39Q Prof. S. P. Thompson on Binaural Audition. 



tion upon a hypothesis of a pure association of previous sen- 

 sations; but such cannot be framed until the physiological 

 points involved are more distinctly recognized. 



11. The Binaural Estimation of Combinational Tones. — In 

 my paper of last year I remarked that I had been unable to ob- 

 serve the existence of combinational (differential) tones when 

 two simple tones ordinarily capable of yielding such tones 

 were led separately to the two ears. Thus the forks e and g f 

 (MI 3 and SOL3) when struck together give a differential tone 

 of 64 vibrations, or C,( = UT,), which is perfectly recognizable; 

 but when led separately to the ears no such tone is to be heard. 

 But if the two tones be allowed to mingle and their joint sound 

 be led to the two ears by a Hughes's transmitter and two tele- 

 phones arranged to yield opposite vibrations, then the differ- 

 ential tone of 64 vibrations is heard accompanying the two 

 simple tones, and is localized with them at the back of the 

 head. 



Other experiments with forks of higher pitch have convinced 

 me that the absence of differential tones is general when the 

 two separate simple sounds do not mingle before reaching the 

 ears. During a recent visit to the workshop of Dr. E. Konig of 

 Paris, I had the opportunity of hastily repeating this experi- 

 ment upon forks of shrill tones belonging to a series purposely 

 constructed for observations upon the combinational tones; 

 and the result fully confirmed previous observations. 



My paper of last year mentioned a case which seemed to 

 indicate the presence of summational tones. The difficulty of 

 observing these is ordinarily much greater than that of obser- 

 ving the differential tone, and in binaural experiments is still 

 greater. Further observations are still wanting on this point. 

 It was remarked in my former paper that in binaural audition 

 dissonances are excessively disagreeable, and the ordinary 

 consonant intervals harsh. No subsequent experiments have 

 been made upon this matter, which is probably connected with 

 the absence of the differential tones. 



12. One other phenomenon, as yet only partially investi- 

 gated, remains to be recorded. Let a small tuning-fork, such 

 as that used by pianoforte-tuners or violin-players, be struck, 

 and its stem pressed against any portion of the back of the 

 head. If the fork be pressed against the parietal or occipital 

 regions of the head on the right of the median line, its sound 

 appears to be heard in the left ear ; if held against a region of 

 the head on the left, it appears to be heard in the right ear. 

 If, however, the fork be pressed against the region immedi- 

 ately above either temporal bone, its sound is heard in the ear 

 of the same side. 



