618 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
it represents the locus of the exciting cause of this local action. We, therefore, have 
direct indications of this being a locus where the positive electricity (which alone can 
be the exciting cause of these negative discharges) obtains relief from the negative 
given off in the tube. 
It will here be the place to mention an observation of a very remarkable kind which 
was made upon the shadow thrown by a crooked wire within the tube upon this 
attendant phosphorescence. The portion casting the shadow w r as nearly straight and 
had a direction lying in a normal section of the tube somewhat inclined to the radius 
of that section on which its foot lay. As the narrow green line was driven round the 
tube towards the ware the shadow of the wire appeared upon it, but not crossing it. 
When it was driven still nearer to the foot of the wire the shadow, instead of 
remaining fixed, and thus more and more completely crossing the green line, was 
found to shrink in and grow shorter, and continued only partially to cross the green 
line, although the latter had moved so much that it would have been completely 
divided had the shadow retained its original length. When, however, the green 
line was driven still closer up to the foot of the wire the shadow ceased to shrink 
below a certain length, and finally passed quite across the green line. The solution 
of the peculiarities here exhibited will be found, we believe, in the hypothesis that 
the streams of molecules come off from a considerable segment'" of the surface of the 
tube situated in the region opposite to the green line, and they by their mutual 
action, or by the direction in which they originally start, stream towards the thin 
column of positive luminosity without their paths crossing at the axis of the tube or at 
any other part of their path. 
We have said that this thin luminous column with its attendant phosphorescence is 
sensitive in the highest degree, and is strongly repelled when the hand is brought 
anywhere in the neighbourhood of the tube. It might be thought, therefore, that it 
is nothing but a special form of relief phosphorescence, and that it differs from what 
we have treated of in a previous section only in that it is the portion first formed and 
not needing actual contact for its formation. But this is by no means the case. The 
two are wholly different in genesis and in properties. Nothing is easier than to dis¬ 
tinguish them. Belief phosphorescence is quite fixed in position unless another centre 
of production is formed by touching the tube in its immediate neighbourhood, in which 
case the two systems of streams of course interfere the one with the other. But even 
wdien contact is made with the tube it is quite easy to drive this attendant phosphor¬ 
escence about by bringing other conducting systems into the neighbourhood of the 
tube, although they do not produce the slightest effect upon the relief phosphor- 
* This is confirmed by the fact that it is impossible to cast a shadow upon this attendant phospho¬ 
rescence unless the object casting it be very near to the side of the tube where the phosphorescence 
appears. And even then it has a tendency to be blurred and hazy. This indicates that the molecular 
streams that cause it do not proceed from any one spot, but that they converge upon it from a consider¬ 
able region, all their directions being, however, approximately situated in a normal section of the tube. 
