Photography has been used with great success in studying the aurora, especially by the Norwegian physicists Störmer, Vegard, and Krogness. Simultaneous photographs of a single detail are taken from two points several miles apart against a background of stars. The apparent position of the auroral detail among the stars will differ in the two pictures, and a comparison of them makes it possible to determine the actual position of the aurora in space. A slow-moving cinematograph has also been used to obtain series of pictures. The measurements of these observers show that the base of the aurora is, generally between 60 and 70 miles above the earth with a strong tendency to assume a definite location at an altitude of about 61 or 67 miles. Its upper limits are not well defined, but it has been photographed up to an altitude of more than 300 miles. Earlier observers reported seeing the aurora at altitudes of only a few miles, and even down to the earth’s surface, but recent authorities are inclined to discredit these observations. One more phenomenon of atmospheric electricity requires brief mention, viz., the electric waves that produce the erratic disturbances known to wireless telegraph operators as “strays” or “static.” As heard in the receiver of a wireless outfit the noise of strays has been described as “like hailstones beating against a sheet of tin,” or “short hisses from a steam pipe,” or “periodic discharges of coal down a chute.” Another characteristic sound is a sharp “click.” The study of strays has been carried out on a world-wide scale by a committee of the British Association for the Advancement of Science, but their nature is not yet fully understood. Some strays are undoubtedly due to near or distant discharges of lightning, and special forms of wireless apparatus, known as “thunderstorm recorders” or “ceraunographs,” have been used to give notice of the approach of thunderstorms. On the other hand, strays seem frequently to have no connection with thunderstorms, and their principal origin is now sought in electrical disturbances in the upper atmosphere, perhaps similar to those which cause the aurora, and, as in the case of the aurora, having their ultimate source in the sun. CHAPTER X ATMOSPHERIC OPTICS