136 
MR. W. CROOKES OK THE ILLUMINATION OF LINES OF 
ending outside in loops and inside in aluminium poles ; the positive pole being a wire 
and the negative pole a disk about 10 millims. diameter, bare in front and covered with 
mica at the back. The bulb being full of dry air and connected with the Sprengel 
pump, was exhausted. An induction coil capable of giving sparks 68 millims. long in 
air when actuated by 3 Grove’s cells, was connected with the terminals, the disk being 
always negative except when otherwise stated. 
487. At a pressure of 20 millims. of mercury a soft velvety halo of violet light com¬ 
menced to form on the edges of the disk ; as the exhaustion proceeded, this glow 
flickered over different parts of the surface, till at a pressure of 15 millims. it remained 
steady all over the front of the disk. 
Plate 14, fig. 2, shows the appearance at a pressure between 15 and 20 millims. : a 
shows the glow creeping over the disk as the exhaustion increases ; b is the appearance 
on the mica side of the disk ; and c shows the edge of the disk. By close examination 
edge-wise it can be seen that the glow and the metal are not in contact, but are separated 
by a very minute interval. 
At a pressure of 7'5 millims. the glow is thicker, and the black space easily visible. 
At 1*6 millims. the appearance is as shown in Plate 14, fig. 3. The dark space is 
half a millim. in thickness, and the glow is very bright. As the exhaustion proceeds 
the appearance round the negative disk goes through the stages shown in Plate 14, 
figs. 4, 5, 6, and 7, the glow diminishing in intensity and the dark space widening 
out ; the outline still being distinctly shown. Plate 14, fig. 7, shows the appearance 
at a pressure of '078 millim., the thickness of the dark space being 8 millims. 
488. At any particular pressure the dark space separating the glow from the metal 
disk can be slightly altered by varying the power of the contact breaker. When it is 
screwed so as to increase the intensity of the spark the dark space slightly contracts, 
and when the intensity of the spark is diminished the dark space slightly expands, the 
variation in size being about 1 millim. For the same intensity of spark and degree of 
exhaustion the dark space is always uniform in size. 
489. Another bulb was now made of a different construction, to ascertain if the 
size of the dark space varied with the distance separating the poles. Each pole 
consisted of a flat metal disk, one being fixed and the other capable of sliding along a 
glass tube fitting loosely into a wider tube. By tapping the tube, the adjustable pole 
can be brought close to the fixed pole or removed some distance from it, contact between 
the disk and the outer loop being always preserved by wires in the sliding tubes.* 
As the exhaustion proceeded the phenomena observed were similar to those already 
described. It was found that the dimensions and thickness of the dark space did not 
vary, whatever was the distance separating the fixed and sliding pole. 
490. The battery power actuating the coil was also varied between 1, 2, 3, and 4 
* The device of making one of the poles of a vacuum tube extensible so as to alter the distance 
between the two poles has already been adopted by Mr. Spottiswoode. Experiments with such a tube 
are described by Messrs. De La Rue and Muller in the Phil, Trans, for 1878, p. 163. 
