616 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
of form. As it approaches the high state of exhaustion which is usually associated 
with phosphorescence, the positive column will be found to diminish in diameter until 
it is reduced to a small pencil-like column very sensitive to the approach of a conductor, 
which, on account of the difficulty of keeping the tube free from all disturbing influences, 
usually stretches along the glass on one side of the tube instead of taking the normal 
centrical position. Its extreme sensitiveness often causes it to be crooked, and it may 
sometimes be seen crossing the tube from side to side in a zigzag manner. 
This luminosity can be driven against the opposite side of the tube by the approach 
of the hand or any other conductor to the tube. Contact with the tube is so far from 
being necessary, that the violent action which it causes entirely masks the phenomena 
of which we are about to speak. The best effects are got when the hand is from three 
to six inches from the tube. When this pencil-like column is thus in contact with the 
tube it will be found that a long streak of the well-known phosphorescence marks its 
line of contact, and if the hand be brought a little nearer, so as to force the column, as 
it were, with more violence against the side, this green streak becomes very bright and 
vivid (Plate 29, fig. 26). It is extremely mobile, for it moves with the positive column, 
and is beyond all doubt attendant upon it. We shall hereafter see that they can be 
made to separate slightly by special precautions being taken, but this only serves to 
prove more clearly how closely they are attendant the one on the other, for it is with 
difficulty that the separation takes place, and as soon as the disturbing influence is 
removed they come together again. 
But it is not only in this way that the phosphorescence attendant upon the positive 
luminosity becomes visible. If from any cause the positive column, taking a zigzag 
course, impinges, so to speak, on the side of the tube, a green patch of phosphorescence 
appears. And when the finger is placed upon the tube so as to form a patch of what 
we have termed relief-phosphorescence, although this seems to blot out the feebly 
luminous positive column, yet its presence is shown by its attendant phosphorescence. 
If some other conductor be brought near the tube, not on the same side as the finger, 
it will be found that the true relief-phosphorescence remains unmoved, but that there 
is an extremely mobile portion of the phosphorescent image of the finger which by 
its sensitiveness shows that it has a different origin and nature to the immovable 
relief-phosphorescence. If this portion be traced out, as can be easily done since its 
mobility enables it to be readily distinguished, it will be found to be in the line of a 
prolongation of the positive column, and thus it is evident that it is the attendant 
phosphorescence of which we have been speaking. 
Before we proceed to examine in detail this most peculiar phenomenon it will be 
well to mention that one of the most convenient modes of obtaining it is by means 
of the positive unipolar or double unipolar discharge. Using this arrangement, the 
thin pencil-like form of the positive column appears in tubes whose exhaust is not 
sufficiently perfect, or in which other circumstances are not sufficiently favourable to 
give it when a current passes through the tube. And just as in less perfectly 
