the two Ears in the Perception of Space. 409 



ing: — Let /3 be the angle between the plane of the ear and the 

 line of sight, and a the angle which the line of sight makes 

 with the direction of the sound ; and let i x and i 2 represent 

 the relative intensity with which the sound is heard in the two 

 ears ; then 



tan a. : tan /3 : : i±— i 2 : ^ + i 2 . 



The operation, therefore, of finding the direction of a sound 

 — say a lark singing high up in the air — will, according to 

 Steinhauser, be as follows : — First, the head is rotated on its 

 axis horizontally until the sound is equally loud in both ears; 

 then the head is moved up and down until a maximum of 

 loudness is discovered, when the lark will be found in the line 

 of sight. Steinhauser's theory, it will be noted, takes into 

 account no differences of phase, pitch, or quality, but of inten- 

 sity only; and it fails, as will be seen, to account for the fact 

 that we have, without moving the head at all, a very fair per- 

 ception of the direction of sounds. It also presupposes (what 

 the experiments of Crum Brown and M'Kendrick have pretty 

 definitely shown to exist) that we possess a rotation-sense, a 

 means of perception of angular movement in horizontal and 

 meridional planes. Steinhauser devised an instrument, which 

 he named the homophone, wherewith to test his theory. It 

 consisted of a system of wooden tubes for bringing to the ears 

 the sounds of two organ-pipes tuned to unison, whose respec- 

 tive intensities could be regulated by stopcocks. It was held 

 by its inventor to confirm his theory. 



A similar theory has been implicitly adopted by Prof. 

 Graham Bell, in the paper on Experiments relating to Bin- 

 aural Audition which he read before the American Association 

 for the Advancement of Science in 1879 *. Graham Bell 

 investigated the matter with telephones connected to the ears, 

 with some curious and valuable results. He also experimented 

 on the degree of accuracy with which the ear can determine 

 the direction of a sound, and found it to be most accurately 

 defined for those sounds the direction of which approximates 

 to the axial line of the ears. 



5. Theories of A. M. Mayer and {provisionally) of S. P. 

 Thompson. — A modification of the preceding theory was pro- 

 visionally adopted by the present writer in the earlier part of 

 his researches. He had discovered that when two sounds 

 agreeing in every respect, save in phase, are led to the ears, a 

 singular phenomenon occurs, complete difference of phase 

 producing an acoustical illusion as if the sound were localized 

 in the back of the skull. He therefore suggested that differ* 



* American Journal of Otology, July 1880, p. 109< 



