The lines (adopting Dr. Vogel’s wave-lengths for the H lines) were, when compared, as under:—

Aurora556953905233518950044694
to
4629		band.
Hydrogen55555422...518950084632

I remarked that a line (5596) described by Dr. Vogel as “very bright” in his H spectrum does not appear in my tube, though in most other respects our H spectra agree.

Effect of distance on the spectrum.

I thought this tube afforded a good opportunity for testing the effect of distance upon the spectrum. The slit was made rather fine. At 6 inches distance from it the line α in the blue-green (F solar) was very bright. The lines marked β, γ, δ, ε, and ζ also survived, but were faint. At 12 inches from the slit α and γ were alone seen, and at 24 inches α stood by itself upon a dark ground. I also noticed that the red and yellow parts of the spectrum first lost their light on the tube being withdrawn from the slit; and this appeared to account for β disappearing while γ survived. For drawing of the hydrogen-tube spectrum see Plate XIV. spectrum 1.

The question of effect of distance upon the spectroscopic appearance of a glowing light, as tested for this and other tubes, seems an important one. It may possibly account for the generally faint aspect of the lines in the more refrangible part of the Auroral spectrum.

Plate XIV.

Carbon- and Oxygen-tubes.

Carbon- and oxygen-tubes. Tubes described. Carbon-tubes lighted up. Spectra of the carbon-tubes described.