12 



Light and Sound. 



[ Jan , 



the flame are rendered parallel by s and converged upon the watch 

 by s, where a brilliant spot of light is seen. Similarly the sound- 

 waves from the ticking of the watch follow the opposite course, 



Fig. 3 



and an acoustic focus is cast upon the flame. The flame throbs 

 in exact time with the beats of the watch. Moving the watch 

 or the flame nearer together places them out of the focus, and the 

 flame by its stillness announces the fact. 



It is easy for any one to repeat this experiment with very 

 homely apparatus. Circular dish-covers of earthenware or metal 

 may be used as mirrors, and for the flame may be substituted a 

 little runnel with a short tube leading to the ear. 



§ 7. Refraction of Light and Sound. 

 Light on passing from a dense medium to a rare one, or vice 

 versa, is bent out of its original course. So also is sound. Light 

 may by refraction be converged and focussed by lenses. Sound- 

 lenses may also be made.* Filling a thin india-rubber balloon with 

 a dense gas, like carbonic acid, a double convex acoustic lens is 

 produced. By means of such an arrangement divergent sound 

 rays, from, for example, the ticking of a watch, can be converged 

 and focussed. Placing a sensitive flame in the focus, this refrac- 

 tion of sound may be rendered apparent to a large audience. 



§ 8. Inflection of Light and Sound. 



This is an effect produced upon divergent waves merely by the 

 presence of an obstacle in their path. When a sea-wave meets an 

 isolated rock it breaks, spreads itself around the rock, and clasps 

 itself again at a short distance behind. Thus only comparative rest 

 is found behind such a breakwater. Similarly a sonorous wave 

 meeting an obstacle, say a large pillar, girdles the pillar and thus 



* As first demonstrated by Sondhauss. 



