30 DISPLACEMENT INTERFEROMETRY. 



volume (fig. 41) the 4-foot octave should appear at all. Finally, the highest 

 peaks in figure 41 exceed those of figure 38. 



fct, The perplexing question as to what resonates in case of the small resona- 

 tor was approached by lengthening the tubing between the telephone and 

 U-tube from its usual length of 48cm. to 85 cm. and 120 cm. successively. 

 The shell-like volume of the resonator IV can not much have exceeded 10 or 

 15 cm. 3 . The tubes added 10, 17, and 24 cm. 3 respectively. To this the disk- 

 like volume in the telephone (about 2 cm. 3 ) is to be added ; nevertheless the 

 greater part of the total volume seems to reside in the tubes. 



The results obtained in the survey maxima and minima are given by the 

 curves in figure 42 . The corresponding maxima and minima are easily recog- 

 nized in the three cases, though there has been a little shifting, part of which 

 is necessarily mere subjective impression. What has changed actually is the 

 amount of fringe displacement, both at the maxima and the minima. This is 

 particularly true for the order of pitch above c" . In other words, the overtones 

 have been differently accentuated. Thus the minimum near d" passes from 

 positive to negative values as the tube-length increases, while the reverse is 

 observed at the maximum near g". The case is, then, analogous to the 

 occurrences of the vowel sounds, where different overtones are accentuated 

 by variation of the mouth cavity. 



