616 PEOPESSOE STOKES ON THE LONO SPECTETJM OE ELECTEIC LIGHT. 
mode of discharge, in which the negative electrode, if at all thin, quickly becomes red-hot 
and fuses, the jar has not much influence, and the lines in question are still better seen 
when it is suppressed altogether. To show them to perfection, it is best to take a flat 
negative electrode, so as to carry off the heat, and not to hide from the prism any part 
of the blue negative light, and a sharp positive electrode almost touching the former. 
In this way the visible discharge is reduced almost wholly to an insignificant-looking 
star of blue light ; but it is wonderful how strong an effect it is capable of producing in 
the invisible region. The most striking part of the invisible spectrum consists of four 
bright lines, numbered 1, 2, 3, 4 in fig. 1, situated not far from the visible spectrum. 
These are followed, after a nearly dark interval, by light arranged in masses resembling 
in its general aspect the groups of copper lines (from which, however, it differs), but not 
strong enough to be resolved or accurately measured. The figure represents also a 
couple of blue bands (b, b') seen by projection. These are not seen on looking at the 
blue light directly with a flint-glass prism of 60°, because everything is seen in too great 
detail. Most of the air-lines in the invisible spectrum, especially the bands beyond 
line 4, have an ill-defined look, and would probably be resolved did the intensity of the 
light permit. 
The appearance just described is independent of the nature of the electrodes, and 
therefore is to be referred to the air, and not to the metal. On viewing in a moving 
mirror the star of light producing this effect, it is found to have a considerable duration. 
On slightly separating the electrodes, forming an image of the discharge with the 
2’5-inch lens, and receiving it on a cake of the uranium salt, a very strong fluorescence 
was seen over the image of the blue disk when the lens was focused for a point a little 
beyond the visible spectrum. On moving the lens onwards, the fluorescence produced 
by the rays belonging to this image spread out into a ring ; and on moving still frirther, 
a tolerably well-defined image of the whole discharge was perceived. Of this the part 
belonging to the blue disk was the brightest, and was surrounded concentrically by the 
ring before mentioned, now still further widened. The image of the remainder of the 
discharge was brightest where it was most contracted at the positive electrode. The 
discharge generally was perhaps of slightly higher refrangibility than the blue disk, even 
excluding from the latter the rays belonging to the ring. It thus appears that the four 
bright lines figured were produced mainly by the blue negative light. 
The mode of transition of the discharge may be studied by placing the electrodes at 
the greatest striking-distance and making them gradually approach. At first there 
passes a clean bright spark making a sharp report, and not resolved by a revolving 
mirror. The invisible spectrum which this shows is too faint for precise observation ; 
the visible spectrum shows chiefly air-lines. As the electrodes approach, the spark 
becomes clothed by the well-known yellowish envelope capable of being blown aside, 
and the blue negative light begins to appear. A moving mirror, as M. Lissajous has 
already observed*, shows an instantaneous spark at the commencement, in point of time, 
* See Dtt Mokcel, ‘ Eecherches sur la non-homogeneite de I’etinceUe d’ induction,’ p. 107. 
