234 BELL SYSTEM TECHNICAL JOURNAL 



which, much later, there will be a reminiscence: the ear can detect the 

 amplitude, but not the phase, of the vibrations. It follows, then, that 

 we must devise tests of the wave-theory in which the amplitude of the 

 waves shall vary from time to time, or from place to place. 



Such tests are easily arranged. Let a pair of tuning-forks be set up 

 not too close together and not too far apart. If sound consists of 

 waves, the spherical undulations broadening outward from each of 

 these separately must fall off steadily in amplitude as they recede, and 

 the sound grow steadily fainter as the listener moves away. So it 

 does; but the fact is equivocal, and cannot be taken as evidence for 

 the waves; if the fork emitted corpuscles of sound, they would scatter 

 apart as they flew away, and fewer would enter the ear the farther off 

 it was placed. If, however, both of the forks are giving voice at once, 

 and trains of spherical waves expanding outward from both, then the 

 amplitude in the air must vary in a curious and striking manner from 

 place to place, alternating between maxima and minima. This is just 

 the sort of test which the ear is excellently fitted to make ; being moved 

 (or the mouth of its listening-tube being moved) from place to place 

 in the field where the streams of sound overlap, it reports the fluctua- 

 tions of loudness which are predicted from the wave-theory. By 

 properly choosing the conditions one or more of the minima may be 

 reduced to zero; loudness added to loudness makes silence. By 

 properly choosing the conditions, maxima and minima may be caused 

 to move in succession across a fixed point, listening whereat the ob- 

 server hears "beats." All of these are phenomena of interference, and 

 many like them are realized with light. 



But it is not necessary to produce two overlapping streams of sound, 

 in order to find evidence favouring the wave-theory. One suffices, 

 provided that we try to separate from it a narrow jet or ray. Near 

 one of the forks let a wall be placed, and perforated with a little hole. 

 This seems to be an artifice for producing a constricted beam of sound 

 proceeding like a searchlight straight outward along the line passing 

 from the source through the hole; but it does not work that way. 

 Instead, the tone of the fork is heard everywhere beyond the wall; 

 sound is radiated from the hole in all directions. The aperture be- 

 comes itself a sort of secondary source, from which sound emanates 

 sidewise as well as forward. 



Precisely similar is the visible behaviour of water-waves (and in- 

 cidentally of the violent compressional pulses produced in air by ex- 

 plosions, which have been photographed; but we are assuming that 

 our imaginary pupil knows nothing of sound but what he hears!). 

 Circular ripples expand over the surface of still water until one of them 



