Our Astronomical Column

Abstract

COMET-SPECTRA AS AFFECTED BY WIDTH OF SLIT.—The unaccountable differences between the spectrum of burning or electrically glowing carbon and the carbon, bands observed in comets are successfully explained by Prof. H. Kayser in the Astronomische Nachrichten. The chief differences observed between the cometary and terrestrial spectra are the following:—The carbon flutings in the laboratory have a bright edge on the red side, which in the comet spectrum is displaced towards the red. But the maximum of luminosity in the latter is more refrangible than the bright edge in the former. Whereas in the true carbon spectrum the first fluting is the brightest, in cometary spectra the second has often appeared brighter than the first. It is suggested that all these anomalies are due to the fact that in astronomical spectroscopy the slit cannot be closed so far as in the laboratory, when the objects observed are as faint as comets usually are., If we suppose the true spectrum to be that produced by a very narrow slit, we may reproduce the impure cometary spectrum, by Sliding a wide slit along the true spectrum, and adding up for every position the strips of the true spectrum covered by the slit. We shall thus obtain he portion of the impure spectrum corresponding to the centre of the slit. When the wide slit encounters a band with a bright edge towards the red, it will at once begin to indicate a brightness, which will gradually in crease until the slit is completely filled with light. The maximum will then have been obtained, and we see that it does not correspond to the bright edge, but to a line within it. Thus the first two anomalies are accounted for. Finally, if the slit is so wide that it comprehends two carbon bands at the same time, the maximum will not be obtained when the first or the second band occupies its centre, but when the first is just leaving and the third just entering. This accounts for the third anomaly. The experiment may be easily performed in the laboratory, by observing the arc spectra of calcium or iron. On widening the slit the line spectra of these elements show the same positions for the widened lines, but the carbon bands are diffused towards the red, and their maxima are displaced towards the violet.