r
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 l1/2 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