Page:The story of the comets.djvu/232

178 Hasselberg reviewing the observations of the first 18 comets subjected to prismatic analysis concluded :—

(1) That all observed cometary spectra belong to one type, with the exception of the two doubtful cases of Comets i. 1868, and iii. 1877, noted above.

(2) That this type is that of the hydro-carbons.

(3) That they deviate from the type in being incomplete; and, in general, in the relative brightness of the bands.

(4) That they are incomplete in so far as the red and violet bands of the hydro-carbons are wanting, and also that the maximum brightness of the bands is not at the less refrangible edge, but somewhat towards the violet.

(5) That this circumstance explains why in the case of faint comets the connection with hydro-carbon spectra has appeared doubtful.

(6) That the displacement of the maxima of the bands of most comets, with regard to those of hydro-carbons, is approximately the same; therefore it seems probable that the differences of the physical conditions of the hydro-carbons in the comets, from those which have hitherto been obtained in observations of hydro-carbon spectra, are approximately the same.

The observation of the red and violet bands in Coggia's Comet and in several recent bright comets meets the allegation of incompleteness made above in (4); whilst the shift of the maximum brightness from the edge of the cometary band towards the violet admits often of a very simple explanation. To observe a faint comet the slit of the spectroscope must be opened wide, and probably the light from the whole of the head or from a considerable portion of it will be embraced within the opening. This will not be of one homogeneous brightness throughout, but will fade outwards from the centre; there will be more light near the centre of the slit opening than close to either jaw, and consequently the cometary bands will not only degrade towards the violet, but to a slight extent towards the red as well, throwing the maximum brightness