Very few photographs were obtained of the eclipse of August 19, 1887, in Russia, owing to the unfavorable weather. The English observers intended to use similar instruments to those employed in 1886, but the weather did not permit.
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.
Several new departures in eclipse photography were introduced. Chief amongst these was the remarkable apparatus by means of which no less than twenty-three objectives and two mirrors were accurately pointed at the sun and caused to follow it by one large clock. A large duplex polar axis (the old English form as used for the 12.5 inch reflector at Greenwich) was mounted on solidly constructed stone piers and very carefully adjusted. This axis is constructed of 6–in. wrought-iron tubing, the total weight being about 2000 lbs. In it the cameras were fixed by set screws, the optic axis of the instruments being adjusted parallel to each other, and at an angle equal to the south polar distance of the sun at the time of totality. The carefully regulated and very powerful clockwork attached to the instrument caused the polar axis to rotate, and thus the whole battery of instruments followed the sun. Each lens was fitted with a pneumatic shutter regulated to give the required exposure in each case. The cameras themselves were enclosed in a dark-room, the lenses only being exposed to the sun, so that dark slides were not required, the plates being held on open rotating frames, these frames being rotated at the proper time by pneumatic arrangements. When the cameras were once pointed, and the clock driving properly, all the operations of exposure and changing of plates were performed without personal superintendence by means of the pneumatic apparatus, and a chronograph attached to the valve system of this apparatus recorded the exact time at which each exposure was begun and ended.
It is to be regretted that this ingenious and elaborate apparatus did not have a satisfactory trial, owing to the dense clouds; but Professor Todd assures us that he was thoroughly satisfied with the success of the pneumatic movements during the three minutes ten seconds he brought it into operation at the time of totality.
It is not improbable that (in spite of the great strength and weight of the axis and the solidity of the supporting piers) with this plan of fixing a large number of cameras and spectroscope on one polar axis, the constant opening and shutting of shutters, and the changing of the plates, may produce so much shake that none of the long exposure photographs will be satisfactory. This, of course, can only be ascertained by the use of the instrument on the corona, and several years must elapse before the trial can be made.
Another unusual instrument was a photo-heliograph of five inches aperture and forty feet focus, mounted on a combination of the equatorial stand and tripod.
The long tube was made of iron, coiled spirally and strongly riveted, the necessary rigidity being attained by strong wires extending from end to end, and tightly stretched by a disc in the middle of the tube. Close to one end of the tube the polar axis was attached by a universal joint; the other end of the tube being supported by two rods, one on the east and one on the west side, these rods being also attached by universal joints. By means of these rods the proper inclination was given to the tube. The east rod was the declination rod, and was capable of sliding along the polar axis. The west rod was for giving motion in right ascension, being terminated at the free end in the form of a piston of a sand clock fixed in an inclined position. The rate at which the sand escaped from the cylinder could be accurately regulated, so that the rate of descent of the piston was completely under control, and was, of course, such as would cause the instrument to follow the sun.