of water. Those bright, flashing pieces of glass called "prisms"
are also spectroscopic.
A spectroscope consist essentially of four parts:
(1) A very narrow slit, through which passes the beam of light, (2)
a small telescope called a collimator, at the focus of which the slit
is placed, (3) a prism, or a very closely ruled glass plate to disperse
the light into its component colors, and (4) an observation telescope to
produce a magnified image of the spectrum.
Light is an electromagnetic wave motion in
the ether of space. It differs from the vibrations that produce heat,
wireless and chemical effects only in its wave length. Wireless waves are
very long, frequently a mile or more, while light waves are measured in
millionths of an inch. Like musical tones, one is of a very low pitch,
the other very high. Color is simply pitch, and, within the
very short range of the ether waves visible to the human eye, there is a
whole scale of color, starting with the red and ending with the violet.
The red waves are longest, the violet shortest. When light waves fall
obliquely upon one of the faces of a glass prism that portion which
enters the glass is retarded most. Therefore when white light passes
through a prism the red waves are retarded least and the violet length
most with the colors of other wave lengths lying between. The result of
this unequal retardation, or refraction as it is called, is to separate
the light into its component colors.
In 1815 Fraunhofer,
an eminent optician, using a spectroscope of higher magnifying power than
his predecessors, mapped certain dark lines crossing the sun's spectrum.
For many years the meaning of these lines was a mystery. Then,
in 1858, Kirchhoff and
Bunsen, bringing this instrument to a much
higher degree of perfection, discovered the following principles of
spectrum analysis: (1) Incandescent solids and liquids and also gases
under high pressure give a continuous spectrum, or solid band of color.
(2) Gases under low pressure give a series of bright lines whose number
and position depend upon the elements present. (3) When white light
passes through a gas of lower temperature than its source, this gas
will absorb from the white light those colors which it would produce,
if viewed by itself in the incandescent state.
At once the meaning of the Fraunhofer
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lines became apparent. These dark bands were due to elements in the
state of incandescent vapor in the sun's atmosphere and they neutralized
in the sun's spectrum the colors which they themselves would have
emitted. Now every element gives its own characteristic bright line
spectrum, the lines being of absolutely definite color and position.
Therefore by devising a spectroscope with a comparison prism so that
the solar spectrum or that of a star might be viewed side by side with
the spectra of terrestrial elements, it at once became possible to
determine the chemical composition of any heavenly body whose light
would reach our telescopes. And more, this instrument reveals the
physical state of stars, nebulæ and comets, for a continuous
spectrum means an incandescent solid or a gas under great pressure,
while a discontinuous, or bright line, spectrum proves the presence of a
light, vaporous firemist. Most of the nebulæ, those worlds in the
process of formation, have been shown to be of the latter composition.
By replacing the eyepiece of the spectroscope with a photographic plate
these spectra may be photographed and studied at leisure. Every great
telescope carries a spectroscopic attachment and the light gathered by
the great lens or mirror is dispersed by the prism.
The spectroscope discloses the motion of a
distant star. When a star is approaching our solar system its spectrum
is shifted from the normal position it would occupy if the star were
stationary toward the violet. When the star is receding from us the
spectrum is shifted toward the red. By the amount of the shifting the
velocity may be determined with an accuracy of within two to three miles
per second. This shifting of the spectral lines has led to the discovery
of twin stars, revolving about a common center of gravity and periodically
eclipsing each other. Each star gives its own spectrum and as the lines
alternately shift first toward the red and then the violet there can be
but one conclusion, that is, companion stars in mutual revolution.
In the hands of the chemist the spectroscope
has proved the most delicate means of detecting minute quantities of
chemical elements. So small a quantity as one two-hundred-thousandth
of a grain of sodium may be detected by this means.
A marvelous instrument is the spectroscope,
and its possibilities are not yet exhausted.
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NE of the most remarkable adaptations of
magnifying glass to the unraveling of the secrets of the universe is
the spectroscope. The world's original and largest spectroscope is to
be found in nature itself, for every rainbow that paints the sky is an
immense spectrum produced by sunlight shining through falling drops