In 1917 Lord Rayleigh and Prof. A. Fowler observed that when the vapour of a carbon compound is introduced into the afterglow of active nitrogen, the two CN systems—the so-called "red" and "violet" systems—are developed, but their appearance in this source is strikingly different from the more familiar appearance of the same bands in the carbon arc in air. The "violet" system as ordinarily observed in the arc, comprises the four well-known groups of bands which degrade towards the further ultra-violet from prominent heads at λλ 4606, 4216, 3883 and 3590 respectively, and four groups of weaker bands, the so-called "tail" bands, degrading towards the red. The modifications of the λ 4216 and λ 3883 groups are shown in figs. 4 and 5 of Plate 6, in Rayleigh and Fowler's paper. The description of these modifications will be much facilitated by reference to Table I, which shows the wave-lengths and wave-numbers of the band-heads, and the initial and final vibrational quantum numbers (
n', n"
) for the bands, as well as other particulars to which frequent reference will be made later. The modifications are conveniently described as two effects:— First Effect.—In the typical case of the λ 4216 (
n"
-
n'
= +1) group in the afterglow, there is a modified development of the lines of the first (0, 1) and the third (2, 3) bands, and a partial suppression of the heads of the second (1, 2) and fourth (3, 4) bands. The lines near the head of the (0, 1) band in the afterglow are apparently identical with lines in the corresponding part of the same band in the arc, but at a short distance, about 9 A. U., from the head there is a conspicuous gap of about 3 A. U., beyond which the lines recover in intensity and overrun the weakened (1, 2) head. The (2, 3) band, though not nearly so strong as the (0, 1), shows a similar gap and regain of intensity, its lines over-running the weakened (3, 4) head. A similar effect occurs in the λ 3883 (
n"
-
n'
= 0) group.
The more refrangible band system of cyanogen as developed in active nitrogen