THE GENERAL FIELD OF PHYSICAL INQUIRY.
We have now gone down the stream of time, from Hipparchus to our own
days. We find now enormous telescopes which enable us to see and examine
celestial bodies lying at distances so great that the mention of them
conveys little to the mind. We find also perfect systems of determining
their places. The following chapters will show, however, that modern
astronomy has not been contented with annexing those two branches of
physics which have enabled us to make the object-glass and the clock,
and another still which enables us to make that clock record its own
time with accuracy.
These applications of Science have been effected for the purpose either
of determining with accuracy the motion and positions of the heavenly
bodies or of enabling us to investigate their appearances under the best
possible conditions. The other class of observations to which we have
now to refer, have to do with the quantity and the quality of the
vibrations which these bodies impart to the ether, by virtue of which
vibrations they are visible to us.
We began by measurement of angles, we end with a wide range of
instruments illustrating the application of almost every branch of
physical as well as of mathematical science. In modern observatories
applications of the laws of Optics, Heat, Chemistry and Electricity, are
met with at every turn.
Each introduction of a new instrument, or of a new method of attack, has
by no means abolished the preexisting one; accretion rather than
substitution has been the rule. On the one hand, measurement of angles
goes on now more diligently than it did in the days of Hipparchus, but
the angles are better measured, because the telescope has been added to
the divided arc. Time is as necessary now as it was in the days of the
clepsydra, but now we make a pendulum divide its flow into equal
intervals and electricity record it. On the other hand, the colours of
the stars are noted as carefully now as they were before the
spectroscope was applied to the telescope, but now we study the spectrum
and inquire into the cause of the colour. The growth of the power of the
telescope as an instrument for eye observations has gone on, although
now almost all phenomena can be photographically recorded.
The uses to which all astronomical instruments may be put may be roughly
separated into two large groups:—
I. They may be used to study the positions, motions, and sizes of
the various masses of matter in the universe. Here we are
studying celestial mechanics or mechanical astronomy, and with
these we have already dealt.
II. They maybe used to study the motions of the molecules of which
these various masses are built up, to learn their quality,
arrangement, and motions. Here we are studying celestial
physics, or physical astronomy.
It is with this latter branch that we now have to do.
First we have to deal with the quantity and intensity of the ethereal
vibrations set up by the constituent molecules of these distant bodies.
We wish to compare the quantity of light given out by one star with that
given out by another. We wish, say, to compare the light of Mars with
the light of Saturn; we are landed in the science of photometry, which
for terrestrial light-sources has been so admirably investigated by
Rumford, Bouguer, and others.
Here we deal with that radiation from each body _which affects the
eye_—but by no means the total radiation. This is a point of very
considerable importance.
Modern science recognises that in the radiation from all bodies which
give us white light there is so great a difference of length of wave in
the vibrations that different effects are produced on different bodies.
Thus white light is a compound thing containing long waves with which
heat phenomena are associated, waves of medium length to which alone the
eye is tuned, and short waves which have a decided action on some
metallic salts which are unaffected by the others.
To thus examine the constituents of a beam of light a lantern, with a
lime-light or electric light, may be used for throwing a constant beam;
we may then produce an image of the cylinders of lime or the carbon
points in the lantern on a piece of paper or a screen, and our eyes will
tell us that this is an instance of how the radiations from any
incandescent substance are competent to give us light. We receive all
the rays to which our eyes are tuned and we see a white image on the
screen. We shall see also that the light is more intense than that of a
candle, in other words that the radiation from the light-sources we have
named is very great.
[Illustration:
FIG. 164.—Thermopile and Galvanometer.
]
Now let us insert in front of the lantern a piece of deep red glass,
that is, glass which allows only the red constituents of the white light
to pass. Now if a thermo-electric pile, Fig. 164, be introduced into the
beam we shall see that the needle of the galvanometer will alter its
position. Now, why does the needle turn? This is not the place for
giving all the details of this instrument, but it is sufficient to say
(1) that the needle moves whenever a current of electricity flows
through the coil of wire surrounding the needles, and (2) that the pile
consists of a number of bars of antimony and bismuth joined at the
alternate ends, and whenever one end of the pile is heated more than the
other, a current of electricity is caused to flow. Such is the delicacy
of the instrument, that the heat radiated from the hand, held some yards
away from it, is sufficient to set the needle swinging violently; this
then acts as a most delicate thermometer. In this case it shows that
heat effects are produced by the red constituents of the light from the
lamp.
Now replace the thermopile by a glass plate coated with a salt of silver
in the ordinary way adopted by photographers. No effect will be
produced.
Replace the red glass by a blue one. If the light is now allowed to fall
on the photographic plate, its effect is to decompose, or alter the
arrangement of, the atoms of silver, so that on applying the developing
solution, the silver compound is reduced to its metallic state on the
places where the light has acted; and thus, if the image of the
light-source has been focussed on the plate, a photograph of it is the
result. If the thermopile is brought into the beam it will be now as
insensitive to the blue light as the photographic plate was to the red
light in the former case. We have therefore three kinds of effects
produced, viz., light, heat, and chemical or actinic action, and when
light is passed through a prism, these three different radiations, or
energies, are most developed in three different portions of the
spectrum.
If indeed a small spectrum be thrown on the screen and the different
colours are examined with the thermopile, it will be found that as long
as we allow it to remain at the blue end of the spectrum, there will be
no effect on the galvanometer, but if instead of holding it at the blue
end we bring it towards the red, the galvanometer needle is deflected
from its normal position, to that it had when the red rays fell on it,
showing that it is beyond all doubt the red rays and not the blue to
which it is sensitive. Where then in the spectrum are the rays which
affect the photographic plate? We can at once settle this point. If one
be placed in the spectrum for a short time, and then developed, it will
be found to be affected only in the part on which the blue rays have
fallen. Indeed to demonstrate this no lamp is necessary.
If for half-an-hour or so two pieces of sensitive paper are placed in
the daylight, one covered with red glass, and the other with violet, so
that the sunlight is made to travel, in the one case, through red glass,
and in the other through violet, it will be found that the violet light
will act, and produce a darkening of the paper, while the red glass will
preserve the paper below it from all action. This is a proof that the
blue end of the spectrum has another kind of energy, a chemical energy,
by means of which certain chemicals are decomposed, this is the basis of
photography.
These different qualities of light have been utilized by the astronomer.
He attaches a thermopile to his telescope and establishes a celestial
thermometry. The radiations repay a still more minute examination, and
aided by the spectroscope, he is able to study with the utmost certitude
the chemical condition of the heavenly host, while the polariscope
enables him to acquire information in still another direction. Nor does
he end here. He replaces his eye by a sensitive plate, which not only
enables him to inquire into the richness of the various bodies in these
short waves, but actually to obtain images of them of most marvellous
beauty and exactness.
These various lines of work we have to consider in the remaining
chapters.