1870.] Light and Sound. 11 



source of sound. If a bundle of different tuning-forks be struck, 

 sonorous pulses of varying length will be emitted. Allowing this 

 composite pulsation to fall on a single quiescent fork, only that 

 pulse, which corresponds to the rate of vibration of the fork, will be 

 absorbed, and by this means the hitherto silent fork will be thrown 

 into vibration. 



§ 6. Reflection of Light and Sound. 



This is one of the best known, and at the same time one of the 

 closest analogies between light and radiant sound. 



When a beam of light, i, falls upon a polished surface, m m, it 

 rebounds in the same plane, and at an equal angle on the other 

 side of an imaginary perpendicu- 

 lar, p, drawn from the surface of 

 the mirror. This is expressed by i v 

 saying the angle of reflexion is 

 equal to the angle of incidence. 

 The same law is rigorously true 

 for sound. It is easy to observe 

 this reflexion from the sides of a 



7P 



cliff, a blank wall, or the roofs of 

 churches. Echo is due to this reflexion of sound. Standing be- 

 tween two parallel mirrors, successive and gradually fainter reflexions 

 of the body will be seen. The same is true with regard to sound, as 

 can be observed when speaking between two perpendicular walls, 

 sufficiently distant from each other. 



Under certain conditions light is "internally or totally reflected," 

 as it is termed ; that is, at a considerable obliquity of incidence light 

 cannot emerge from a dense medium into a rarer one : this may be 

 well observed when a glass rod is made red hot at one end, the 

 distant extremity glows by internal reflexion. In the same manner 

 sound can be internally reflected. When a sound made under 

 water strikes the surface of the water very obliquely, the greater 

 part of the sound is unable to escape into the air, owing to the total 

 reflexion occurring at the upper surface of the water* 



With curved reflectors sound is affected in the same way as 

 light. A sound-focus or a sound-image may be obtained by a con- 

 cave reflector. The following lecture illustration of this fact was 

 shown by the writer two years ago at the Royal Society of Dublin. 

 Two parabolic mirrors s s were placed as in Fig. 3. At c, in the 

 focus of, s, a watch is suspended; in the conjugate focus c' of 

 the other mirror, s', a sensitive flame was caused to burn. The 

 flame moves at the slightest sound. The light rays bk from 



* This fact has been established by the experiments of Colladon and Sturm on 

 the Lake of Geneva. 



