For many years experiments have been made upon the use of the invisible rays which accompany visible rays. The practicability of signaling with invisible rays depends upon producing them efficiently in sufficient quantity and upon separating them from the visible rays which accompany them. Some successful results were obtained with a 6-volt electric lamp possessing a coiled filament at the focus of a lens three inches in diameter and twelve inches in focal length. This gave a very narrow beam visible only in the neighborhood of the observation post to which the signals were directed. The beam was directed by telescopic sights. During the day a deep red filter was placed over the lamp and the light was invisible to an observer unless he was equipped with a similar red screen to eliminate the daylight. It is said that signals were distinguished at a distance of six miles. By night a screen was used which transmitted only the ultraviolet rays, and the observer's telescope was provided with a fluorescent screen in its focal plane. The ultraviolet rays falling upon this screen were transformed into visible rays by the phenomenon of fluorescence. The range of this device was about six miles. For naval convoys lamps are required to radiate toward all points of the compass. For this purpose a quartz mercury-arc which is rich in ultraviolet rays was surrounded with a chimney which transmitted the ultraviolet rays efficiently and absorbed all visible rays excepting violet light. The lamp appeared a deep violet color at close range, but the faintly visible light which it transmitted was not seen at a distance. A distant observer picks up the invisible ultraviolet "light" by means of a special optical device having a fluorescent screen of barium-platino-cyanide. This device had a range of about four miles.