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Popular Science Monthly Oct, Nov, Dec, 1915 — Volume 86 By: Anonymous |
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NOTE: degrees A (Absolute?) is the same as the current
degrees K (Kelvin). THE POPULAR SCIENCE MONTHLY VOLUME LXXXVI JULY TO SEPTEMBER,
1915 THE SCIENTIFIC MONTHLY VOLUME I OCTOBER TO DECEMBER, 1915 EDITED BY J. McKEEN CATTELL
THE SCIENTIFIC MONTHLY OCTOBER, 1915 THE EVOLUTION OF THE STARS AND THE FORMATION OF THE EARTH. II BY DR. WILLIAM WALLACE CAMPBELL DIRECTOR OF THE LICK OBSERVATORY, UNIVERSITY OF CALIFORNIA THE PRINCIPLES OF SPECTROSCOPY THUS far our description of the stellar universe has been
confined to its geometrical properties. A serious study of the
evolution of the stars must seek to determine, first of all,
what the stars really are, what their chemical constitutions
and physical conditions are; and how they are related to each
other as to their physical properties. The application of the
spectroscope has advanced our knowledge of the subject by leaps
and bounds. This wonderful instrument, assisted by the
photographic plate, enables every visible celestial body to
write its own record of the conditions existing in itself,
within limits set principally by the brightness of the body.
Such records physicists have succeeded to some extent in
duplicating in their laboratories; and the known conditions
under which the laboratory experiments have been conducted are
the Rosetta Stones which are enabling us to interpret, with
more or less success, the records written by the stars. It is well known that the ordinary image of a star, whether
formed by the eye alone, or by the achromatic telescope and the
eye combined, contains light of an infinite variety of colors
corresponding, speaking according to the mechanical theory of
light, to waves of energy of an infinite variety of lengths
which have traveled to us from the star. In the point image of
a star, these radiations fall in a confused heap. and the
observer is unable to say that radiations corresponding to any
given wave lengths are present or absent. When the star's light
has been passed through the prism, or diffracted from the
grating of a spectroscope, these rays are separated one from
another and arranged side by side in perfect order, ready for
the observer to survey them and to determine which ones are
present in superabundance and which other ones are lacking
wholly or in part. The following comparison is a fair one: the
ordinary point image of a star is as if all the books in the
university library were thrown together in a disorderly but
compact pile in the center of the reading room: we could say
little concerning the contents and characteristics of that
library; whether it is strong in certain fields of human
endeavor, or weak in other fields. The spectrum of a star is as
the same library when the books are arranged on the shelves in
complete perfection and simplicity, so that he who looks may
appraise its contents at any or all points. Let us consider the
fundamental principles of spectroscopy. 1. When a solid body, a liquid, or a highly condensed gas is
heated to incandescence, its light when passed through a
spectroscope forms a continuous spectrum: that is, a band of
light, red at one end and violet at the other, uninterrupted by
either dark or bright lines. 2. The light from the incandescent gas or vapor of a chemical
element, passed through a spectroscope, forms a bright line
spectrum; that is, one consisting entirely of isolated bright
lines, distributed differently throughout the spectrum for the
different elements, or of bright lines superimposed upon a
relatively faint continuous spectrum. 3. If radiations from a continuous spectrum source pass through
cooler gases or vapors before entering the spectroscope, a
dark line spectrum results: that is, the positions which the
bright lines in the spectra of the vapors and gases would have
are occupied by dark or absorption lines. These are frequently
spoken of as Fraunhofer lines. To illustrate: the gases and vapors forming the outer strata of
the Sun's atmosphere would in themselves produce bright line
spectra of the elements involved... Continue reading book >>
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