The most notable advance in the design of arc search-lights was achieved in recent years by Beck, who developed an intensive flame carbon-arc. His chief object was to send a much greater current through the arc than had been done previously without increasing the size of the carbons and the unsteadiness of the arc. In the ordinary arc excessive current causes the carbons to disintegrate rapidly unless they are of large diameter. Beck directed a stream of alcohol vapor at the arc and they were kept from oxidizing. He thus achieved a high current-density and much greater beam intensities. He also used cored carbons containing certain metallic salts which added to the luminous intensity, and by rotation of the positive carbon so that the crater was kept in a constant position, greater steadiness and uniformity were obtained. Tests show that, in addition to its higher luminous efficiency, an arc of this character directs a greater percentage of the light into the effective angle of the mirror. The small source results in a beam of small divergence; in other words, the beam differs from a cylinder by only one or two degrees. If the beam consisted entirely of parallel rays and if there were no loss of light in the atmosphere by scattering or by absorption, the beam intensity would be the same throughout its entire length. However, both divergence and atmospheric losses tend to reduce the intensity of the beam as the distance from the search-light increases. Inasmuch as the intensity of the beam depends upon the actual brightness of the light-source, the brightness of a few modern light-sources are of interest. These are expressed in candles per square inch of projected area; that is, if a small hole in a sheet of metal is placed next to the light-source and the intensity of the light passing through this hole is measured, the brightness of the hole is easily determined in candles per square inch. BRIGHTNESS OF LIGHT-SOURCES IN CANDLES PER SQUARE INCH Kerosene flame 5 to 10 Acetylene 30 to 60 Gas-mantle 30 to 500 Tungsten filament (vacuum) lamp 750 to 1,200 Tungsten filament (gas-filled) lamp 3,500 to 18,000 Magnetite arc 4,000 to 6,000 Carbon arc for search-lights 80,000 to 90,000 Flame arc for search-lights 250,000 to 350,000 Sun (computed mean) about 1,000,000