220 W. Crookes—Lines of Molecular Pressure. 
When aluminium cups are used for the vanes instead of 
disks backed with mica, similar appearances are seen. The 
velvety violet halo forms over each side of the cup. On 
increasing the exhaustion the dark space widens out, retaining 
almost exactly the shape of the cup. The bright margin of 
the dark space becomes concentrated at the concave side of the 
cup to a luminous focus, and widens out at the convex side. 
On further exhaustion, the dark space on the convex side 
touches the glass, when positive rotation commences, becoming 
very rapid as the dark space further increases in size and ulti- 
mately flattens against the glass. 
Convergence of Molecular Rays to a Focus.—The subject next 
investigated is the convergence of the lines of force to a 
ocus, as observed with the aluminium cup. As this could not 
This greenish-yellow phosphorescence, characteristic of high 
j . It must 
be remembered, however, that the particular color is due to 
the special kind of soft German glass used. Other kinds of 
es 
k 
place only under the influence of the negative pole. At an 
a difficulty in passing, and the green light appears occasionally 
in flashes only. At 0° 
ductive; and a spark can be forced through only by increasing 
* M signifies the millionths of an atmosphere. 
