ible for great strides in certain directions in
the development and use of artificial light and the era of peace will
inherit these developments and will adapt them to more constructive
purposes.

XV

SIGNALING

From earliest times the beacon-fire has sent forth messages from
hilltops or across inaccessible places. In this country, when the Indian
was monarch of the vast areas of forest and prairie, he spread news
broadcast to roving tribesmen by means of the signal-fire, and he
flashed his code by covering and uncovering it. Castaways, whether in
fiction or in reality, instinctively turn to the beacon-fire as a mode
of attracting a passing ship. On every hand throughout the ages this
simple means of communication has been employed; therefore, it is not
surprising that mankind has applied his ingenuity to the perfection of
signaling by means of light, which has its own peculiar fields and
advantages. Of course, wireless telephony and telegraphy will replace
light-signaling to some extent, but there are many fields in which the
last-named is still supreme. In fact, during the recent war much use was
made of light in this manner and devices were developed despite the many
other available means of signaling. One of the chief advantages of light
as a signal is that it is so easily controlled and directed in a
straight line. Wireless waves, for example, are radiated broadcast to be
intercepted by the enemy.

The beginning of light-signaling is hidden in the obscurity of the past.
Of course, the most primitive light-signals were wood fires, but it is
likely that man early utilized the mirror to reflect the sun's image and
thus laid the foundation of the modern heliograph. The Book of Job,
which is probably one of the oldest writings available, mentions molten
mirrors. The Egyptians in the time of Moses used mirrors of polished
brass. Euclid in the third century before the Christian era is said to
have written a treatise in which he discussed the reflection of light by
concave mirrors. John Peckham, Archbishop of Canterbury in the
thirteenth century, described mirrors of polished steel and of glass
backed with lead. Mirrors of glass coated with an alloy of tin and
mercury were made by the Venetians in the sixteenth century. Huygens in
the seventeenth century studied the laws of refraction and reflection
and devised optical apparatus for various purposes. However, it was not
until the eighteenth century that any no