PHOTOGRAPHY OF SOUND WAVES. 3fi7 



♦ 



below the plate in No. 8 of the series a gap in the wave is found, which 

 constitutes a shadow. But presentl}^, by diffraction, the wave curls in, 

 closing up the gap and obliterating the shadow entirely. In the last 

 one of the series it is interesting to note how the diffracted waves have 

 their centers at the edges of the obstacle, the edges acting as secondary 

 sources, as in the case of the diffraction of light. 



The passage of a wave through a diffraction grating is shown in fig. 

 1(), PL IV. The grating is made of strips of glass arranged on a cyl- 

 indrical surface, the wave starting at the center of curvature. In No. 

 2 of the series the union of the secondary disturbances coming from 

 the openings into a new wave front is beautifully shown. In No. 8 

 the reflected wavelets have converged to the center, but as each one 

 is a complete hemicylinder, we see them radiating from the center. 

 This form can be constructed by describing semicircles around points 

 on a circle of such radius that they all pass through the circle's center. 

 These semicircles represent secondary wavelets starting simultaneously 

 from the various grating elements. In the last three pictures of the 

 series the wave passes down, strikes the table, and is reflected up 

 again, *ind it is interesting to see how the medium is broken up into 

 meshes by the crossing and recrossing of the secondar}^ waves. 



Fig. 17, PI. IV. shows the form of the secondary wavelets formed b}^ 

 the reflection of a wave from a corrugated surface and is interesting 

 in connection with reflection gratings. 



The formation of a musical note b}' the reflection of a single pulse 

 from a flight of steps is shown photographed in fig. 18, PI. IV. This 

 phenomenon is often noticed on a still night when walking on a stone 

 pavement alongside a picket fence, the sound of each footstep being- 

 reflected from the palings as a metallic squeak, which Young has 

 pointed out to be analogous to the power of a diffraction grating to 

 construct light of a definite wave length. 



It occurred to me, while making some geometrical constructions to 

 aid in unraveling some of the complicated forms reflected from surfaces 

 of circular curvature, that a very vivid idea of how these curious wave 

 fronts are derived one from another could l)e obtained if a complete 

 series could be prepared on the film of a cinematograph, and projected 

 in motion on a screen. 



Having been unable to so control the time interval ])etween the two 

 sparks that a progressive series could be taken, I adopted the simpler 

 method of making a large number of geometrical constructions and 

 then photographing them on a cinematograph film. 



As a very large number of drawings (one hundred or so) nuist be 

 made if the result is to be at all satisfactory, a method is desirable that 

 will reduce the labor to a mininuim. I may be pcM-mitted to give, as 

 an instance, the method that I devised for buikling the series illustrat- 



