430 Mr. R. H. M. Bosanquet on the Beats 



inserted, 2 inches long, which tapered down to an orifice J inch 

 in diameter; this served to communicate with the interior of 

 a resonator by means of a small flexible tube. The extremi- 

 ties of the semicircle were turned inwards and upwards; and 

 into them two brass tubes were inserted, f inch long and £ inch 

 in internal diameter, screwed on the outside. Over each of 

 these was fitted a brass tube, screwed inside, carrying an ivory 

 nipple, such as is used for ear-trumpets. I generally covered 

 the nipple with a couple of thicknesses of thin india-rubber 

 tube. 



38. When used, the semicircle is passed under the chin with 

 the resonator-attachment projecting in front. The nipples are 

 at first screwed back as far as possible, brought opposite the 

 orifices of the ears, and then screwed forward until they enter 

 the ears. They are then gradually advanced until the passages 

 are closed to external sounds. Something depends on the 

 way the tube is held. With practice it is possible to hold it 

 so that the passages are closed to external sounds without 

 screwing the nipples in very tight. When they are screwed 

 very tight, it is rather unpleasant, and even painful. But it 

 is necessary constantly to be on one's guard against being 

 deceived by an occasional entrance of external sounds if the 

 nipples are not quite tight. This instrument was made for 

 me some time ago by Mr. Walters of Moorgate Street ; it 

 has already been described (Proc. Mus. Assoc. 1879-80, 

 p. 18). 



39. The resonators I employ are bottles fitted w T ith corks 

 having apertures of various sizes. I sometimes tune them 

 with water, in the same way as the bottle-notes; sometimes 

 I insert tubes into the apertures to lower the pitch. A bit of 

 small glass tubing passed through the cork is connected by an 

 india-rubber tube with the above-described ear-piece. 



40. By means of these arrangements I some time ago 

 examined the nature of the ordinary first difference-tone, and 

 convinced myself that it is not capable of exciting a resonator 

 (I. c. p. 20). This conclusion has also been arrived at by 

 others*. In short, the difference -tone of Helmholtz, or first 

 beat-note of Konig, as ordinarily heard, is not objective in its 

 character. It is therefore subjective. (See Helmholtz, Ton- 

 empftndungen, 4th ed. p. 259.) In making the experiment 

 of listening for the difference-tone through a resonator, it is 

 necessary to be careful that the ears are both closed to external 

 sounds; otherwise the external notes will penetrate through, 

 the difference -tone will appear, and the completeness of the cut- 

 off effected by the resonator will be entirely lost. 



* Preyer, Akustische Untersuchungen, p. 13. 



