Abstract
AT the meeting of the Astronomical Society on November 8, a sketch was given of Lord Lindsay's preparations for the forthcoming transit of Venus. Lord Lindsay has selected the island of Mauritius as his station, on account of its highly favourable meteorological conditions. He intends, if possible, to combine the following methods of observation:—I. Observations of the internal contacts to be worked out on the plans of Halley and Delisle. 2. Observations of the first external contact at the chromosphere, to be made with the spectroscope. 3. Photographic pictures. 4. Heliometric measures. For the longitude it is at present intended to use the transits of the moon with an altazimuth made by Simms. As it is expected that the Germans will also have a station on the Mauritius, Lord Lindsay will connect his station with theirs by triangulation. The transit instrument is by Cooke, and has four inches aperture. The chronograph, which can be kept in motion for four hours, has four barrels, each of which can be worked separately, thus avoiding all confusion. The photographic method to be used is that of Prof. Winlock, who suggests a telescope of 40 feet focal length, placed horizontally, and a heliostat to reflect the sun's image along it. The lens is to be an achromatic one. It is intended to have two planes to the heliostat, one mounted on a polar axis, and another to send the rays down the tube. Lord Lindsay has ordered a Foucault siderostat with 16-inch mirrors, and has obtained a 13-inch unsilvered mirror to fit the telescope to be taken out. He intends to use a heliometer, though it is not much in favour in this country, Messrs. Respald, of Hamburgh, having undertaken to make one for him with all the improvements used in the Oxford,instrument, as well as in some others. The Germans intend to send one to Ker-guelen Land, and the Russians will use it at Lake Baikal and the mouth of the Amoor. Lord Lindsay's will include the motion of the halves of the object-glass in curved slides, so that the images will remain in focus; unlimited rotation of the tube in the cradle; the measurement of the position angle at the eye end, and measures of the micrometer read there also. Some new points are;'the graduation of the slides of the object-glass side by side, so as to be read by the same microscope; an arrangement to shut off light from half the object-glass, so as to equalise the light of the images; and the introduction of a thermometer at the end of the tube. Lord Lindsay proposes to eliminate errors of division as affected by temperature, by placing the instrument on one of the collimating piles of his transit circle at home, and heating the room by gas to different temperatures. It is hoped that, by taking a large number of measures, and by taking the most careful precautions, the original error of observation may be reduced to less than 0″.5, and thus make the result one of extreme accuracy.
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The Transit of Venus . Nature 7, 109 (1872). https://doi.org/10.1038/007109a0
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DOI: https://doi.org/10.1038/007109a0