Sound- Waves by the " ' SchUeren-Methode." 225 



In fig. 8 we have the refraction of the wave through a 

 hydrogen prism, the front being bent forward instead of 

 backward, corresponding to the passage of light through a 

 prism of air in glass. 



The formation of a sound-shadow and the diffraction of 

 sound is beautifully shown in rig*. 9, a small strip of glass 

 having being mounted just below the balls. In No. 1 the 

 wave has just struck the plate, and the reflected portion is 

 starting off. In No. H the wave has quite passed the obstacle, 

 and a distinct shadow, or region of no disturbance, is seen 

 close under the plate, while in No. 4 the wave has, by 

 diffraction, curled round the edge and joined together again, 

 obliterating all trace of the shadow. 



The principle of Huygens, that any point on a wave-front 

 may be considered as the centre of a new disturbance, is 

 illustrated in fig. 10, two slits having been mounted below 

 the source of the wave. The slits were made by fastening 

 rather broad strips of glass together, with a gap of 3 mm. 

 w:d:h between them. (They do not show distinctly as slits 

 in the photographs on account of two narrow strips used to 

 fasten the broader pieces together.) In No. 1 the wave has 

 struck the upper slit, and the small notch which marks the 

 aperture is seen to be the centre of a new disturbance, which 

 is nearly a complete hemisphere. This secondary wave then 

 encounters the second slit, which in its turn becomes the 

 centre of a new disturbance, while that portion of the wave 

 which does not pass through the aperture bounces back and 

 forth between the two plates, as is seen in the last two 

 pictures of the series. 



The union of a number of secondary waves from several 

 parallel slits into a new wave, a rough example of a diffraction- 

 grating, is shown in fig. 11, where we have a number of 

 narrow strips of glass mounted so as to form a hemi-cylinder, 

 at the centre of which the wave starts. There are also 

 secondary wavelets reflected from the strips, and these unite 

 into a converging wave which returns to the origin. 



Fig. 12 illustrates the reflexion of a wave from a grooved 

 surface, the grooves being rectangular in form. No. 1 of 

 the series is interesting, as the time-interval between the two 

 sparks was exceptionally short, and the wave has scarcely 

 emerged from behind the balls. In the last one of the series 

 two wave-fronts are seen, one arising from the union of the 

 secondary wavelets reflected from the tops of the ridges, the 

 other formed by the disturbances coming from the bottom of 

 the grooves. Reflexion from a corrugated surface is shown 



