PERCEPTION OF THE DIRECTION OF SOUNDS. 819 



3. Harmony. If the beats take place more rapidly than 33 times per second, 

 the sensation of dissonance gradually diminishes, and it does so the more rapidly 

 the beats occur. The sensation passes gradually from moderately inharmonious 

 relations (which in music have to be resolved by certain laws) towards consonance 

 or harmony. The tone relations are successively the Second, Seventh, Minor Third, 

 Minor Sixth, Major Third, Major Sixth, Fourth, and Fifth. 



4. Action of the Musical Tones (" Klange"). Two musical "klangs," or com- 

 pound tones, falling on the ear simultaneously, produce a result similar to that of 

 two simple tones ; but in this case we have to deal not only with the two funda- 

 mental tones ; but also with the overtones. Hence the degree of dissonance of 

 two musical tones is the more pronounced the more the fundamental tones and the 

 overtones (and the " differential " tones) produce beats which number about 33 

 per second. 



5. Differential Tones. Lastly two " klangs," or' two simple musical tones 

 sounding simultaneously, may give rise to new tones when they are uniformly and 

 simultaneously sounding in corresponding intensity. We can hear, if we listen 

 attentively, a third new tone, whose number of vibrations corresponds to the 

 difference between the two primary tones, and hence it is called a " differential 

 tone" 



Summational Tones. It was formerly supposed that new tones could arise from the sum- 

 mation or addition of their number of vibrations, but it has been shown that these tones are in 

 reality differential tones of a high order (Appunn, Preyer). 



418. PERCEPTION: OF SOUND OBJECTIVE AND SUBJECTIVE 

 AUDITION AFTER-SENSATION. Objective and Auditory Perceptions. 



When the stimulation of the terminations of the nerves of the labyrinth is referred 

 to the outer world, then we have objective auditory perceptions. Such stimulations 

 are only referred to the outer world as are conveyed to the membrana tympani by 

 vibrations of the air, as is shown by the fact that if the head be immersed in water, 

 and the auditory meatuses be filled thereby, we hear all the vibrations as if they 

 occurred within our head itself {Ed. Weber), and the same is the case with our own 

 voice, as well as with the sound-waves conducted through the bones of the head, 

 when both ears are firmly plugged. 



Perception of Direction. As to the perception of the direction whence sound 

 comes, we obtain some information from the relation of both meatuses to the 

 source of the sound, especially if we turn the head in the supposed direction of the 

 sound. We distinguish more easily the direction from which noises mixed with 

 musical tones come than that of tones {Rayleigh). When both ears are stimulated 

 equally, we refer the source of the sound to the middle line anteriorly, but when 

 one ear is stimulated more strongly than the other, we refer the source of the 

 sound more to one side (Kessel). The position of the ear-muscles, which perhaps 

 act like an ear-funnel, is important. According to Ed. Weber, it is more difficult 

 to determine the direction of sound when the ears are firmly fixed to the side of 

 the head. Further, if we place the hollow of both hands in front of the ear, so as 

 to form an open cavity behind them, we are apt to suppose that a sounding body 

 placed in front is behind us. The semicircular canals are said also to be concerned, 

 as sound coming from a certain direction must always excite one canal more than the 

 others. Thus, the left horizontal canal is most stimulated by horizontal sound- 

 waves coming from the left (Preyer). Other observers assert that the membrana 

 tympani localises the sound, as only certain parts of it are affected by the sound- 

 waves. 



The distance of a sound is judged of partly by the intensity or loudness of the 

 sound, such as we have learned to estimate from sound at a known distance. But 

 still we are subject to many misconceptions in this respect. 



