Physiological Perspective. 321 



applies to variations resulting from tension in the rectus 

 muscles, external and internal, as well as from ciliary change. 



10. Binaural Audition and Binocular Vision. 

 The discoveries in relation to binaural audition, made during 

 the last few years independently by Professor Silvanus P. 

 Thompson*, of England, and Professor Alfred M. Mayerf, of 

 this country, are interesting, not only as additions to our 

 knowledge of physiological acoustics, but also in connexion 

 with the phenomena of physiological perspective. The locali- 

 zation of sounds has been found to be much affected by the 

 mode in which the waves are conveyed to the separate ears. 

 The same tone may be perceived as coming from the back of 

 the head, or from the two sides, or from a point obliquely in 

 front, while the position of the true source is unchanged — the 

 perception being involuntary, while the conditions are adjusted 

 at will. The judgment of distance by the ear is far more un- 

 certain than by the eye, there being no criterion other than 

 intensity of the sound perceived ; but the perception of direc- 

 tion may be modified by imposing unusual physiological con- 

 ditions, such as fatiguing one ear with a given tone and then 

 listening to the same with both ears. For a fixed position of 

 the eye, the perception of direction may be modified by chan- 

 ging the medium or pressing upon the eyeball ; while that of 

 distance is subject to variable physiological conditions, largely 

 controllable by the will. Although the binaural estimate of 

 direction and distance may be made less uncertain by properly 

 adjusting the position of the head to the wave-fronts of the 

 successive vibrations, no one has attempted to apply geometry 

 to the localization of sounds through binaural audition. 

 Brewster's attempt to do this for binocular vision was based 

 upon two assumptions, each of which has been proven incor- 

 rect. The first was, that " the line of visible direction does 

 not depend on the direction of the ray, but is also perpen- 

 dicular to the retina"?. The latter clause has been disproved 

 by Helmholtz § . The second was, that convergence of visual 

 lines is necessary in binocular vision, whether normal or ste- 

 reoscopic || . The falsity of this has been abundantly shown by 

 recent experiments, in addition to those of Burckhardt, Himes, 

 and others. A third fundamental statement by him was, that 

 "distinct vision is obtained only on one point of the retina "IT. 

 "While this opinion is still supported by many, it is incapable 



* Philosophical Magazine, Oct. 1877, Not. 1878, and Not. 1881. 

 t •' Researches in Acoustics.' 



% Brewster on Optics, p. 246. § Optique Physiohgique, p. 93. 



|| Brewster on the Stereoscope, pp. 90-106. % Ibid. p. 50. 



