dye have been used with some success, but such a method reduces the luminous efficiency of the lamp considerably.  The dye fluoresces red under the illumination of ultra-violet, violet, and blue rays; that is, it has the property of converting radiation of these wave-lengths into radiant energy of longer wave-lengths.  By the use of electric incandescent filament lamps in conjunction with mercury-arcs, a fairly satisfactory light is obtained.  Many experiments have been made by adding other substances to the mercury, such as zinc, with the hope that the spectrum of the other substance would compensate the defects in the mercury spectrum.  However no success has been reached in this direction.

By the use of a quartz tube which can withstand a much higher temperature than glass, the current density can be greatly increased.  Thus a small quartz tube of incandescent mercury vapor will emit as much light as a long glass tube.  The quartz mercury-arc produces a light which is almost white, but the actual spectrum is very different from that of white sunlight.  Although some red rays are emitted by the quartz arc, its spectrum is essentially the same as that of the glass-tube arc.  Quartz transmits ultra-violet radiation, which is harmful to the eyes, and inasmuch as the mercury vapor emits such rays, a glass globe should be used to enclose the quartz tube when the lamp is used for ordinary lighting purposes.

It is fortunate that such radically different kinds of light-sources are available, for in the complex activities of the present time all are in demand.  The quartz mercury-arc finds many isolated uses, owing to its wealth of ultra-violet radiation.  It is valuable as a source of ultra-violet for exciting phosphorescence, for examining the transmission of glasses for this radiation, for sterilizing water, for medical purposes, and for photography.

X

THE ELECTRIC INCANDESCENT FILAMENT LAMPS

Prior to 1800 electricity was chiefly a plaything for men of scientific tendencies and it was not until Volta invented the electric pile or battery that certain scientific men gave their entire attention to the study of electricity.  Volta was not merely an inventor, for he was one of the greatest scientists of his period, endowed with an imagination which marked him as a genius in creative work.  By contributing the electric battery, he added the greatest impetus to research in electrical science that it has ever received.  As has already been shown, there began a period of enthusiastic research in the general field of heating effects of electric current.  The electric arc was born in the cradle of this enthusiasm, and in the heating of metals by electricity the future incandescent lamp had its beginning.