The Open court . e wavesproceeded. A body which is thrown against an elas-tic wall, furnishes the best demonstration : for examplea ball moving about upon a billiard table. It cannotbe difficult, upon the basis of such a law, to constructa surface whichwill concentrate therays in a commonpoint. This is donefor sound - waves,for example, by theear-trumpet. Yousee here a pair ofmirrors (Fig. 3)with whose help asimilar phenomenamay be we generatesound - waves infront of the centreof the mirror stand-ing at the left, they,agreeably to theparticular construc-tion of the latter,will be
The Open court . e wavesproceeded. A body which is thrown against an elas-tic wall, furnishes the best demonstration : for examplea ball moving about upon a billiard table. It cannotbe difficult, upon the basis of such a law, to constructa surface whichwill concentrate therays in a commonpoint. This is donefor sound - waves,for example, by theear-trumpet. Yousee here a pair ofmirrors (Fig. 3)with whose help asimilar phenomenamay be we generatesound - waves infront of the centreof the mirror stand-ing at the left, they,agreeably to theparticular construc-tion of the latter,will be so reflected as to proceed onward in parallel soon strike the second mirror, 5 metres distant,are there a second time reflected, and finally concen-trate themselves at a single point in front of the cen-tre. The two points that thus correspond to eachother, and whose position is determined by the con-struction of the mirrors, are called the foci. We willperform the experiment for heat-rays by placing a. glowing-hot ball in one of the foci. An easily in-flammable substance, placed in the. focus of the sec-ond mirror, is ignited as you see at once. If I nowreally wanted to convince you that the law of the re-flection for these heat-rays is the same for sound-rays,I should have to bring a sotmd into the one focus andbeg you to step up here singly and be personally con-vmced whether the sound is clearly concentrated inthe other focus. I think you will excuse me from theperformance of this experiment. Light-rays have the same characteristics as heat-rays, only their undulations are somewhat shorter, andtheir vibrations follow one another more quickly: wehave here, to some extent, higher notes in the tone-scale. But to us human beings they offer a muchgreater multiplicity and variety than sound, inasmuchas we are, by a wonderful sense-organ, placed in aposition to distinguish from one another a whole se-ries of these sounds, namely the various colors. The assertion that we
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Keywords: ., bookcentury1800, bookdecade188, booksubjectreligion, bookyear1887
