Page:Popular Science Monthly Volume 48.djvu/371

Rh

HE famous "D$3$," so called because it is very near the D lines of sodium, is a bright yellow line in the spectrum of the solar chromosphere, in which it is more conspicuous than anything except the C and F lines of hydrogen. Unlike them, however, it has no corresponding dark line in the ordinary solar spectrum, a rather perplexing fact which has caused much discussion, and has not even yet found an explanation in which all authorities agree.

It was discovered in 1868, when the spectroscope was for the first time directed upon a solar eclipse. Most of the observers supposed it to be the sodium line, but Janssen noted its non-coincidence; and very soon, when Lockyer and Frankland took up the study of the chromosphere spectrum, they found that the line could not be ascribed to hydrogen or to any other known terrestrial element. As a matter of convenient reference Frankland proposed for the unknown substance the provisional name of "helium" (from the Greek "helios," the sun), and this ultimately, though rather slowly, gained universal acceptance.

Within a year, two other lines (λ 7,065 and λ 4,472) were discovered in the chromosphere spectrum by Rayet and Respighi respectively, which like D$3$ are always present in the prominences, but have no corresponding dark lines. It was of course early suggested, but without proof, that these lines might also be due to helium. Since then some eight or ten other lines have been found, frequently, but not always, presenting themselves in the chromosphere spectrum, and, like the first three, also without dark analogues. Moreover, still more recently, D$3$ and its congeners have been detected in the stellar spectra—dark. in the spectra of the "Orion stars," bright in the spectra of certain variables and of the so-called Wolf-Rayet stars; and both bright and dark in β Lyræ and the "new star" of Auriga which appeared in 1892.

Naturally there has been much earnest searching after the hypothetical element, but until very recently wholly without success; though it should be mentioned that in 1881, Palmieri, the director of the earthquake observatory upon Vesuvius, announced that he had found D$3$ in the spectrum of one of the lava minerals with which he was dealing. But he did not follow up the announcement with any evidence, nor has it ever received any confirmation, and from what we now know as to the conditions necessary to bring out the helium spectrum, there is every reason to suppose that he was mistaken.