Page:The story of the comets.djvu/247

XII. rendered incandescent, is but another step in accordance with the law of continuity.

Schiaparelli and others have taught us to associate closely meteors with comets, and we now know of numerous instances in which a comet and a meteor stream are actually travelling on the same orbit. It might be expected that there would be some resemblance between the spectra of the two classes of bodies. But the rapid motion and evanescent character of meteors makes their spectroscopic observation exceedingly difficult. Browning, however, succeeded in observing no fewer than 70 in August and November 1866, with an instrument constructed by himself for the purpose. This consisted simply of a direct-vision compound prism, and a plano-concave cylindrical lens; the latter being intended to diminish the apparent angle through which the meteors fell. The heads of the meteors gave spectra mostly continuous, though with frequent differences in the relative preponderance of the colours. In the tails, in every instance, orange-yellow light predominated, from which the presence of sodium may probably be inferred. Konkoly looks upon this presence of the sodium line as possibly due rather to particles floating in our air and becoming incandescent with the meteor than to any constituent of the meteor itself. But the same observer on the spectrum of a magnificent fireball on Oct. 13, 1873, observed not merely the sodium lines, but also bands which he was able to identify by direct comparison with the spectrum of a hydrocarbon, thus affording an evidence from the spectroscopic side of the connection between comets and meteors; and remembering the brilliant sodium lines of the comets of 1882, it does not seem improbable that meteors should show this metal also.

Other metallic lines have also been observed in meteors: those of Magnesium frequently, and the lines of lithium and potassium sometimes.