. A text book of physics, for the use of students of science and engineering . p, but is very much less along the axis. J M 546 LIGHT CHAP. If the illuminating power of a lamp be P, this is the intensity ofillumination at unit distance, so that, by the inverse square law, theintensity of illumination I at distance d, is P/d2, or Photometers.—In order to compare the illuminating powers oftwo sources, the principle always made use of is that of adjustingtheir distances from a screen until they both produce the sameintensity of illumination at the screen. Thus if Pl and P2 are theilluminat
. A text book of physics, for the use of students of science and engineering . p, but is very much less along the axis. J M 546 LIGHT CHAP. If the illuminating power of a lamp be P, this is the intensity ofillumination at unit distance, so that, by the inverse square law, theintensity of illumination I at distance d, is P/d2, or Photometers.—In order to compare the illuminating powers oftwo sources, the principle always made use of is that of adjustingtheir distances from a screen until they both produce the sameintensity of illumination at the screen. Thus if Pl and P2 are theilluminating powers of the sources, and f/x and d2 their distancesLfromthe screen when they produce equal illuminations, then c^ a? or dld/ There are many devices for carrying out the required adjustment,but they nearly all depend upon illuminating one part of the screen byone source, and another part by the other source. The eye can thendetect any inequality in the two illuminations, and the adjustment to]equality can be made by varying the distance of one or other of FIG. 494.—Shadow photometer. Rumfords shadow photometer.—Two sources, such as a lamp anda candle, are placed upon an optical bench, as shown in Fig. anda vertical rod A is placed in front of a white screen, so that two sharpshadows are cast, one of which is illuminated only by the lamp andthe other by the candle. The positions of the lamp and candleare adjusted until the shadows are equally intense, and the distancesBC and BD are then measured. From the relation P(lamp) _BD2P(candle) BC8the ratio of the illuminating powers can be found. XLII PHOTOMETERS 547 Expt. 115.—Shadow photometer. Place a candle and a lamp in the posi-tions shown in Fig. 494. Keep one of them fixed and move the other aboutuntil the shadows are equally intense, making six trials and taking themean position. Repeat at four different distances ; in each case, calculatethe illuminating power of the lamp in terms of that of the
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