Gilbert light experiments for boys . Fig. 28. The spot is dark by re-fleeted light Fig. 27. The light is one-ninth as strong onB as on A when B is 3 feet from the candle square inches. Place A 1 foot fromthe candle and B 2 feet. Does thelight which passes through 1square inch in A cover 4 squareinches on B? Is it, therefore, onlyone-fourth as intense on B as it ison A? Place B 3 feet from thecandle. Does the light now cover9 square inches? Is it, therefore,only one-ninth as intense? Thisproves that the intensity of lightvaries inversely as the square ofthe distance. Experiment No. 16. A grease
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Gilbert light experiments for boys . Fig. 28. The spot is dark by re-fleeted light Fig. 27. The light is one-ninth as strong onB as on A when B is 3 feet from the candle square inches. Place A 1 foot fromthe candle and B 2 feet. Does thelight which passes through 1square inch in A cover 4 squareinches on B? Is it, therefore, onlyone-fourth as intense on B as it ison A? Place B 3 feet from thecandle. Does the light now cover9 square inches? Is it, therefore, only one-ninth as intense? Thisproves that the intensity of lightvaries inversely as the square ofthe distance. Experiment No. 16. A greasedspot. Rub a small piece of butteron the center of a piece of paperand melt it. Look at the spot by reflectedlight (Fig. 28). Is the spot dark?Look at it by transmitted light 22 GILBERT BOY ENGINEERING. (Fig.29). Is it bright? It is darkerthan the paper in the first case andbrighter in the second, because morelight goes through the greased spotthan through the paper. Experiment No. 17. Four can-dles against one. Put four candles2 feet from the greased-spot screenand one candle 1 foot on the otherside (Fig. 30). Trim the wicksuntil the flames are of equal size. Is the greased spot as brightas the paper? It is, because thelight which goes through from oneside is exactly equal to that whichgoes through from the other, thatis, the one candle throws as muchlight on the screen as do the fourcandles. This is explained by the Fig. 29. The spot is bright by trans-mitted light law of intensity men-tioned above. Each ofthe four candles is 2 feetfrom the screen andtherefore throws justone-fourth as much lighton the screen as onecandle does at 1 foot. Experiment No. 18.Candle power of alamp. The candlepower of a lamp is the