492 



Physiology. — ''On hearing-apparatus examined after Lord Ray- 

 LEiGH'.y mode of arrangement." By Prof. H. Zwaardemaker. 



(Communicated in the meeting of September 27, 1913). 



In an earlier publication ^) we were guided by the pi-inciple that, 

 before endeavouring to find a fitting a|)paratus for a diseased organ 

 of hearing, we should first of all examine its auditory range in the 

 zone a^ to e^ • The apparatus has to supply the deficiency of the 

 organ. The experimental part of our previous researches was performed 

 with a microphone (in camera plumbica-)) and a string-galvanometer. 

 Lord Rayleigh's mode of arrangement is unquesiionably a better 

 method. 



§ 1. Non-resonating arrangement. 



Zernov placed Lord Rayleigh's small mirror^) obliquely to the sound- 

 wave and encircled it by a gauze screen. In this way all resonance is 

 pre^•ented; however, the sensitiveness is relatively small. That was why 

 I took an afferent tube of (he dimensions of the auditory canal and 

 drew up before it a funnel, whose tone of resonance lies beyond the 

 speechzone and whose mouth corresponds in width with the auricle. 

 Hereby resonance is excluded, except such as the human ear is 

 always subject to. The place where the measuring mirror is suspended 

 corresponds to that of the tympanic membrane, with this difference 

 that in the case of stationary waves, there is always a node on the 

 membrane and a loop on the mirror, for the former is strongly 

 damped by the auditory ossicles, whereas the latter is damped only 

 by the air ^). 



The width of the artificial auditory canal is 6 mm., the diameter 

 of the mirror, used in the experiment, is 4 ram. It is placed at an 

 angle of 45° immediately in front of the canal. To keep off the 

 streams of air a small plug of cotton-wool is put in the afferent 

 tube. The stopped wooden pipes, sounded with maximum power 

 Avithout causing a deflection or an anomalous vacillation, yield the 

 following results as registered at 1 m's distance in the camera silenta 

 (mirror completely stationary ; absence of resonance ; the wall of the 



1) "On hearing-apparatus" Nederl. Tijdschr. v. Geneesk, 1912. 11. p. 1101. 

 2^ "Eine Camera plumbica fiir Mikrophone". Zeitschr. fur biol. Technik. Bd. 11. 

 S. 340. 



3) Zernov used a magnet I o ensure a constant position of rest. I adopted a 

 simpler method by applying a flat Wollaston fibre. (See tliese Procs. XVI p. 195. 



4) ViOLLE, Acouslique, 1892, p. 108. 



