202 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
there is a positive discharge, which gets satisfaction from a source of negative 
electricity, or, as we may term it, a negative terminal, situated within the tube at the 
surface of the glass, and we find precisely the same formation present itself. The 
luminous positive column is separated from the negative terminal by a dark space, and 
its head becomes bright, rounded, and definite in outline, and assumes a,11 the charac¬ 
teristics of the bright head of a stria. And the experiment may be made still more 
striking and instructive by placing a second finger a little on the negative side of the 
first. The luminosity which previously commenced close to the first finger and 
stretched away towards the negative end of the tube, becomes divided exactly as hr 
the previous case, and the portion intercepted between the fingers resembles in all 
respects a stria, except that the hazy blue negative surface is imperfectly developed, 
being really replaced by the haze under the first finger, which is the place at which 
the interchange of discharges takes place. The length of the body of this stria can 
be varied at will by increasing or decreasing the distance between the fingers so as to 
leave a more or less prolonged positive column behind the bright head, but the real 
structure is in nowise altered thereby. 
The unit of a striated vacuum discharge is therefore composed of the body of a stria 
terminating in its bright surface, the dark space in front of it, and the hazy interior 
surface of the stria on the further side of that dark space. In this unit* we have a 
positive terminal, with a positive luminous column starting from it, a space across 
which the discharge passes non-luminously, and a negative terminal; so that in each 
unit we have represented all the elements of a complete discharge. And in the 
opinion of the authors of this paper, all striated vacuum discharges are composed of 
reduplications of this unit, and any phenomena connected with the negative terminal 
which seem to contradict this view, and to point to a special structure of the discharge 
near the negative terminal unlike anything that exists in other portions of the 
discharge, are merely modifications due to the local circumstances of the terminal in a 
manner now to be explained. We allude, of course, to the phenomena known as the 
negative dark space, the negative glow, and Crookes’ spa.ce; and we now proceed to 
examine these phenomena, taking first the negative glow, as the one presenting the 
greatest difficulty on the foregoing theory. 
It is well known that the bright surfaces of strise are usually convex. But this is 
not always the case. If a striated discharge be produced in a tube which has a bulb 
in the middle of its length, of diameter considerably greater than that of the tube, 
it will be found that the strise in the bulb towards the side nearest the negative are 
concave on their bright surface, especially in that part of the bulb that rapidly narrows 
* It must be borne in mind.tbat the analogue of the positive terminal in this unit is a surface within 
the body of the striae, at which the separation of the electricities takes place; and that the body of the 
striae is to be regarded as a short positive column terminating in the bright luminous head or conical 
surface of the striae. In artificially produced striae this column can, as we have seen, be made of any 
length; and even in the natural striae its length varies within very wide limits. 
