568 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
only a very minute period of time, is yet, in all probability, many times as long’ as the 
time occupied by the impulse itself. This is precisely the action which takes place 
within a tube with positive intermittence when the positive special is being produced ; 
the sole difference being that in the present case there are no ecpiivalent discharges of 
positive electricity synchronously advancing along the tube towards the tinfoil which 
get satisfied by the negative electricity left behind at the tinfoil, while the positive 
discharges that are produced there pass out at the negative terminal of the tube. 
What, then, is experimentally found to take place in the present case ? It is found 
that if the distance between the terminals, or as it might be termed the air-spark 
of the interfering system, be properly adjusted, we have exactly the same visible 
appearances as we have in the case of the most perfect form of the positive relief. 
There is the same complete separation of the positive column, the same sharp bright 
termination of the truncated portion of it, the same hollow cone of positive discharge 
separated from the truncated portion of the positive column by the familiar dark 
space. Nor is it only when the adjustment is very precise that the resemblance 
exists. With other lengths of air-spark we have other forms of these positive effects, 
all equally characteristic, although not so readily recognisable as the typical form 
above described. Just as in producing the positive special a proper adjustment of 
the length of the air-spark was necessary to produce the typical form, and any excess 
or defect from this exact length of spark caused the appearances to deviate to a 
greater or less degree from the typical form, so it is in the present case. And there 
is so striking a resemblance between the other forms in the two cases that we may 
express it by saying that they are substantially the same, differing only in the pro¬ 
minence which they respectively give to certain kinds of variance in the details. We 
thus see that the presence of the synchronous discharges proceeding from the positive 
terminal is not essential to the formation of these typical forms which we have 
hitherto associated only with the positive special, and its correlative phenomenon the 
negative relief. 
The explanation of this result is very simple. The electricity of the interfering 
system rushing in accumulated charges to the tinfoil, drives off equal or at all events 
comparable charges into the tube. These fly to the negative terminal as the goal 
most suitable for them, thus, as it were, anticipating a portion of the continuous 
current that would have passed along that portion of the tube in the succeeding 
interval. This portion of that current is arrested and satisfied by the negative that 
is left behind on the inner surface of the tube under the tinfoil; and, inasmuch as this 
negative proceeds from a fixed region (which therefore acts for the moment as a 
negative terminal or the negative end of a stria), we have the usual phenomenon of 
a fixed head of luminosity (or, as we may now term it, stria-head), sharp and bright in 
outline, indicating its reception, such head being separated from the region from which 
the negative electricity comes by the accustomed dark space.* 
* See Phil. Trans., 1879, Plate 1G, fig. 10. 
