SPECTEA OE IGNITED GASES AND VAPOUES. 
i 
(measured by means of the manometer) being from 40 millims. to 80 millims., the spec- 
tral tube was melted off and hermetically sealed. 
16. When we send through our nitrogen-tube the direct discharge of Ruhmkorff’s 
large induction coil, without making use of the Leyden jar, we observe a beautiful 
richly coloured spectrum. This spectrum is not a continuous one, but divided into 
bands, the character of which differs essentially at its two extremities ; its middle part 
is in most cases less distinctly traced. Towards the more refracted part of the spectrum, 
the bands, illuminated by the purest blue or violet light, present a channeled appear- 
ance *. This effect is produced by a shading, the intensity of which decreases from the 
more to the less refracted part of each band. On applying four prisms instead of two, 
we perceive a small bright line, forming an interstice between two neighbouring chan- 
nels, and the shading is, by the telescope of the spectral apparatus, resolved into dark 
lines. The number of such dark lines of one of the brightest bands (of the eighth band, 
we always count from the red to the violet) was found to be thirty-four, or nearly so. 
Their mutual distance is nearly the same, but their darkness decreases towards the least- 
refracted limit of each channeled band. Hence we concluded, the breadth of the band 
having been measured, that the angular distance of two contiguous shading-lines was 
nearly equal to the distance of the two sodium-lines. The breadth of the channeled 
bands varies, but the character of all is absolutely the same ; only if foreign bright 
lines like those of hydrogen are simultaneously seen, it becomes slightly disturbed. 
We may distinguish seventeen bands of this description ; the first three are smaller 
ones, the fourth is traversed by H/3, the eleventh by Hyf. At the violet extremity the 
light is very faint. 
17. The bands of the less refracted part of the spectrum are all of nearly the same 
breadth, but smaller than those just described, and of quite a different appearance. 
Making use of only a single prism, and of a small magnifying power, we count eighteen 
such bands, starting from the extreme red and extending to the greenish yellow, 
where they are bounded by a dark space. H a falls within the fourth, and the double 
sodium-line (Na) within the fourteenth of these bands. Under favourable circum- 
stances, both extremities of the spectrum being equally developed, these bands extend 
to the channeled part, their number rising to thirty-five. All have the same general 
character, but not the same brightness. From the extreme red the intensity of light 
* Under favourable conditions such a band appears furrowed semicircularly ; but psychological effects of this 
description may be quite different : partly by our own will, partly by exterior circumstances, the bands may be 
seen convex as well as concave. Even the engraving of the bands (Plate I.) shows it. Let it be illuminated by 
daylight through a window, you will see the bands concave if their more refracted and shaded part be directed 
towards the window ; if in the opposite direction, the bands will appear convex. The shade passes from one 
side to the other if really concave and convex bands are replaced by one another ; so it does if the illuminating 
light pass to the opposite side. Accordingly, the stereoscopic appearance depending upon the direction from 
which the light comes, the mind passes judgment on it unconsciously. 
t "We denote by Ha, H/3, and Hy the three bright lines of the spectrum of hydrogen (the red, the bluish 
green, and the violet one). See 57. 
