PRESIDENT’S ADDRESS—SECTION A. 29 
do not influence the velocity in any experiments we _ haye 
been able to make, we consider it likely that the velocity 
of light is the same throughout the whole universe—which, 
be it noted, is only known to us from the light which comes 
to us from the stars. As a consequence, it seems fair to assume 
that whatever the agent which—if we may use such a term—carries 
the energy from Jupiter to the earth, it is the same as the agent 
which carries it throughout space as we know it. That this 
carrlage is not due to the actual motion of some substance we 
may be certain, since we can assure ourselves that the stars are 
visible in all directions at once, and it is unlikely that there 
should be a convergence of anything from all points of space 
towards our very insignificant earth. We will also note that the 
velocity of light through transparent substances at the earth’s 
surface is not the same as a rule as it is through air—while in 
our so-called vacua, the velocity is very nearly the same as it is 
through air. Turning to electric and magnetic action, we note 
that the attractions and -repulsions we observe are not the same 
in degree through all substances, but depend on the nature of 
the substances, and finally in the induction of currents on one 
another, where we have energy transferred from one circuit to 
another—the circuits not being in any conducting connection— 
we have evidence both as to the storing of energy in the space 
surrounding the circuits, and the transferring of it from one 
circuit to another depending on changes taking place in the cur- 
rents themselves, and being independent of the nature of the 
conductors in which the currents exist. And these facts lead us 
to imagine that space is filled with something or other by and 
through which the aforesaid actions take place. There may be 
more than one kind of substance for all we know to the contrary, 
but one at least there certainly must be. We are led to this 
conclusion by reasoning based on the interpretation of our sen- 
sations, and thus come to know of the existence of the ether—or 
medium—as we shall call it, in a manner completely analogous to 
the manner in which we know of the existence of matter. The 
evidence in the latter case is rather more complete—as we have 
an additional confirming sense, that of touch—to appeal to. The 
point which I wish to make however, is that the evidence in both 
cases 1s of the same kind, and open to exactly the same _ philoso- 
phical criticism in one case as in the other. In what follows I 
shall assume the action of a medium in order to account for 
electrical and magnetical effects. This is the first great point in 
Maxwell’s theory. We can reach the point at which we aim 
most quickly by considering the case of a Leyden jar, or of any 
condenser, whose plates we will suppose are separated by a layer 
of dielectric which we will not further particularise. If we 
electrify the plates of the condenser, a point will be reached 
beyond which we cannot go, for a spark will take place between 
