232 



Professor Silvanus P. Thompson 



[June 13, 



8 = j, and maximum with 8 = ^. Inspection of the first and third 

 lines of curves in Fig. 6 shows that in these wave-forms that phase 

 which is the most forcible is that in which the maximum displacement, 

 and resulting condensation, is sudden and brief. 



Observing that wave-forms in which the waves are symmetrical — 

 steeper on one side than on the other — are produced as the resultant 

 of a whole series of compounded partial tones, it occurred to Dr. 

 Koenig to produce from a perfect and symmetrical sinusoidal wave- 

 curve a complex sound by the very simple device of turning into an 

 oblique position the slit through which the wind was blown against it. 

 In Fig. 13 is drawn a simple symmetrical wave-form, eglnjprtv. If a 



Fig. 13. 



Eflfect of Tiltins: the Slit. 



series of such vs^ave-forms is passed in front of a vertical slit, such as 

 ah, B. perfectly simple tone, devoid of upper partials, is heard. But 

 by inclining the slit, as at ab\ the same effect is produced as if the 

 wave-form had been changed to the oblique outline e'g'l'n'p'r't'v', the 

 slit all the while remaining upright. But this oblique form is pre- 

 cisely like that obtained as resultant of a decreasing series of partial 

 tones (Fig. 6, a). If the slit be inclined in the same direction as the for- 

 ward movement of the waves, the quality produced is the same as if 

 all the partial tones coincided at their origin, or with S = ; while if 

 inclined in the opposite direction the quality is that corresponding to 

 8 = J. It is easy to examine whether the change of phase produces 

 any effect on the sound. Before you is a simple wave-disk, and air 

 is being blown across its edge through a slit. On tilting the slit 

 forward to give 8 = 0, you hear a purer and more perfect sound ; and 

 on tilting it back, giving 8 = J, a sound that is more nasal and 

 forcible. 



All the preceding experiments agree then in showing that 

 differences of phase do produce a distinct effect upon the quality 

 of compound tones : what then must we say as to the effect on the 



