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Page:Popular Science Monthly Volume 57.djvu/313

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THE SOLAR ECLIPSE.

Hill Meteorological Station, occupied a part of the grounds of the Smithsonian party.

The main object of investigation was, of course, the corona, and of this, (first) a photographic and visual study of its structure; with, (second) a determination by the bolometer whether appreciable heat reaches us from it, and, if possible, an examination of the form of its spectrum energy curve.

The writer had been particularly struck, when observing the eclipse of 1878 on Pike's Peak, by the remarkable definiteness of filamentary structure close to the sun's limb, and had never found in any photographs, not even in the excellent ones of Campbell taken at the Indian eclipse of 1898, anything approaching what he saw in the few seconds which he was able to devote to visual observations at the height of fourteen thousand feet. His wish to examine this inner coronal region with a more powerful photographic telescope than any heretofore used upon it, was gratified by the most valued loan by Prof. E. C. Pickering of the new 12-inch achromatic lens of 135 feet focus, just obtained for the Harvard College Observatory. This lens, furnishing a focal image of more than 15 inches diameter, was mounted so as to give a horizontal beam from a cœlostat clock-driven mirror by Brashear, of 18 inches aperture, and used with 30-inch square plates. To supplement this great instrument, a 5-inch lens of 38-feet focus, loaned by Professor Young, was pointed directly at the sun. This formed images upon 11 x 14 plates moved in the focus of the lens by a water clock. Specially equatorially mounted lenses of 6, 4 and 3-inch aperture, driven by clock work, were provided for the study of the outer corona, and the search for possible intra-mercurial planets.

For the bolometric work, the massive siderostat with its 17-inch mirror, and a large part of the delicate adjuncts employed at the Smithsonian Institution in recent years, to investigate the sun's spectrum, was transported to Wadesboro. The excessively sensitive galvanometer reached camp without injury even to its suspending fibre, a thread of quartz crystal 1-15,000 inch in diameter.

Besides these two chief aims (the photography and bolometry of the inner corona), several other pieces of work were undertaken, including the automatic reproduction of the 'flash spectrum' by means of an objective prism with the 135-foot lens, the photographic study of the outer coronal region, including provision for recognizing possible intra-mercurial planets, already alluded to, visual and photographic observations of times of contact, and sketches of the corona, both from telescopic and naked-eye observations.

The assignment of the observers was as follows: Mr. Langley, in general charge of the expedition, observed with the same 5-inch telescope used by him on Pike's Peak in 1878, which was most kindly lent