Page:Popular Science Monthly Volume 30.djvu/41

Rh In 1883 Egoroff, a Russian physicist, succeeded in showing that the great A and B groups of the solar spectrum are due to the oxygen in our atmosphere.

Cornu, by a very ingenious arrangement, in which he makes a small image of the sun, four or five millimetres in diameter, oscillate across the slit of a powerful spectroscope three or four times a second, has succeeded in bringing out conspicuously, and at a glance, the difference between the true solar and the telluric lines in the spectrum. The solar lines oscillate slightly as the eastern and western, the advancing and receding, limbs of the sun come alternately to the slit, while the telluric lines stand fast.

Mr.Lockyer has called in question the existence of the so-called "reversing layer" of the chromosphere, being disposed to hold that certain of the lines which we identify as belonging to the spectrum of any given substance, say iron, are due to absorption in upper and cooler regions of the solar atmosphere, while others are produced low down. In support of this idea, he adduces the observation that, at the Egyptian eclipse of 1882, certain of the so-called "iron-lines," between b and F, were much longer, though no brighter, than other "iron-lines" close by them, and remained much longer visible as the moon advanced to cover the chromosphere. There is not room to discuss the matter here. Those who believe that the Fraunhofer lines are mainly produced by that portion of the solar atmosphere which bathes and sustains the photospheric clouds, or lies immediately above them, would not quarrel with the idea that the upper regions also co-operate to a certain extent; but we see no proof from observation, as yet, that lines which are produced by the absorption of the upper regions of the solar atmosphere are not also found in the lower. The question undoubtedly is interesting and important: Does each region of the solar atmosphere have its own spectrum, peculiar and distinct from those of other regions above and below; all of them co-operating by simple summation to form the spectrum as we see it—or, on the other hand, as has been usually admitted, does the spectrum of the lowest stratum contain everything, while the spectra of the higher regions differ from it merely by defect? Eclipse observations may possibly decide it. Of course, if Mr.Lockyer is right, the fact would be a very effective argument for the theory of "compound elements," which theory, notwithstanding the failure of its "basic-line" defense, seems to be, decidedly gaining ground in scientific opinion.

As to the bright-line spectrum of the chromosphere, no great discoveries have been made; a number of lines, probably fifteen or twenty, have been added by the writer to the two hundred and seventy-three long ago catalogued as constantly or occasionally appearing. Most of the new lines are in the violet and ultra-violet. Not one of them is below C.

Trouvelot has observed (or thinks he has) dark prominences—i.e.,