Page:The story of the comets.djvu/233

XII. towards the violet. Thus Konkoly found, when examining Sawerthal's Comet (1888 i.), that the lines were not sharply defined on either side. Consequently his measures of the middle of the maximum light-intensity of the hydro-carbon bands did not coincide with the laboratory wave-lengths. In bright comets when a very narrow slit can be used, and when therefore the entire breadth of the slit is filled with light of practically the same intensity, this discrepancy between the spectra of the comet and of the hydro-carbon disappears. Maunder writes thus in regard to Tebbutt's Comet of 1881:—

"With the spectra of the Comet and of the Bunsen-flame arranged one above the other, and the flame adjusted until the bright sharp edge of the green band was of the same intensity in each, the resemblance between the two spectra was exceedingly striking, the three principal bands corresponding exactly in position, in brightness, and in the manner and degree in which they shaded off towards the violet."

If this be the correct explanation, there will be no reason to suppose, as in paragraph (6), that the displacement arises from any peculiarity in the physical condition of comets. The displacement will be apparent only, and due to the difficulties of observation.

Beside these shaded hydro-carbon bands, comets generally give a continuous spectrum from the nucleus and the immediate neighbourhood. This is largely due to reflected sunlight, the Fraunhofer lines having been detected on favourable occasions.

The years 1881 and 1882 were especially remarkable for the fine comets which were then visible. Comet iii. of 1881, discovered by Tebbutt, proved especially interesting as the first which was itself successfully photographed, and the first of which the spectrum was photographed, the former feat being accomplished by Janssen, the latter by Huggins and by H. Draper. As first seen in Northern latitudes, the spectrum of Tebbutt's Comet was almost purely a continuous one; it was some few days before the cometary bands began to show themselves. But as the nucleus faded they became more apparent, and their precise coincidence with the bands from a Bunsen-flame was rendered evident. The continuous