The science of light . -sions and rarefactions in the airor any other medium throughwhich the sound is the compressions and rarefactions follow one an-other perfectly, regularly a sustained musical note isproduced. If the sound is a very short sharp one, ablow on wood with a hammer, the number of com-pressions and rarefactions is extremely small, and insome cases only a single compression may be produced a single compression by means of thecrack of an electric spark. Any compressed or rarefiedportion of air surrounded by undisturbed air will cast afairly defi
The science of light . -sions and rarefactions in the airor any other medium throughwhich the sound is the compressions and rarefactions follow one an-other perfectly, regularly a sustained musical note isproduced. If the sound is a very short sharp one, ablow on wood with a hammer, the number of com-pressions and rarefactions is extremely small, and insome cases only a single compression may be produced a single compression by means of thecrack of an electric spark. Any compressed or rarefiedportion of air surrounded by undisturbed air will cast afairly definite shadow of itself on a screen placed closebehind it. For instance, if the shadow of a hot pokerbe received on a screen, the shadow of the hot air stream-ing upwards from the poker is seen quite hot air is less dense than the surrounding air, andconsequently casts a shadow. If the poker be verycold instead of very hot, if it has been placed in iceor snow before being brought into a warm room, the. Fig. 2. PROPAGATION OF LIGHT 15 shadow of the colder denser air can be seen streamingoff downwards from the poker. Wood thereforearranged another spark so that it cast a shadow of thecompression produced by the first spark on to a photo-graphic plate. By adjusting the interval of timebetween the two sparks, the compression was allowedto travel different distances before being photographedand so show different stages in its development. Inthe path of the compression, two screens with slits inthem were placed, and it was observed that the com-pression passing through each slit spread out from itjust as if from a new disturbance. The following fivediagrams are drawn from Woods photographs. The RRRRS Fig. 3. small black circle with the two fines coming obliquelyfrom it at the top of each diagram represents the shadowof the two knobs, one behind the other, between whichthe spark which produces the compression passes. The two horizontal lines with the small gaps in th
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1910
