698 
Proceedings of the Royal Society 
nature, touching its power and manner of showing the two D lines of 
the solar spectrum. 
The condensed induction spark filled all the field of view with 
continuous fervid glow, and quaking air-hands almost fearful to 
behold. 
But a vacuum tube of Hydrogen, with a trace of sodium chloride, 
showed of the latter only the two D lines, each of them intensely 
bright, exactly defined, without any stray light; and, in short, 
exactly as they should be to serve any case of Micrometical 
measure. 
CH again^ hut now in Vacuum Tubes. 
Trying the vacuum tube method, therefore, on Coal-gas, the first 
tube, though bright enough, was yet a failure ; for it was not bright 
with the coal-gas spectrum, but with a variety of impurities and 
decomposed materials. After this, however, had been got over to a 
considerable extent by increasing the density of the gas put into 
these so-called vacuum tubes from OT" to 2 '5 inches of Barometric 
pressure, the Citron and the Green bands of CH were produced in 
a condition for examination. They ran quite parallel with their 
blow-pipe congeners, but were far more refined : the linelets being 
often like exquisitely thin spider lines, in place of broad hazy 
threads. But they did not last ; for day after day they grew fainter, 
more and more of them became first double, then treble, and then 
faded away, lines of pure Hydrogen coming in their stead. 
The leading Orange band of the CH spectrum was particularly 
difficult to observe, on account of the intrusion of these H lines. 
But after having appealed from Coal-gas to Olefiant gas, and having 
increased the pressure of that up to 4 Mercurial Inches, the cynosure 
was obtained at last, resulting in what may be termed a perfect view 
of the beginning of the CH spectrum, and of that alone. 
There, for instance, was the Orange band, with its leading lines 
brilliant, and its linelets clear and distinct, though continually widen- 
ing in distance apart, but in unbroken series not only to the usual place 
of vanishing but right up to the strong beginning of the Citron band. 
The Citron band’s linelets similarly continued in exquisite perfec- 
tion of gradation right up to the Green band. And the Green 
band’s linelets continued without a break, or an interference, or 
