v THE SENSE OF HEABING 263 



These general conclusions of Venturi are borne out by all the 

 scientific researches made later on with different methods. 



Preyer, in a series of observations, endeavoured to bring the 

 perception of the direction of sounds into relation with the 

 orientation of the semicircular canals a theory already pro- 

 pounded in Italy by Lussana. Mlinsterberg supported this view 

 with certain variations. But since it' has been proved that no 

 auditory function can be assigned to the vestibular apparatus as 

 a whole, the theory that the semicircular canals are responsible 

 for the immediate sensation of the direction of sound has fallen 

 through completely. On the other hand, v. Kries showed that 

 the facts stated by Preyer and by Miinsterberg may be explained 

 quite simply by the different intensity of the sensations in the 

 two ears that is, by the old theory of Venturi. 



Bloch, under the guidance of v. Kries, made a special study 

 of the subject of binaural audition, and disposed of numerous 

 errors that had accumulated. He found that in binaural hearing 

 there was a reciprocal reinforcement of sensation, greater perhaps 

 than would result from simple summation. With equal stimula- 

 tion of both ears, the field of audition is projected to the middle 

 of the head, with stimulation by tones of unequal intensity, to 

 the side of the stronger stimulation. 



The direction of the source of sound is determined as follows : 

 The subject is blindfolded and placed in the centre of a circle 

 with a radius of 1 metre ; the circle is divided into 16 parts ; 

 at the middle of each part Politzer's acumeter is set going, and 

 is then moved in one direction or the other along the periphery 

 of the circle, while the subject is asked to judge the direction of 

 the movement. 



Bloch found that on moving the source of sound in front of 

 or behind the subject variations of about 4-5 could be perceived. 

 At the sides a displacement 6-7 times greater was necessary 

 before the change could be recognised, no matter if the subject 

 were standing or lying down. The same was the case for 

 uniaural hearing, but the power of distinguishing the direction 

 of displacement was much less. These results can be fully 

 explained by differences in intensity of sensation, and partly also 

 by the form of the external ear (pp. 195 et seq.). The proof of 

 this lies in the fact that when the sound is produced in the 

 sagittal plane the displacement must be about five times greater 

 before it can be recognised. 



The frequency of error in judging the distance of a sound 

 seems to be in relation with the strength of the partial tones. It 

 is usually an indirect estimation, based on the apparent intensity 

 of tones, the absolute strength of which is known to us. Not 

 only is binaural hearing of great importance to our judgment of 

 the direction and the distance of the source of sound, but it 



