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.