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GASEOUS SPECTRA IN VACUUM TUBES. 95 
This capillary, when thus occupied by the incandescent rushing molecules 
and viewed end-on, presents a little disc, smaller than a pin’s head, of light as 
to size; but of exceeding brightness as to anything ordinarily seen in vacuum 
tubes. So bright, indeed, that when viewed under small spectroscopic dis- 
persion, one’s eyes quail before the red and blue hydrogen lines as though they 
were glancing from the sun itself; while carbon bands appear more as solid 
things than haze ; and Nitrogen is simply a many coloured glory to behold. 
Of course that is a symptom in spectroscopy that those subjects will bear 
more dispersion : in which case by all means let them have it ; for only in that 
way can we ascertain the degree of importance of gaseous spectra. From that 
mere name of gaseous you might almost justifiably expect, that if there is 
anything sharp to be seen in them with a low power, it must of course become 
hazy and foggy with a high one, when made thereby to subtend a larger angle ; 
just as the edge of a cumulus cloud on the horizon, however well defined there, 
disappears as an edge, in soft formless vapour when we come close to it. 
But it is not so here in spectroscoping the ten thousandth part of a grain 
of electric-illuminated, rarefied gas. Take the Cyanogen pin’s head of white 
light as an example; stretch that little speck horizontally by spectroscopic 
power, just say to a finger’s-breadth; or, as it can be made to appear in 
angular space in even the smallest spectroscope, to half a degree in length ; 
and we have, with a broad slit, not much more than a very pretty spectrum, 
red at one end, citron in the middle, and violet at the other end; with some 
liazy transverse bars of greater or less than the general brightness. 
Stretch it then a little more, say to 3 degrees; and behold, by means of 
that, only in so far, increased scale of length from red to violet we now behold the 
alternations of more or less brightness, as seen before, explain themselves as a 
beautiful set of bands; sharp as knife edges on one side, if the slit be rather 
narrowed and the focus improved, but indefinitely shading away at the other 
side ; whilst here and there are single lines burning and shining like linear 
suns; only that in place of their being, in colour, all of them like our sun, 
yellowish-white, one is red, another orange, or citron, or green, or glaucous, 
or blue, or violet, or lavender, harmoniously with its spectrum place. 
So stretch the little pin’s head of light more still, say to a length of 12 degrees. 
Why the bands are still more beautiful than ever ; still so sharp and solid on 
one side, but resolving themselves now into close linelets and ranks of the most 
needle-like lines on the other; lines defying the powers of the micrometer to 
count their number, or equal them in thinness, or to separate them fully and 
clearly one from another. 
Wherefore now spare nothing; stretch the luminous pin’s head by prism 
power and magnifying power combined, until it forms an environing circle all 
round the observer, or subtends to him an angle of 360°; and have you now 
