ON the sensitive state of vacuum discharges. 
611 
Now we have already established the fact that the immediate consequence of afford¬ 
ing relief to any portion of the surface of a tube through which a discharge with 
positive air-spark is passing is to cause rapid impulsive discharges of negative elec¬ 
tricity from the inside of the tube, and we have also seen that these discharges 
are accompanied in high vacua by the usual streams of molecules. Hence it is 
natural, in seeking to account for the phenomenon we have described above, to look to 
these negative discharges and their accompaniments for the solution. And that this 
is the proper source is shown by the fact that a similar phenomenon appears with 
negative special where there are also similar impulsive negative discharges, while in 
positive special and negative relief which give rise to positive discharges there is either 
no such phenomenon, or it is manifested on so much more insignificant a scale as to 
point to its being only a secondary effect. 
Considering then that the positive relief and the negative special give effects which 
are as identical in the case of phosphorescent discharges as in that of ordinary dis¬ 
charges, we may fairly consider that we are on safe ground in applying our previously 
obtained results to them; and we therefore conclude that a negative discharge from 
the inside of the tube transversely to its length is the necessary condition for the 
existence of this phenomenon of virtual shadows. We shall now show that it is due 
to a beating down of the streams of molecules coming from the negative terminal 
(which would otherwise impinge on the side of the tube and there cause green light), 
this beating down being caused by the transverse streams of similar molecules coming 
from the inside of the tube. 
In the first place, it is certain that such streams of molecules do interfere with each 
other when their paths cross. The experiments referred to in the last section suffice 
to show this. If two patches of tinfoil giving positive relief be so arranged on the 
tube that their green lines cross, it will be found that they displace each other, and that 
neither of the green patches produced by the pieces of tinfoil is in the position that it 
would be were the other not present. It is true that it is difficult to draw conclusions 
as to the exact nature of this interference from the mode in which they are displaced, 
for both the patches themselves and their displacements are very irregular, but the 
experiment is decisive to show the existence of interference between such streams of 
molecules when they are synchronous and when their paths cross. 
In the instance just given the two sets of molecular streams are both due to the 
relief discharges that come from the side of the tube. We shall now give some in¬ 
stances in which one of the interfering streams is due to the discharge at the negative 
terminal of the tube. 
We have already described relief-phosphorescence, and have shown that it is usually 
situated exactly opposite the place where the relief is given. But if the finger is 
placed in the immediate neighbourhood of the negative terminal, a little in front of it, 
we shall find that the patch of phosphorescence formed by it is no longer immediately 
opposite it, but some distance farther down the tube (Plate 28, fig. 19). In other 
