But we can extend our investigations in the other direction; we find
that the ether carries many waves longer than light-waves. Special
photographic emulsions can reveal the existence of waves five times
longer than violet-light waves. Extending below the limits of visibility
are waves we detect as heat-waves. Radiant heat, like the heat from a
fire, is also a form of wave-motion in the ether, but the waves our
senses recognise as heat are longer than light-waves. There are longer
waves still, but our senses do not recognise them. But we can detect
them by our instruments. These are the waves used in wireless
telegraphy, and their length may be, in some cases, measured in miles.
These waves are the so-called electro-magnetic waves. Light, radiant
heat, and electro-magnetic waves are all of the same nature; they differ
only as regards their wave-lengths.