Our Astronomical Column


    THE D3 (HELIUM) ABSORPTION LINE IN THE NORMAL SOLAR SPECTRUM.—An important statement by Mr. J. Evershed, concerning the presence of the dark helium, D3, line in the solar spectra photographed by Mr. Nagaraja, appears in No. 396 (p. 212, May) of the Observatory. Readers of these columns will remember that various observers have criticised copies of Mr. Nagaraja's photographs, and have arrived at the conclusion that the dark line shown thereon is probably not the absorption line (D3) of helium. But Mr. Evershed has carefully measured this dark line shown on a number of plates, and the results of his measures lead him to the belief that the line is really the helium absorption line. Micrometer measures on the actual photograph from which the copies were taken (May 4, 1907) give 5876.15 and 5876.17 as the wave-lengths of the bright line at the limb and of the dark line respectively, whilst from six determinations of each of a series of photographs obtained during 1907 Mr. Evershed obtains the mean values 5875-96 and 5875-97 respectively. The coincidence is indeed very close, and the mean value is in good accordance with the values obtained by Prof. Rowland and Prof. Hale. Mr. Evershed suggests that the prolongation of the dark line across the umbral areas, a phenomenon which proved a difficulty with the visual observers who criticised the results, may be due to the unsteadiness of the image on the spectrograph slit during a long exposure.

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    Our Astronomical Column . Nature 78, 38 (1908). https://doi.org/10.1038/078038a0

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