Somewhat analogous to this application of luminous paint is the use of blue light at night on battle-ships and other vessels in action or near the enemy.  Several years ago a Brazilian battle-ship built in this country was equipped with a dual lighting-system.  The extra one used deep-blue light, which is very effective for eyes adapted to darkness or to very low intensities of illumination and is a short-range light.  Owing to the low luminous intensity of the blue lights they do not carry far; and furthermore, it is well established that blue light does not penetrate as far through ordinary atmosphere as lights of other colors of the same intensity.

The war has been responsible 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