200 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
stripe tend to assume when they deviate from the conical type, it may be fairly stated 
that all the forms of natural striae can be artificially reproduced : a fact which is 
strongly in favour of the identity of the two phenomena. 
But it is not only in structure that these luminous shells resemble strise. There is 
also an identity of function. We know that when the positive pulses arrive at the 
glass they drive off similar positive pulses from the interior of the tube, and thus form 
the luminous shells. And knowing, therefore, that each luminous shell signifies a 
positive discharge, and also that no electricity passes through the glass, it is absolutely 
certain that a like amount of negative electricity must be collected at the surface of 
the glass within the tube, and must ultimately satisfy an equal amount of the original 
positive discharge— i.e., of that which comes to it from the positive terminal, or from 
the shell immediately on the positive side of the one we are considering, as the case 
may be. We have then a negative discharge from the side of the tube, or from the gas 
immediately within it, satisfying a positive discharge advancing towards it along the 
tube; and we find that it causes the luminosity of this discharge to stop short and 
terminate in a bright, clearly-defined rounded head, which is separated by a dark 
space from the seat of the negative discharge. This, then, is the function of the shell: 
the bright part is to serve as the place of departure of the positive electricity that is 
about to pass across the dark space (or the place of arrival of the negative electricity 
after so doing), and the hazy interior of the cone is to form the place of its arrival (or 
the place of departure of the negative electricity); and, so far as we know, this is the 
sole function of these elements of the shell. Now let us take the case of the striated 
discharge. Here, also, we know that the positive electricity in the current must leave 
the bright head of the stria, and, after passing the dark space, arrive at the hazy 
inside of the next, and the negative electricity must take a reverse course. There is 
an absolute identity in the functions of the corresponding parts of the two structures, 
the only difference being that we know, from independent extrinsic evidence, that the 
electricity in the artificially segmented discharge does not flow continuously, but in 
intermittent discharges. This independent testimony is absent in the case of the 
ordinary striated discharge. We shall refer to this point again at the conclusion of 
this paper, but at present we shall assume what we have already stated to be so 
highly probable, viz.: that all vacuum discharges are in reality intermittent. Any who 
do not wish to admit this must take the reasonings of this section as applicable only 
to those striated discharges which are known to be intermittent. 
Returning, then, to the case of the artificially produced conical shells, the modus 
operandi of the discharge is as follows :—When the pulse of positive electricity arrives 
at the terminal and causes the discharge into the tube, a positive discharge equal * to 
* It may seem an unwarranted assumption to assume that each of these artificially produced discharges 
is equal to the whole original discharge, but the appearances (with suitable adjustments) seem to warrant 
it, and as the reasoning is simplified, and the validity of the theory is not affected by this assumption, we 
shall, through the rest of this section, suppose such to be the case. 
