. The science-history of the universe . each light from the shadows cast, hefound the lamp to be four times as far away as the candle,from which, by the law of inverse squares, he perceivedthat the lamp was twice as bright as the candle. Some fifty years later another light-measuring instru-ment was produced by the famous chemist Robert WilhelmBunsen. This admirably simple device consisted of asheet of white paper with a grease spot on it. The ex- THE SPEED OF LIGHT 77 periment may easily be made by any one. If the paper isequally illuminated from both sides the grease spot willbe hardly visib
. The science-history of the universe . each light from the shadows cast, hefound the lamp to be four times as far away as the candle,from which, by the law of inverse squares, he perceivedthat the lamp was twice as bright as the candle. Some fifty years later another light-measuring instru-ment was produced by the famous chemist Robert WilhelmBunsen. This admirably simple device consisted of asheet of white paper with a grease spot on it. The ex- THE SPEED OF LIGHT 77 periment may easily be made by any one. If the paper isequally illuminated from both sides the grease spot willbe hardly visible, but if the light upon one side is made,ever so little brighter than upon the other, the spot willat once appear on the darker side brighter (and on thebrighter side darker) than the rest of the paper. Theobvious reason of this is that the matt surface of the whitepaper reflects back more and transmits less of the lightwhich falls upon it than does the part covered with a filmof grease. If now a standard light be placed on one side. Fig. 13 —Simple Mode of Measuring Intensity of Light. of this paper, any other light whose candle-power is to bedetermined may be shifted back on the other side until thegrease spot is no longer visible, when by measuring thedistances of the two lights from the paper screen therelative intensity may easily be determined. Incandescent electric lamps, arc lights and in fact allcommon illuminants are measured in candle power. OneBritish standard candle power is the rate at which light isemitted by the flame of a sperm candle weighing 1/a of apound and burning 120 grains per hour. The amount oflight from such a source, however, has been shown tovary as much as 20 per cent., hence the standard is some-what unsatisfactory. Ordinary electric glow lamps areequivalent to 16 standard candles and are therefore called 78 PHYSICS 16 (candle-power) lamps. Other varieties of pho-tometer (light-measurers) have subsequently been in-vented, one of which, Wh
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