PUBLICATIONS OF THE Astronomical Society of the Pacific. VOL. III. SAN FRANCISCO, CALIFORNIA, JUNE 13, 1891. No. 16. THE SOLAR CORONA OF JANUARY, 1889, FROM THE PHOTOGRAPHS. BY PROFESSOR H. S. PRITCHETT.* The photographic testimony as to the form of the Sun's corona, obtained on the occasion of the eclipse of January 1, 1889, exceeds in amount and completeness of detail the results obtained on any similar occasion, with the single exception that the observing parties were confined to a small part of the path of totality. Of the photographic parties, but three, so far as my information goes, were provided with equatorial cameras, equipped with driving-clock, making long exposures possible. These were the Lick Observatory party at Bartlett Springs, the Harvard College party at Willows, and the Washington University party at Norman, all three places being in the State of California. Up to the present the only publication of photographic results from these observing parties has been that of the Lick Observatory. The report of the work of the Washington University party is now going through the press. As a large number of the members of the Astronomical Society of the Pacific were engaged in observations of that eclipse, a brief statement of the results obtained at Norman, so far as the form of the corona is concerned, may be of interest. STATION AND EQUIPMENT. a small hamlet on the Southern Pacific Railway. The Washington University party was stationed at Norman, graphical co-ordinates of the station are: Latitude, 39° 24′ 58′′.8 808 45-39 Washington University, St. Louis, Missouri. The geo The photographic equipment consisted of an equatorial camera, having for its objective one of DALLMEYER'S patent portrait and group lenses of the size known as No. 8 D. The instrument is the property of the U. S. Naval Observatory, and was used in the eclipse of 1878 at La Junta, Col. It has a clear aperture of 6.0 inches, and an equivalent focal length of 37.9 inches. The tube was mounted upon a equatorial stand belonging to one of the fiveinch equatorials used in the transit of Venus observations. This stand is very firm; has divided circles on both axes, and was provided with an excellent clockwork. The visual and photographic foci of this lens coincide; the instrument was, therefore, adjusted by the eye, and this adjustment tested by numerous photographic experiments before the day of eclipse. The plates used were SEED'S 26. With these it was found almost impossible to make the tube light-tight without using an automatic shutter. The largest shutter which could be procured in St. Louis was one worked by the ordinary air-bulb, having a clear aperture of 4.5 inches. This cut down the effective aperture of the lens from 6 to 4.5 inches, but probably increased the sharpness of the negatives. The orientation of the plates was provided for by reading with a clinometer the inclination of the edge of the plate-holder to the horizon. THE NEGATIVES. It is manifestly impossible to obtain on a single negative a good representation of both the outer and inner corona. A negative properly exposed for the bright inner corona would be under-exposed for the outer, and vice versa. It was decided, therefore, to make exposures of various lengths, from nearly instantaneous up to about thirty seconds. Seven plates were exposed, but the last one was lost by the sticking of the slide in the plate-holder. The length of exposure for each negative is given in the description of the negatives which follows. The photographic equatorial was in the hands of Professors NIPHER and CHARROPPIN. The negatives were developed at Norman by Professor CHARROPPIN. Those of short exposure were lacking in density for printing purposes, but this was rather an advantage than otherwise, as it brought out admirably the details of the filaments and streamers. Negative No. 1.-This negative, taken immediately after the beginning of totality, had an exposure of less than half a second. The polar filaments form the prominent feature of the negative. The corona is shown to a distance of three-fourths of a diameter from the sun's disc. Negative No. 2.-The time of exposure was 3.0 seconds. The polar filaments are a marked feature of this negative, also, the corona being shown to a distance of a diameter and a quarter from the disc. Negative No. 3.-This negative, which was the first one developed, had an exposure of a little more than six seconds. The equatorial streamers can be seen to a distance of one and a half diameters from the disc. Two large prominences are shown on the western line and five on the eastern line. Negative No. 5.—This negative had a longer exposure than any of the rest-about twenty-eight seconds. The corona is shown to a distance of more than two diameters from the disc. Negatives No. 4 and No. 6 of eighteen seconds and thirteen seconds exposure, respectively, differ but little from No. 5. All the negatives of long exposure show a curious diffraction ring in the center of the black disc of the Moon. REPRODUCTION FROM THE NEGATIVES. As just stated, several of these negatives, while very sharp and showing great detail in the structure of the corona, are lacking in density for printing. Furthermore, it is impossible for any one negative to give a fair representation of the entire corona. In the publication of the observations of the Washington University, four of the negatives described above are reproduced by the artotype process. The negatives chosen for reproduction were those numbered 1, 2, 3 and 5, and include those of short exposure, of medium exposure and that of longest exposure. It seems desirable to reproduce from the negatives a representative picture which should put into a form capable of study the entire result of the photographic work at this station; a picture, in other words, which should represent the corona as determined by all the photographs taken. The following plan was adopted, at the suggestion of Professor ENGLER: A positive made from the negative of longest exposure was placed in an ordinary stereopticon, and the image thus produced thrown upon a screen. The lantern was then placed in the position, determined by trial, which gave the best definition to the |