XX
325
LIGHT AND ELECTRICITY
But in other respects it is really a beam of light. By rotat-
ing the mirror we can send it in various directions, and by
examining the path which it follows we can prove that it
travels in a straight line. If we place a conducting body in its
path, we find that the beam does not pass through-it throws
shadows. In doing this we do not extinguish the beam but
only throw it back: we can follow the reflected beam and
convince ourselves that the laws of its reflection are the same
as those of the reflection of light. We can also refract the
beam in the same way as light. In order to refract a beam of
light we send it through a prism, and it then suffers a deviation
from its straight path. In the present case we proceed in the
same way and obtain the same result; excepting that the dimen-
sions of the waves and of the beam make it necessary for us
to use a very large prism. For this reason we make our
prism of a cheap material, such as pitch or asphalt. Lastly,
we can with our beam observe those phenomena which hitherto
have never been observed excepting with beams of light-the
phenomena of polarisation. By interposing a suitable wire
grating in the path of the beam we can extinguish or excite
the sparks in our resonator in accordance with just the same
laws as those which govern the brightening or darkening of
the field of view in a polarising apparatus when we interpose
a crystalline plate.
Thus far the experiments. In carrying them out we are
decidedly working in the region of optics. In planning the
experiments, in describing them, we no longer think electric-
ally, but optically. We no longer see currents flowing in the
conductors and electricities accumulating upon them: we only
see the waves in the air, see how they intersect and die out
and unite together, how they strengthen and weaken each
other. Starting with purely electrical phenomena we have
gone on step by step until we find ourselves in the region of
purely optical phenomena. We have crossed the summit of
the pass our path is downwards and soon begins to get level
again. The connection between light and electricity, of which
there were hints and suspicions and even predictions in the
theory, is now established: it is accessible to the senses and
intelligible to the understanding. Froin the highest point to
which we have climbed, from the very summit of the pass, we