Review of reviews and world's work . helaws of light, too, are well understood andare comparatively simple. The source of light (except that of theglow-worm and the fire-fly) is a substancewhich is raised to such a temperature that itsets up waves in the surrounding ether,which, when falling upon the eye, producethe sensation we know as light. It isacknowledged that the source of light in thesun is a great mass of white-hot matter. Theatmosphere enables us to see sunlight, andbeyond the earths atmosphere sunlightis said to be invisible to human eyes. Thesource of light in an arc lamp is the he
Review of reviews and world's work . helaws of light, too, are well understood andare comparatively simple. The source of light (except that of theglow-worm and the fire-fly) is a substancewhich is raised to such a temperature that itsets up waves in the surrounding ether,which, when falling upon the eye, producethe sensation we know as light. It isacknowledged that the source of light in thesun is a great mass of white-hot matter. Theatmosphere enables us to see sunlight, andbeyond the earths atmosphere sunlightis said to be invisible to human eyes. Thesource of light in an arc lamp is the heatedparticles of carbon floating between thewhite-hot tips of the electrodes, which to a high temperature by an incandescent lamp the light source isa thin filament maintained at a high tempera-ture inside the glass globe by the passage of acurrent of electricity. In gas and oil lampsthe light is thrown ofif by the myriad par-ticles of carbon heated to incandescence in 834 THE AMERICAN REl^fElV OF REI^ BOSTON. THE BEST LIGHTED CITY.(Latest types of arc lamps on Commonwealth Avenue.) the flame. In the new gas lamps it is thewhite-hot mantle which produces the light. The human eye can withstand ordinarilj,without fatigue, a brilliancy of about fivecandlepower per square inch of intensity of light sources ranges all theway from the two or three candlepower persquare inch in the ordinary candle flame to600,000 candlepower of the sun when atzenith. This means that a square inch ofcandle flame gives off from three to fourcandlepower, while every square inch of thesuns surface gives 600,000 intensity of the arc light ranks next tothat of actual sunlight, being about 10,000candlepower per square inch. The newmetal filament lamps give about looo candle-power, which means that if we had a ballof tungsten as big as the sun, heated by elec-tricity, it would throw off a thousand candle-power of light for every square inch of itssur
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