Prof. S. P. Thompson on Binaural Audition. 385 



very clearly recognized. I have not been able hitherto, how- 

 ever, to devise any crucial experiment to decide whether the 

 interference is an interference of the sensations, or whether 

 it is to be attributed to the physical conveyance of the sounds 

 through the bones of the skull, and their mechanical inter- 

 ference. 



5. Phenomena of Localization. — Almost all persons who 

 have experimented with the Bell telephone, when using a pair 

 of instruments to receive the sounds, one applied to each ear, 

 have at some time or other noticed the apparent localization 

 of the sounds of the telephone at the back of the head. Few, 

 however, seemed to be aware that this was the result of either 

 reversed order in the connexion of the terminals of the instru- 

 ment with the circuit, or reversed order in the polarity of the 

 magnet of one of the receiving-instruments. When the two 

 vibrating disks execute similar vibrations, both advancing or 

 both receding at once, the sound is heard as usual in the ears ; 

 but if the action of one instrument be reversed, so that when 

 one disk advances the other recedes, and the vibrations have 

 opposite phases, the sound apparently changes its place from 

 the interior of the ear, and is heard as if proceeding from the 

 back of the head, or, as I would say, from the top of the cere- 

 bellum. So distinctly marked is the apparent localization, that 

 it has been regularly employed, I am informed, by Professor 

 D. E. Hughes to ascertain whether a pair of receiving-tele- 

 phones are rightly adjusted or not. 



6. My recent experiments have been directed to determining 

 how this apparent localization is affected by variations of the 

 sounds in respect of (a) pitch, (b) phase, (c) intensity, (<i) qua- 

 lity, and whether it is to be accounted as a physical, physio- 

 logical, or psychological phenomenon. I have made, finally, 

 a few experiments on the binaural estimation of combinational 

 tones. 



(a) After employing the simple sounds of tuning-forks of 

 various pitches, and conveying their vibrations to the two ears 

 in opposite phases by three distinct methods, I find the appa- 

 rent locality of the sound (the acoustic " image ") to occupy 

 an invariable position near the top of the cerebellum. These 

 three methods are: — First, employing two india-rubber tubes 

 of equal length, armed with either glass funnels or box reso- 

 nators, in front of which two tuning-forks are held, differ- 

 ence of phase being obtained by rotating one fork round on 

 its axis, or by loading it to obtain a continuously varying 

 phase. Secondly, by employing one tuning-fork, but having 

 a branching tube to the ears, and making the lengths of the 

 two branches differ by half a wave-length. Thirdly, by em- 



Phil. Mag. S. 5. Vol. 6. No. 38. Nov. 1878. 2 C 



