320
XX
LIGHT AND ELECTRICITY
electric or magnetic force capable of being propagated after
the manner of light waves. This pillar again would directly
support the electrical side, and indirectly the optical side.
In order to complete the structure symmetrically, both pillars
would have to be built; but it would suffice to begin with
one of them. With the former we have not as yet been able
to make a start; but fortunately, after a protracted search, a
safe point of support for the latter has been found. A suffi-
ciently extensive foundation has been laid down: a part of
the pillar has already been built up; with the help of many
willing hands it will soon reach the height of the arch, and
so enable this to bear the weight of the further structure
which is to be erected upon it. At this stage I was so
fortunate as to be able to take part in the work. To this I
owe the honour of speaking to you to-day; and you will
therefore pardon me if I now try to direct your attention
solely to this part of the structure. Lack of time compels
me, against my will, to pass by the researches made by many
other investigators; so that I am not able to show you in
how many ways the path was prepared for my experiments,
and how near several investigators came to performing these
experiments themselves.
Was it then so difficult to prove that electric and magnetic
forces need time for their propagation ? Would it not have
been easy to charge a Leyden jar and to observe directly
whether the corresponding disturbance in a distant electro-
scope took place somewhat later? Would it not have sufficed
to watch the behaviour of a magnetic needle while some
one at a distance suddenly excited an electromagnet?
As a
matter of fact these and similar experiments had already been
performed without indicating that any interval of time elapsed
between the cause and the effect. To an adherent of Max-
well's theory this is simply a necessary result of the enormous
velocity of propagation. We can only perceive the effect of
charging a Leyden jar or exciting a magnet at moderate dis-
tances, say up to ten metres. To traverse such a distance,
light, and therefore according to the theory electric force like-
wise, takes only the thirty-millionth part of a second. Such
a small fraction of time we cannot directly measure or even
perceive. It is still more unfortunate that there are no