THE ILLUMINATING POWER OF ETHYLENE.
Dr. Percy Frankland has obtained results which may be thus briefly summarized: (1.) That pure ethylene, when burnt at the rate of 5 cubic feet per hour from a Referee's Argand burner, emits a light of 68.5 standard candles. (2.) That the illuminating power of equal volumes of mixtures of ethylene with either hydrogen carbonic oxide or marsh-gas is less than that of pure ethylene. (3.) That when the proportion of ethylene in such mixtures is above 63 per cent. the illuminating power of the mixture is but slightly affected by the nature of the diluent. When, on the other hand, the proportion of ethylene in such mixtures is low, the illuminating power of the mixture is considerably the highest when marsh-gas is the diluent, and the lowest when the ethylene is mixed with carbonic oxide. (4.) That if 5 cubic feet of ethylene be uniformly consumed irrespectively of the composition of the mixture, the calculated illuminating power is in every case equal to or actually greater than that of pure ethylene until a certain degree of dilution is attained. This intrinsic luminosity of ethylene remains almost constant when the latter is diluted with carbonic oxide, until the ethylene forms only 40 per cent. of the mixture, after which it rapidly diminishes to zero when the ethylene forms only 20 per cent. of the mixture. When the ethylene is diluted with hydrogen, its intrinsic luminosity rises to 81 candles when the ethylene constitutes 30 per cent. of the mixture, after which it rapidly falls to zero when the ethylene amounts to only 10 per cent. In the case of mixtures of ethylene and marsh-gas, the intrinsic luminosity of the former is augmented with increasing rapidity as the proportion of marsh gas rises, the intrinsic luminosity of ethylene, in a mixture containing 10 per cent. of the latter, being between 170 and 180 candles.
DIFFRACTION PHENOMENA DURING TOTAL SOLAR ECLIPSES.[1]
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.
A paper read before the American Astronomical Society, May 5, 1884.
A CATALOGUE containing brief notices of many important scientific papers heretofore published in the SUPPLEMENT, may be had gratis at this office.