How rapidly the streamers of the corona vary is not known. Occasionally an observer reports having seen the filaments vibrate rapidly as in the aurora borealis, but this is not verified by others who saw the same corona perfectly unmoving. Comparisons of photographs taken at widely separate stations during the same eclipse have shown that at least the corona remained stationary for hours at a time. Whether it may be unchanged at the end of a day, or a week, or a month, is not known; because no two total eclipses can ever happen nearer each other than within an interval of 173 days, or one-half of the eclipse year. And usually the interval between total eclipses is twice or three times this period. Theories of what the solar corona may be are very numerous. The extreme inner corona is perhaps in part a sort of gaseous atmosphere of the sun, due to matter ejected from the sun, and kept in motion by forces of ejection, gravity, and repulsion of some sort. Meteoric matter is likely concerned in it, and Huggins suggested the débris of disintegrating comets. Schuster was in agreement with Huggins that the brighter filaments of the corona might be due to electric discharges, but it seems very unlikely that any single hypothesis can completely account for the intricate tracery of so complex a phenomenon. [Illustration: SOLAR CORONA AND PROMINENCES. Photographed during a total eclipse of the sun, June 8, 1918. (_Courtesy, American Museum of Natural History._)] [Illustration: VENUS, SHOWING CRESCENT PHASE OF THE PLANET. Venus is the earth's nearest neighbor on the side toward the sun. (_Photo, Yerkes Observatory._)] [Illustration: MARS, THE PLANET NEXT BEYOND THE EARTH. The photograph shows one of the white polar caps. The caps are thought to be snow or ice and may indicate the existence of atmosphere. (_Photo, Yerkes Observatory._)] Elaborate spectroscopic programs have been carried out at recent eclipses, affording evidence that certain regions are due to incandescent matter of lower temperature than the sun's surface. A small part of the light of the corona is sunlight reflected from dark particles possibly meteoric, but more likely dust particles or fog of some sort. This accounts for the weakened solar spectrum with Fraunhofer absorption lines, and this part of the light is polarized.