Page:The American Cyclopædia (1879) Volume XV.djvu/494

 SUN away at 12h. 80ra., and on returning at 12h. 65m. "found that the whole thing had been lit- FIG. 1. Prominence as it appeared at half-past 12 o'clock, Sept. 7, 1871. erally blown to shreds by some inconceivable up-rush from beneath." Fig. 2 represents the appearance when the up - rushing hydrogen had at- tained its great- est height, ex- ceeding 200,000 m. " The whole phenomenon, " he says, " suggest- ed most forcibly the idea of an explosion under the great prom- inence, acting mainly upward, but also in all directions out- ward, and then after an interval FIG. 2.-A, the above appeared foll wed by a half an hour later" corresponding m- rush." A strange circumstance remains to be mentioned : " The same afternoon a portion of the sierra on the Fio. 8 Relative Frequency of Protuberances and Sun Spots. opposite limb of the sun was for several hours in a state of unusual brilliance and ex- citement, and showed in the spectroscope more than 120 bright lines whose position was determined and catalogued all that I had ever seen before and some 15 or 20 besides." Before passing from the prominences it may be well to indicate the laws of their numer- ical distribution, as determined by Secchi and others. This is shown in fig. 3. On the left side the results of Carrington's observation of 1,414 spots between 1853 and 1861 are indi- cated, and on the right the result of Secchi's observations of 2,767 protuberances in 1871, the number of spots or prominences being of course shown by the length of the radial lines. The dotted line on the right-hand side repre- sents in the same manner the distribution of the larger prominences, viz., those exceeding 1' or 27,000 m. in height. During a total eclipse there appears around the black body of the moon a halo or glory of light, bright, close to the place of the concealed sun, but gradually fading away outward, until its light is lost in the general tint of the sky. In this glory of light, which is called the solar corona, ra- diations are also sometimes seen, and under favorable atmospheric conditions complicated series of streaks can be seen extending to a considerable distance outward from the promi- nence region. Various theories were advanced in former times to explain the corona. Ac- cording to one theory, it is a phenomenon caused by the solar light falling on our own atmosphere; another theory ascribed it to a lunar atmosphere. In the opinion of Lever- rier and Foucault (among others), the corona is an example of the interference of light (see LIGHT), the phenomenon being analogous to the colored fringes seen on a screen in a darkened room when a solar beam is admitted through a chink. To this theory Airy raised the objection that if, in order to make the analogy perfect, the eye is placed in the position of the screen, no colored fringes are seen. It is shown that the corona is partly polarized, and hence part- ly consists of reflected light. It has been further proved that the plane of polarization passes through the sun and the observer. This was regarded by Airy as pointing to the existence of an atmospheric medium capable of reflecting light, and extending from the earth to the moon. But in more recent times astronomers began to perceive that no other theory can be admitted than that which regards the corona as a true solar appendage. (Of course, it must be admitted that a portipn of the light around the eclipsed sun comes from our own atmosphere, which must necessarily be illuminated by the true corona