612 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
words, the streams of molecules that were crossing the tube to form the relief- 
phosphorescence have been swept down the tube by the streams that were proceeding 
from the negative terminal.* 
A much more striking form of what is substantially the same phenomenon was 
observed in a tube the negative terminal of which consisted of a straight wire fix ed 
at right angles to the axis of the tube and passing through it to a point about 
half-way between the axis of the tube and the opposite side (Plate 28, fig. 20). The 
discharge passing through the tube had a positive air-spark of considerable size. On 
placing the finger upon the tube the usual relief-phosphorescence appeared. But 
when the finger was placed upon the tube at the spot where the wire forming the 
negative terminal of the tube would have, if produced, cut the surface of the tube, it 
was found that the relief-phosphorescence took the form of an annulus round the root 
of the negative terminal. The inner boundary of this annulus was well defined and 
formed approximately a circle round the root of the negative terminal as centre, but 
the external boundary was of course irregular. This showed beyond a doubt that the 
streams of molecules from the sides of the negative terminal had caused the streams 
from the interior of the glass beneath the finger to deviate from their course, and, 
instead of passing along parallel to the negative terminal, to be inclined at an angle 
to it, and thus to form the annular patch already described. And the truth of this 
conclusion was made still more evident when the finger was placed on the side of the 
tube so as to be at the point on the normal section through the negative terminal at 
the greatest distance from that terminal. The relief-phosphorescence then appeared 
to be cut in two by a broad and comparatively black space with roughly parallel 
sides, showing that the molecular streams from the sides of the negative terminal had 
diverted the streams that were going to form the relief-phosphorescence.t 
We will now describe certain experiments which although they closely resemble the 
case which we have just mentioned have got an individual value from the remarkable 
way in which they support the whole theory of the intermittent discharge as put 
* A splendid example of the interference of molpcular streams is obtained by the same means when the 
negative terminal is in the very usual and convenient form of a hollow cone. The molecular streams that 
proceed from it first strike the sides of the tube at a little distance from the negative terminal, thus 
leaving a zone quite destitute of phosphorescence. If the finger be placed upon this zone (the air-spark 
being of suitable length) the whole phosphorescence pn the tube is affected. The molecular streams 
from the finger coming across the cone of molecular streams from the negative terminal cause them to 
deviate en masse from their previous course, and thus throw the phosphorescence upon the other side of 
the tube. The effect is generally very striking - , and this property of the dark zone near the negative 
terminal in such tubes (and also in a lesser degree in tubes that have their negative terminal in the form 
of a disc) has led ns to give to it the name of the sensitive zone. 
t It is instructive to compare this with the behaviour of the positive pole under similar circumstances. 
If relief-phosphorescence be thrown across the positive terminal its shadow is as fine and sharp as though 
it were a non-conductor. The reason is obvious. If it has at the moment any electrical function at all 
it is that of receiving and not of giving forth negative electricity. Hence there are no molecular 
streams proceeding from its surface which could cause those that pass near it to deviate from their course. 
