfectly certain that matters do
really happen in conformity with either one or
the other of these systems? It is curious that
no one, as yet, is able to affirm it positively.
And, what is still more strange, for a long while
both systems accounted equally well for every
known luminous phenomenon. At present, certain
phenomena, lately discovered, are inexplicable
by the Corpuscular theory, whilst the
Undulatory explains them perfectly. The latter
theory may, therefore, be the true one. But,
one of these days, new phenomena may be
discovered which are no more explicable by the
Undulatory theory than those just alluded to
are by the Corpuscular. For the sake of having
some system as a groundwork for our reasonings,
rather than to assert implicit faith in its
truth, it is convenient to assume that matters
happen according to one or the other system.

In the system of emission, the luminous
corpuscules shot out in all directions are veritable
projectiles, which, in obedience to the general
laws of motion, invariably move in a straight
line—so long as their motion suffers no modification,
by meeting, for instance, with material
substances. The speed with which these
projectiles traverse space, is what is called the
Speed of Light.

In the Undulatory system, light consists in a
succession of vibrations of the ether, emanating
from a source, and propagating themselves
around that source to an indefinite distance in.
space. A clear idea of the vibrations is formed
by watching the circular waves as they spread