228 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
themselves visible to the eye without needing to he demonstrated by the tests we 
have described in the paper. 
The tube in which this was first observed was one of considerable length, but only 
of moderate diameter and moderately good exhaust. The appearance of the discharge 
is shown in Plate 20, fig. 30. The break current alone passed. The luminous column 
starting from the positive terminal was much smaller in diameter than the interior of 
the tube, and consequently left a dark space between it and the sides of the tube. 
The portion in the half of the tube nearer to the negative terminal, on the contrary, 
appeared wholly to fill up the tube. On touching the positive terminal the discharge 
filled the tube throughout; on touching the negative terminal the discharge of smaller 
section extended throughout the whole length of the tube, and when a small condenser 
was used, as described on page 211, the portion of the length occupied by each could 
be varied at will. It should be added that the narrow and broad discharges showed, 
by the nature of their relief-effects, that they were respectively caused by positive and 
negative electricity, and the part of the tube where they were seen to join was found 
in all cases to be the neutral zone. It also should be added that either column could 
be shortened by producing relief-effects near its root as though such a proceeding 
delayed the discharge, just as the small condenser would do. 
The cause of the difference in the two discharges is not difficult to explain. Both 
the discharges get some portion of relief from the inner surface of the glass even when 
the finger is not placed thereon, and this in the case of the positive discharge naturally 
results in the formation of a dark space at the surface of the glass, while in the case 
of the negative discharge it leads to the formation of positive luminosity close up to 
the' surface of the glass. 
A similar difference in appearance in the two discharges has since been observed 
in other tubes, and it is probably not an uncommon phenomenon. It is doubtless 
analogous to what is very frequently seen in the case of a discharge with a positive 
air-spark, in which it is found that the luminous column is very slender and does not 
nearly fill the tube while the air-spark is small. 
C .—On the interference of intermittent discharges. 
In making certain experiments with intermittent discharges, it happened that a 
discharge was sent through two tubes placed in series, but lying parallel to one 
another, and at no great distance apart. It was observed that the discharge in the 
one tube strongly affected that in the other, and from this the authors were led to 
examine more closely the question of the interference' of discharges which do not take 
place in the same tube. 
For this purpose intermittent discharges were sent through two parallel tubes in 
series, the current passing sometimes in the same, and sometimes in contrary 
directions through the two tubes. The two discharges were found to be greatly 
