Lord Rayleigh's Acoustical Observations. 457 



view as to the functions of the two ears be correct, there must 

 be other ambiguous cases besides those already experimented 

 upon. To the right of the observer, and probably nearly in 

 the line of the ears, there must be one direction in which the 

 ratio of the intensity of sound as heard by the right ear to the 

 intensity as heard by the left ear has a maximum value greater 

 than unify. For sounds coming from directions in front of 

 this the ratio of intensities has a less and less value, approach- 

 ing unity as its limit, when the sound is immediately in front. 

 In like manner, for directions intermediate between the direc- 

 tion of maximum ratio and that immediately behind the ob- 

 server, the ratio of intensities varies continuously between the 

 same maximum value and unity. Accordingly, for every 

 direction in front there must be a corresponding direction be- 

 hind for which the ratio of intensities has the same value ; and 

 these two directions could not be distinguished in the case of 

 a pure tone. The only directions as to which there would be 

 no ambiguity are the direction of maximum ratio itself, and a 

 corresponding direction of minimum ratio on the other side of 

 the head. 



The attitude of my mind with respect to this result was, I 

 confess, one of considerable scepticism. A great number of 

 miscellaneous experiments had been made with forks as well 

 as with other sources of sound; and I thought that, if these 

 ambiguities had existed, indications of them must have been 

 perceived already. It was therefore with some curiosity that 

 I took the first opportunity, last September, of submitting the 

 matter to the test of experiment, the same forks (making 256 

 vibrations per second) being used as on previous occasions. 

 The decision was soon given. An observer facing north, for 

 example, made mistakes between forks bearing approximately 

 north-east and south-east, though he could distinguish without 

 a moment's hesitation forks bearing east and west. In all 

 such experiments it is necessary that the observer keep his 

 head perfectly still, a very slight motion being sufficient in 

 many cases to give the information that was previously 

 wanting. 



A suggestion was made, in the discussion that followed the 

 reading of my paper before the Musical Association, which I 

 thought it proper to examine, though I had not much doubt 

 as to the result. In order to meet the difficulty in the ordi- 

 nary view as to the functions of the two ears arising out of 

 the fact that a 256-fork seems to be heard nearly as well with 

 the ear turned away as with the ear turned towards it, it was 

 suggested that possibly the discrimination between forks right 

 and left depended on something connected with the commence- 



