[Footnote 1: A paper read before the American Astronomical
Society, May 5, 1884.]
By G.D. Hiscox.
The reality of the sun's corona having been cast in doubt by a leading
observer of the last total eclipse, who, from the erratic display
observed in the spectroscope, has declared it a subjective phenomenon
of diffraction, has led me to an examination and inquiry as to the
bearing of an obscurely considered and heretofore only casually
observed phenomenon seen to take place during total solar eclipses.
This phenomenon, it seems to me, ought to account for, and will
possibly satisfy, the spectroscopic conditions observed just before,
during, and after totality; which has probably led to the epithet used
by some leading observers--"the fickle corona." The peculiar
phenomenon observed in the spectroscope, the flickering bands or lines
of the solar spectrum flashing upon and across the coronal spectrum,
has caused no little speculation among observers.
The diffraction or interference bands projected by the passage of a
strong beam of light by a solid body, as discovered long since by
Grimaldi, and investigated later by Newton, Fresnel, and Fraunhofer,
are explained and illustrated in our text books; but the grand display
of this phenomenon in a total solar eclipse, where the sun is the
source of light and the moon the intercepting body, has as yet
received but little attention from observers, and is not mentioned to
my knowledge in our text books.
In the instructions issued from the United States Naval Observatory
and the Signal Office at Washington for the observation of the eclipse
of July 29, 1878, attention was casually directed to this phenomenon,
and a few of the observers at Pike's Peak, Central City, Denver, and
other places have given lucid and interesting descriptions of the
flight of the diffraction bands as seen coursing over the face of the
earth at the speed of the moon's shadow, at the apparent enormous
velocity of thirty-three miles per minute, or fifty times the speed of
a fast railway train.
From a known optical illusion derived from interference or fits of
perception, as illustrated in quick moving shadows, this great speed
was not realized to the eye, as the observed motion of these shadows
was apparently far less rapid than their reality.
The ultra or diffraction bands outside of the shadow were distinctly
seen and described by Mr. J.E. Keeler at Central City, both before and
after totality. He estimates the shadow bands at 8 inches wide and 4
feet apart.
Professor E.S. Holden, also at Central City, estimated the dark bands
as about 3 feet apart, and variable.
From estimates which he obtained from other observers of his party,
the distances between the bands varied from 6 to l½ feet, but so
quickly did they pass that they baffled all attempts to count even the
number that passed in one second.
He observed the time of continuance of their passage from west to east
as forty-eight seconds, which indicates a width of 33 miles of
diffraction bands stretching outward from the edge of the shadow to
the number of many thousands.
Mr. G.W. Hill, at Denver, a little to the north of the central track
of the shadow, observed the infra or bands within the shadow, alluding
to the fact that they must be moving at the same rate as the shadow,
although their apparent motion was much slower, or like the shadows of
flying clouds. He attributes the discrepancy to optical illusion.
At Virginia City the _colors_ of the _ultra_ bands were observed, and
estimated at five seconds' duration from the edge of the shadow, which
is equal to about 4 miles in width. These are known to be the
strongest color bands in the diffraction spectrum, which accounts for
their being generally observed.
Mr. W.H. Bush, observing at Central City, in a communication to Prof.
Holden alludes to the brilliancy of the colors of these bands as seen
through small clouds floating near the sun's place during totality,
and of the rapid change of their rainbow colors as observed dashing
across the clouds with the rapidity of thought.
All of these bands, both ultra and infra, as seen in optical
experiments, are colored in reverse order, being from violet to red
for each band outward and inward from the edge of the shadow.
It is very probable that the velocity of the passage of all the bands
during a total eclipse very much modifies the distinctness of the
colors or possibly obliterates them by optically blending so as to
produce the dull white and black bands which occupied so large a
portion of this grand panorama.
The phenomenon of these faint colored bands, with the observed light
and dark shadows, may be attributed to one or all of the following
causes:
1. A change in the direction of a small portion of the sun's light
passing by the solid body of the moon, it being deflected outward by
repulsion or reflection from its surface, and other portions being
deflected inward after passing the body by mutual repulsion of its own
elements toward a _light vacuum_ or space devoid of the element of
vibration.
2. The colored spectral bands being the direct result of the property
of interference, or the want of correspondence of the wave lengths due
to divergence; the same phenomenon being also observed in convergent
light. This is practically illustrated in the hazy definition of the
reduced aperture of telescopes, and its peculiarities shown in the
spectral rings within and beyond the focus.
3. Chromatic dispersion by our atmosphere, together with selective
absorption, also by our atmosphere and its vapors, have been suggested
as causes in this curious and complicated phenomena.
In none of the reports descriptive of the phenomena of polarization of
the corona is there the slightest allusion to the influence that the
diffraction bands may possibly have in modifying or producing the
various conditions of polarization observed; although these
observations have been made and commented upon during the past
twenty-five years.
Investigations now in progress of the modifying relation of the
phenomenon of diffraction in its effect upon not only the physical
aspect of the corona, but also in some strange spectroscopic anomalies
that have been observed near the sun at other times than during a
total solar eclipse, will, it is hoped, result in a fuller
interpretation of the physical nature of one of the grandest elements
of creation--_light_; let there be more of it.
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