The first tube I examined was an ordinary Geissler tube charged with rarefied air. The bulbs, on passing the discharge, were filled with the well-known rose-tinged light like to the Aurora-streams. This in the capillary part was condensed into a brighter and whiter thread, while the platinum wire of the negative pole was surrounded by its characteristic mauve or violet glow.
Spectrum described.
The spectrum, even with a weak current, was quite bright, and consisted mainly of the nitrogen-lines and bands, with the lines Hα, Hβ, and Hγ, and some of the intermediate lines of the H tube.
The double line α was undoubtedly the brightest in the spectrum when taken in the capillary part of the tube. After this followed β, and then γ(H), δ, and ε. I was, however, uncertain as to the relative brightness of the last three, and marked their intensities with hesitation. I tested them several times independently with differing results, and suspected them of variability with the current.
The rest of the lines were very much of the same intensity. (For drawing of spectrum of air-tube in capillary part see Plate XV. spectrum 1.)
Violet [negative] Pole, same tube.
Violet (negative) pole: spectrum described.
I next turned my attention to the violet or negative-pole glow; and here a remarkable change took place in the spectrum, not only in the position of the principal bands or lines, but in their relative intensity (see Plate XV. spectrum 2).
The double line α in the capillary part was replaced in the violet glow by a shaded band of second intensity β, the sharp edge of which was extended towards the red, and formed (except for some faint indications) the limit of the spectrum in that direction. The somewhat faint line next α in the capillary tube had its faint representative in the violet pole; but the next two lines (capillary) were represented by the bright band γ in the violet pole lying in a position between them. Next γ in the violet pole came three faint lines, representing β, γ, and δ in the capillary spectrum; and then the bright band α, which was the brightest of the violet-pole group, and represented a medium-intensity band in the capillary spectrum. After this was a faint band near α, representing two rather bright ones in the capillary spectrum, this last being succeeded by other bands in the violet. α, β, and γ in the violet pole were examined carefully for relative brightness, and were, I believe, correctly marked.