540 THE SCIENTIFIC MONTELY
There is a ^eat v&riet3r of comet epectra, indicatiDg as great a variety of cometary contentB or conditioDs. la some cases the spec- tnim seems almost wholly continuous, as in Holmes's comet of 1892 ; in others the light when passed through the spectroscope falls almost wholly into isolated bright lines or bands, as in Morehouse's comet of 1908. Other spectra are a combination of continuous and bright-line light (Fig. 13). The spectrum of the nucleus seems to be always con- tinuous, or continuous except for absorption lines. In eome of the brighter comets the nucleus spectrum as photographed contaioa the well-known absorption lines visible in the sun's spectrum. These ob- servations indicate that the nucleus is shining, at least mainly, by reflected sunlight. In most comets the continuous spectrum is too faint to let us photograph it and thus to prove the presence or ab;eace of the solar absorption* lines. The continuous spectrum in many
��Fio. 1Z Spkctbcm of CoiraT Dahiilb, IBOT.
comets extends also to the head, or at least to the inner strata of the bead. This may or may not mean reflected sunlight. It may mean some other form of luminescence which yields a continuous spectrum. The greater parts of the heads of comets and those parts of the tails of comets which are close to the heads nearly always, and perhaps in every case, give a characteristic spectrum of bright bands, which were for several decades called the hydrocarbon bands. Observations of recent years have made it probable that this spectrum does not indicate a combination of hydrogen and carbon, but that it is either one of the low-pressure carbon vapor bands or that it results from one of the com- pounds of carbon and oxygen, preferably from carbon monoxide. The lines and bands of cyanogen — a nitrogen compound — and of carbon are present without any question in the heads and inner tails of many comets. Several observers have reported that the so-called hydro- carbon spectrum of the heads and inner tails extends far out into the tails. This may have Iwen true for the cases reported, but recent ob- servations are casting doubt upon the presence of that spectrum in the outer extensions of comet tails. Improved methods of photographing comet spectra were applied to the bright comets, Daniels of 1907 and Morehouse of 1908, especially by Deslandres, Evershed and Chretien,
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