Mr. Ws. VAN DER Elst, assistaiit in oiir laboratory, established the 

 resonance curves of such small resonators b^y shutting off one end 

 of tlie ti\be with wax and placing a suitable mirror before the open 

 end. In very long tubes the tone of resonance is so low, that it 

 need not be tai<^en into account. Earlier experiments on the propa- 

 gation of sound in air showed that there is a marked decrease in 

 the velocity of |)ropagation, when the tubes are narrower than 

 4 mm. This at least is the case, when they are made of india-rubber. 

 It must be deemed advisable, therefore, to take glass or metallic pipes of 

 no less than ± 4 mm. in diameter. A mirror of, say 3 mm. diame- 

 ter, placed just in front of a straight-cut aperture, will be found very 

 suitable in most cases. Still, for very high tones even this pipe is 

 too narrow, as was demonstrated by researches years ago ')• The 

 tones of Galton's whistle (six-legerlined octave) change, when passing 

 through a canal of frotn 3 — 5 mm. bore, which after the foregoing 

 need not cause surprise, the tones lying near the upper limit of 

 musical sounds. We found it suitable to provide tiie afferent tubes 

 with leaden taps'). The sound conducted to the measuring apparatus, 

 may be generated at a considerable distance. 



Another simple conduit is the cone. The funnel may be given an 

 angle of 40° and a mouth of 50 cm'. Some American hearing 

 apparatus (operaphone) are provided with a similar funnel. Hklmholtz 

 discusses its resonance in his "Tonempfindungen". The one 

 I used, resounds to d^. This is easy to determine when an opening 

 is left in the apex of 2 mm., before which the Rayleigh mirror is 

 placed. The latter will deflect considerably, when the tone- of 

 resonance is given. With all other tones the waves will be progres- 

 sive, the cone being merely an indifferent receiving funnel. Again 

 a small plug of cotton wool had to be used to arrest disturbing 

 streams of air. 



§ 3. Point-shaped soundsources. 



Outlets in the shape of a mere puncture are obtainable through 

 a fine orifice, say of 1 mm. 1. in a little leaden disc that serves 

 for a septum in a speaking tube; 2. in the covering disc of the 

 air-chamber of a thermoteleplione. In either case the mirror is placed 

 right opposite to the fine opening, through which the sound is con- 



1) H. T. MiNKEMA, On the sensitiveness of the liuman ear to the various tones 

 of the gamut. Dissertation Utrecht 1905. 



3) H. DE Groot, Zschr. f. Sinnesphysiologie Bd. 44 S. 18 (experiments by 

 Dr. VAN Mens) and these Proceedings Vol. 14 p. 75S (experiments by Dr. P. 



NiKIFOROWSKY). 



