Page:Popular Science Monthly Volume 60.djvu/258

would give witness to more rapid changes. From year to year large changes in the form of the corona occur and these appear to be associated with the sun-spot period. This is a natural inference, especially since the solar prominences are thus associated. This is well shown by a comparison of the form of the corona in 1889 and 1900, which occurred near the sun-spot minimum, with the form in 1893, which was near sun-spot maximum. These are given in Figures 6, 7 and 8. The equatorial streamers and the divergent polar streamers are much more pronounced at the time of sun-spot minimum. At maximum the corona is more nearly circular. The polar streamers are beautifully shown in Figure 9, a photograph made by the eclipse party, which was under the direction of Secretary Langley, of the Smithsonian Institution. The true nature of the corona and the complex changes which it undergoes are unknown. The spectroscope is the magician's wand which science generally uses to reveal the constitution of unknown objects, but in this case the revelation is only partial. In 1869 Professor Young found the spectrum to be characterized by a bright line in the green, which he identified as Kirchhoff's line 1474. The unknown substance which produces this line has been given the name 'coronium.' There are also other less conspicuous bright lines. When the name 'helium' was assigned to the origin of certain lines in the solar spectrum, no such terrestrial substance was known. Later it was found by Ramsay. A similar issue for coronium would be very acceptable. The corona also yields a faint continuous spectrum, in which Janssen and others have reported certain dark lines of the solar spectrum. This signifies, that in addition to luminous gases, giving a spectrum of bright lines, the corona contains some substance, like a cloud, which is capable of reflecting ordinary sunlight. A part of the light appears to be polarized. It is thought by some observers that there is also a bright continuous spectrum free from dark lines. If true, this would imply a three-fold origin to the coronal light. For the explanation of the corona we have the diffraction theory of Hastings, the mechanical theory of Schaeberle, the magnetic theory of Bigelow, and others. The complete solution of the problem is of the greatest difficulty and of the greatest importance. At the eclipse of 1900 some experiments with that remarkable instrument, the bolometer, appear to throw new light on this subject. These experiments were made by Secretary Langley's chief assistant, Mr. C. I. Abbott, who reached the following conclusions:

These observations indicate not only that the coronal radiation is very slight, but that the apparent temperature of the inner corona is below 20° C. For it will be noticed that the bolometer lost heat by radiation to the corona, as evidenced by a negative deflection. Hence, when we consider its visual photometric brightness at the point where the bolometric measures were taken,