900 QUALITY OF AUDITORY PERCEPTIONS. 



arrangement produces a characteristic musical tone. If, however, 

 holes are made in another circle of the same disc perforated at unequal 

 distances apart, the current of air directed against the disc gives rise 

 to a whirring, rushing nonmusical noise, because the movements of the 

 sounding body, the condensations and rarefactions of the air, are non- 

 periodic. 



Every sound must last a certain length of time in order to be heard by 

 the ear (the feeblest sound at least two seconds) ; on the other hand, after 

 a sound is once heard, the stimulation of the ear persists for some time. 

 Hence, when sounds recur at short intervals, no intermission can be 

 detected. 



The normal ear distinguishes in every tone three distinct qualities: 



1. The Intensity of the Tone. This depends upon the amplitude of 

 the vibrations of the sounding body. It is well known that a gradually 

 weaker and weaker sounding string exhibits correspondingly smaller 

 amplitude of vibration. The intensity of a sound corresponds to the 

 degree of illumination or brightness in vision. 



2. The Pitch of the Tone. This depends upon the number of vibra- 

 tions that occur in a given unit of time. This also is demonstrated by 

 means of the siren. If the rotating disc have a series of 40 holes, and 

 another of 80 holes, at equal intervals, on blowing a current of air 

 against the rotating disc, two sounds of unequal pitch will be heard, 

 one being an octave above the other. The perception of pitch cor- 

 responds to the sensation of color in vision. 



3. The quality or timbre of the tone, which is peculiar to different 

 sonorous bodies. As will appear later, this depends upon the peculiar 

 form of the vibration of the sonorous body. There is no analogous 

 sensation in the case of light. 



Perception of Pitch. Through the sense of hearing, it is learned that different 

 tones have a different pitch. In this connection the established difference in 

 the pitch of the notes of the so-called musical scale or gamut is characteristically 

 distinct to the normal ear. In addition, there are four tones in the scale that, 

 when sounded together, cause, in the normal ear, the sensation of pleasing sound; 

 and that, when once recognized, may be easily reproduced always with charac- 

 teristic difference in pitch. These are the tones of the so-called accord or major 

 chord, consisting of the first, third, and fifth tones of the scale, to which the eighth 

 tone or octave is added. It is necessary to determine first the pitch of the tones 

 of the accord, and then that of the other tones of the scale. The siren serves for 

 the determination of the first, and from this the others can easily be calculated. 

 Four concentric circles are drawn upon the disc of the siren, the inner one contain- 

 ing 40 holes, the second 50, the third 60, and the outer one 80, all of the holes 

 being at an equal distance from one another. If the disc be rotated, and a current 

 of air be forced against each series of holes in turn, there will be heard successively 

 the four tones of the accord (major chord) . When the entire four series are blown 

 upon simultaneously, the major chord is produced in complete purity. The 

 relative number of the holes in the four series indicates, in the simplest manner, 

 the relative pitch of the tones of the major chord. While 40 condensations and 

 rarefactions of the air in each revolution are necessary to produce the fundamental 

 tone, double this number in the same time (one revolution) are required to pro- 

 duce the octave. Hence, the relation of the number of vibrations of the funda- 

 mental tone or keynote to the octave next above it is i : 2 . In the second series 

 there are 50 holes, which produce the pitch of the third. Therefore, the relation of 

 the fundamental tone to the third in this case is 40 : 50 or i : i = f ; that is for 

 every vibration of the fundamental tone there are f vibration in the third. In 

 the third series there are 60 holes, which, when blown upon produce the fifth. 

 Hence, the ratio of the fundamental tone to the fifth in the disc is 40 : 60, or 

 i : ij f. In this way the pitch of the four tones of the major chord is deter- 

 mined experimentally; it is found that the number of vibrations of the first, third, 

 fifth, and octave are to each other as i : ; ?, ; : 2. 



