Abstract
MOTION OF STARS IN THE LINE OF SIGHT.—Prof. H. C. Vogel, in Monthly Notices R.A.S. for December 1891, fully describes the method used at Potsdam for determining the velocity of stars in the line of sight, and states the chief results that have been obtained since the work was begun in 1887. In order to insure great stability with the smallest possible weight, the frame of the spectroscope is made of cast steel. The camera is also constructed of steel, and the dark slides are of brass. It may be worth remarking, however, in this connection, that stability would have been secured if aluminium had been used instead of steel and brass, and this with a little more than one-third the weight. A spectroscope similar to Prof. Vogel's, but with an aluminium frame, has been made for the Observatory at Kensington, and gives every satisfaction. The comparison spectrum used at Potsdam has been furnished by a Geissler tube placed directly in the cone of rays of the refractor, at a distance of 40 cm. from the slit, the tube being at right angles to the optical axis of the refractor and the slit. The slit is set parallel to the line of the diurnal motion, and width is given to the spectrum by making the driving-clock move slightly slower or faster than its proper rate. A uniform exposure of one hour has been employed, the proper intensity being obtained by changing the rate of the driving-clock, so that the error increases with increase of brightness. The photographs are measured with the aid of a microscope having a sliding apparatus on its table, movable by a fine micrometer screw. One revolution of the screw corresponds to a difference of wave-length of 0˙324 μμ, which, expressed in miles per second, is 139˙13. After describing the methods of measuring the displacement of lines in stars of different types of spectra, Prof. Vogel brings together the results which have formed the subject of several previous communications. It is said that the probable error in the determination of the radial velocity of a star of Class II. is ± 1˙34 miles per second, and for stars of Class 1., ±2˙31 miles. Measurements have been made independently by Prof. Vogel and Dr. Scheiner, and each star has been observed on the average 3˙3 times, wherefore it is concluded “that the probable error of the definitive values for both spectral classes will amount to less than one mile.” A list of the observed velocities of forty-seven stars will soon be published. The mean motion in the line of sight is 10˙6 English miles per second; six stars have a velocity less than 2 miles per second and five greater than 20 miles, α Tauri heads the list with a velocity of about + 36 miles per second. Fifteen of the stars have a positive, and thirty-two a negative motion.
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Our Astronomical Column. Nature 45, 280–281 (1892). https://doi.org/10.1038/045280a0
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DOI: https://doi.org/10.1038/045280a0