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LIGHT AND ELECTRICITY
the electrical equations. This was the bond which Maxwell
set himself to strengthen. He developed the electrical equa-
tions to such an extent that they embraced all the known
phenomena, and in addition to these a class of phenomena
hitherto unknown-electric waves. These waves would be
transversal waves, which might have any wave-length, but
would always be propagated in the ether with the same
velocity that of light. And now Maxwell was able to point
out that waves having just these geometrical properties do
actually occur in nature, although we are accustomed to denote
them, not as electrical phenomena, but by the special name
of light. If Maxwell's electrical theory was regarded as false,
there was no reason for accepting his views as to the nature
of light. And if light waves were held to be purely elastic
waves, his electrical theory lost its whole significance.
if one approached the structure without any prejudices arising
from the views commonly held, one saw that its parts sup-
ported each other like the stones of an arch stretching across
an abyss of the unknown, and connecting two tracts of the
known. On account of the difficulty of the theory the number
of its disciples at first was necessarily small. But every one
who studied it thoroughly became an adherent, and forth-
with sought diligently to test its original assumptions and its
ultimate conclusions. Naturally the test of experiment could
for a long time be applied only to separate statements, to the
outworks of the theory. I have just compared Maxwell's
theory to an arch stretching across an abyss of unknown
things. If I may carry on the analogy further, I would say
that for a long time the only additional support that was
given to this arch was by way of strengthening its two abut-
ments. The arch was thus enabled to carry its own weight
safely; but still its span was so great that we could not
venture to build up further upon it as upon a secure founda-
tion. For this purpose it was necessary to have special pillars
built up from the solid ground, and serving to support the
centre of the arch. One such pillar would consist in proving
that electrical or magnetic effects can be directly produced
by light. This pillar would support the optical side of the
structure directly and the electrical side indirectly. Another
pillar would consist in proving the existence of waves of