The eclipse of January 1, 1889, was very successfully photographed by the American observers, the largest aperture used being thirteen inches. On some of the plates used during this eclipse the standard intensity scale recommended by Captain Abney several years ago was fixed, and for the first time definite conclusions as to the brightness of the corona were obtained.
The expedition sent out by the Royal Astronomical Society for the eclipse of December 22, 1889, were each fitted with a 4–inch photographic lens, belonging to Captain Abney, mounted on the usual equatorial plan, and intended to continue the series so well begun by Dr. Schuster in 1882, and also with a 20–inch mirror of 45 inches focus, specially constructed and mounted for eclipse work, and designed to photograph the outer portions of the corona too faint for ordinary instruments. The plates for use with the 4–inch lenses were specially prepared by Captain Abney, and on each of them he had placed a scale of standard intensity squares for measuring the brightness of the corona. Small squares on each of the plates were exposed to a standard light for various times; these squares were then covered with a strip of black paper, and the plates taken out to the Eclipse Station and exposed on the corona. When the plates were developed the image of the corona and the squares were, of course, developed to the same extent, the squares thus serving as standards for absolutely measuring the photographic intensity of the light of the corona. The density of the deposit in any part of the picture of the corona can be compared with the density of the most similar of the squares on the same plate by Captain Abney’s photometer, and as this photometer depends upon the method of limiting apertures, it gives absolute readings.
The African expedition was entirely unsuccessful, owing to clouds, but the expedition to Salut Isles, under charge of the late Father Perry, obtained successful photographs, which are at present under examination. From them Captain Abney will be able to measure the absolute photographic intensity of the light of the corona.
An American expedition was sent to Cayenne with instruments used on January 1, 1889, and obtained successful photographs, while an American expedition to Southwest Africa was unsuccessful, for the reason already given. This expedition, under the direction of Professor David P. Todd, was located at Cape Ledo, about half a mile from the English Eclipse Station.