572 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
Before leaving this part of the subject we must mention that this arrangement 
enables us to obtain striking examples of artificial striae. The mode of doing so, viz. : 
by connecting two rings of tinfoil upon the affected tube to the negative terminal of 
the machine in the interfering system, will be obvious, after the description given in 
our former paper of the mode of obtaining artificial striae by means of the positive- 
special effect. But in some instances it has not been found necessary to employ two 
rings. So perfectly does the inner surface of the tube beneath the tinfoil and the 
interior of the hollow luminous cone perform the functions of a negative terminal, that 
the portion of the discharge between the positive terminal of the tube and the tinfoil 
lias been found to be organised in every way like an ordinary discharge. When the 
adjustments were suitably arranged, there might be seen standing at the proper 
distance from the tinfoil a perfect stria of a flat disc-like shape (Plate 25, fig. 3). 
This stria was the representative of the negative glow, but it took the ordinary shape 
of a stria because it was not constrained to assume a distorted form by the presence 
of any rigid metallic negative terminal, but had in place of it a system almost, if not 
precisely, what any non-terminal segment of a striated discharge in such a tube would 
naturally have, viz.: a hollow gaseous structure specially framed to receive the positive 
discharge which came from the bright head of the stria next to it. Behind this stria 
came a long dark space, the representative of the ordinary negative dark space; and 
behind this agnin the positive luminous column starting from the positive terminal.* 
It will of course be understood that such perfect effects as these are not common; in 
order to produce them it is necessary that the magnitudes of the interfering and 
affected systems should have due relation to one another as well as that the air-spark 
in the former should be of proper length, t 
tinfoil and the positive terminal. The usual indication of negative intermittence will then appear in the 
form of luminosity proceeding from the inner surface of the tube beneath the finger. 
* The significance of this last result as bearing upon the general theory of the striated discharge given 
in our former paper is very great. In the first place we find the bright termination of the truncated 
positive column (to which our theory assigned the functions of a stria) actually taking the form of a 
single perfectly formed stria. In the next place we see that this stria, having just the same advantages 
that a negative glow has in respect of fixity of conditions, and probably also in respect of the readiness 
with which negative electricity is supplied to it, is like a negative glow, followed by a long negative dark 
space. It is in fact in much the same position, electrically and in all other respects, as the negative glow 
of a tube where the negative terminal is a concave metal plate occupying the whole of the tube. This 
strongly confirms the view that the negative glow and the negative dark space form a physical unit of 
discharge (following the lang-uage of our former paper) identical in nature and function with the unit 
composed of any stria and its dark space, but modified by local circumstances; since we find that when 
we produce a stria under the same circumstances under which a negative glow is formed it is attended by 
a similar long dark space. 
t In connexion with what has been above stated about the proportion between the strength of the 
main and the interfering discharges, and also with the experiments described in Section IX. of our 
former paper (p. 201, and Plate 18, fig. 16), the following fact deserves mention. In some cases where 
two rungs of tinfoil were used, and two strife corresponding to them were formed by connecting them with 
