Perception of Sound Direction. 217 



more than one assistant may be required. The observer, 

 either on his feet, or more conveniently seated upon a rotating 

 stool, is turned round until he loses his bearings. There is 

 no difficulty in this*, but some precautions are needed to 

 prevent the bearing being afterwards recovered. The wind 

 may act as a tell-tale. It is often necessary to cover the eyes 

 with the hands as well as to close the eye-lids in order suffi- 

 ciently to exclude the light. Until all is ready for a judge- 

 ment, the ear* are kept closed by pressure with the thumbs, 

 and it is usually advisable to keep the thumbs in motion and 

 thus to cause miscellaneous noises loud enough to drown any 

 residue of the sound under observation. Pure tones of pitch 

 128 and 256 yielded by this method result in agreement with 

 those already described. 



The turn-table facilitates observation upon the question as 

 to the relative loudness with which a sound is heard according 

 as the source is on the same, or the opposite, side as the ear 

 in use. In my own case 1 thought I could detect an advantage 

 when the source was on the same side as the open ear, while 

 others could detect no difference. This relates to pitch 128. 

 At 256 the advantage is quite marked. 



In considering- whether the discrimination between right 

 and left at pitch 128 can really be attributable to the small 

 intensity-differ ence, it occurred to me that, if so, the judgement 

 might perhaps be disturbed by the introduction of an obstacle, 

 such as a piece of board, near the head of the observer and on 

 the same side as the sound. But it was found that no mistakes 

 could thus be induced, although in each trial the observer did 

 not know whether the board were in position or not. Another 

 circumstance, unfavourable to the intensity theory, may also 

 be mentioned. It was found that the observer on the turn- 

 table could sometimes decide between the right and the left 

 before un-stopping his ears. 



The next step in the investigation appeared to be the 

 examination of pure tones of still graver pitch. A globe, 

 such as are sold to demonstrate the combustion of phosphorus 

 in oxygen gas, was sounded with the aid of a hydrogen flamef . 

 Careful observation revealed little or no trace of overtones. 

 The frequency was about 96 vibrations per second, thus by 

 the interval of the Fourth graver than the 128 forks. At the 

 temperature of the observations this would correspond to a 

 wave-length of about 12 feet. If we use this value in the 

 formula already given, we find for the proportional difference 



* The process is aided by the illusion of a reverse rotation when the 

 real rotation has stopped. 



t Phil. Mag. vii. p. 149 (1879) ; < Scientific Papers/ i. p. 407. 



