The evening telegraph. [volume] (Philadelphia [Pa.]) 1864-1918, 1869-09-29

sun is merely star, a line drawn from the
star exhibiting proper motion to our sun is,
as regards the former, merely line drawn to
star taken at random, and therefore there is
no reason why the stars motion should be,
except accidentally, in a direction perpendi-
cular to the line joining the star with our
sun. We must conclude that the stars, in
cludlng our own sun, or some of them, at
least, are moving in various directions in
space, and that it is merely the transversal
component of the whole motion, or rather of
the motion relatively to our sun, that is re-
vealed to us by a change in the stars appa-
rent place.
How, then, shall we determine whether
any particular star is approaching to or re-
ceding from our sun? It is clear that astro
nomy alone is powerless to aid us here, since
such a motion would be unaccompanied by
change of angular position. Here the science
of optics comes to our aid in a remarkable
manner.
The pitch of musical note depends, as we
know, on the number of vibrations which
reach the ear in a given time, such as
second. Suppose. now, that body, such as
a bell, which is vibrating a given number of
times per second, is at the same time moving
from the observer, the air being calm. Since
the successive pulses of sound travel oil with
the velocity of sound but diverge from diff
ferent centres, namely, the successive points
in the bells path at which the bell was when
those pulses were first excited, it is evident
that the sound-waves will be some what more
spread out on the side from which the bell is
moving and more crowded together on the
side towards which it is moving, than if the
bell had been at rest Consequently the
number of vibrations per second which reach
the ear of an observer situated in the
former of these directions will be some
what smaller, and the number which reach
an observer situated in the opposite direc-
ton somewhat greater than if the bell had
been at rest. Hence to the former the pitch
will be somewhat lower, and the latter some-
what higher than the natural pitch of the
bell And the same thing will happen if the
observer be in motion instead of the bell or
if both be in motion; in fact, the effect de-
pends only on the relative motion of the OBJ
server and the bell in the direction of line
joining the two-in other words, on the velo-
city of recession or approach of the observer
and the bell. The effect may be perceived IN
standing by a railway when train in which
the steam-whistle is sounding passes by at
full speed, or, better still, if the observer be
seated in train which is simultaneously
moving in the opposite direction.
WHAT Is LIGHT?
The present state of optical science is such
as to furnish us with evidence, of force
which is perfectly overwhelming, that light
consists of a tremor or vibratory movement
propagated in an elastic medium filling the
planetary and stellar spaces medium which
thus fultils for Ill, hit an Office similar to that
OF air for sound. In this theory, to difference
OF periodic time corresponds difference of
refrangibility. Suppose that we were in pos
session of a source of light capable, like the
bell in the analogous case of sound, of ex-
citing in the ether supposed at rest vibrations
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to light of definite refrangibillty Then,
just as in the case of sound if the source of
light and the observer were receding from or
approaching to each other with a velocity
which was not insensibly small compared
with the velocity of light, an appreciable
lowering OF elevation of refrangibility would
be produced, which would be capable of de-
lection by means of spectroscope Of high
dispersive power. a. r.-
The velocity of light is so enormous, about
$4,000 miles per second, that it can readily
be imagined that any motion which we can
experimentally produce in a source of light is
as rest in comparison. But the earth in its
orbit round the sun moves at the rate of about
eighteen miles per second; and in the motions
of stars approaching to or receding from our
sun we might expect to meet with velocties
comparable with this The orbital velocity of
the earth is, it is true, only about the one ten
thousandtn p;c,rt Of the velocity of light. Still
the effect of such velocity on the refrangi-
bility of light, which admits of being easily
calculated, proves not to be so insensibly
small as to elude all chance of detection, pro-
vided only the observations are conducted
with extreme delicacy.
KIRcHHoFF's DISCOVERY.
But how shall we find in such distant OBJ
iects as the stars an analogue of the bell
which we have assumed in the illustration
drawn from sound? What evidence can we
ever obtain, even if an examination of their
light should present us with rays of definite
refrangibility, of the existence in those re-
mote bodies of ponderable matter vibrating
in known periods not identical with those
corresponding to the refrangibilities of the
definite rays which we observe? The answer
to this question will involve a reference,
which will endeayor to make as brief as L
can, to the splendid researches of Professor
Kirchhoff. The exact coincidence of certain
dark lines in the solar spectrum with bright
lines in certain artificial sources of light had
previously been in one or two instances OBJ
served; and it is to Kirchhof we owe the
inference from the extension of Pre- ost S
| theory of exchanges, that glowing medium
which emits bright light of any particular re-
Tranquility necessarily (at that temperature
at least) acts as an absorbing medium, ex-
tinguishing light Of the same refrangibilty.
In saying this it is but Just to mention that in
relation to radiant heat Qfrom whence the
transition to light is easy) Kirchhoff was pre-
ceded, though unconsciously, by our own
corntrym0a, MIR. Balfour Stewart. The in
ference Which Kirchhoff drew from Provost
(, theory thus extended led him to make care
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