( -122 ) 



(one determination) for a' average 14.47„ (with 10.8 and 22.5 for 

 extremes). It appeared desirable to perform such an investigation 

 tln'Ough'"tlie~whole scale and to establish it in other regards also on 

 larger foundations. At my request Mr. A. Deenik has executed a 

 very~ great number of observations of this kind, and I take the 

 liberty to communicate his results here in short, and refer the reader 

 to an ample description in a thesis on this subject by Mr. Deenik 

 which will soon be published. 



Experiments ivith the tuning-fork. 



A tuning-fork kept vibrating by electro-magnetism is started in a 

 room at the side of the sound-free cabinet of the physiological 

 laboratory and is kept vibrating at a fixed amplitude. This amplitude 

 may be measured microscopically by means of the triangle of Gradenigo. 

 Normal to the axis of this tuning-fork a circle divided into grades 

 is placed, to which two hearing-tubes are attached in such a way, 

 that their radial prolongations cut the axis of the tuning-fork in the 

 tuning-centre. These hearing-tubes can be moved along the whole 

 circumference of the scale, and can be brought at pleasure into the 

 interference-planes of Kiessling, in the |ilanes of maximum-sound 

 or between. 



The hearing-tubes are led into the interior of the sound-free 

 cabinet by means of thick-walled caoutchouc tubes which were 

 still further acoustically isolated. There by means of a T— *ap 

 alternately the one or the other of the tubes may be listened at or 

 perfect acoustic rest can be obtained by bringing the tap into a 

 closed position. 



An assistant now displaces one of the hearing-tubes, while the 

 other hearing-tube is fixed in the plane of maximum sound, every 

 time through some grades at a time into the direction to the inter- 

 ference plane of Kiessling till a distinct difference has been signa- 

 lised by the investigator (descending method). After the position of 

 the tube has been read this is pushed on and then brought back 

 in the same way till the investigator observes that the existing 

 difference in intensity becomes indistinct (ascending method). Again 

 the position of the tube is read ofï' and the average is taken. 



The observations take place in the above mentioned way "unwis- 

 sentlich" and at five succeeding times. From the ten figures obtained 

 in this way the average is taken at last, which indicates in grades 

 of the scale a lowest "Unterschiedsschwelle" for the concerned 

 amplitude. 



To be able to transpose these angle-values into absolute values, 



