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B54 W. LeConte Stevens—Physiological Optics. 
physiological perspective. The localization of sounds has been 
ound to be much affected by the mode in which the waves 
are conveyed to the separate ears. The same tone may be 
perceived as if produced at the back of the head, or from the 
two sides, or from a point obliquely in front, while the position 
of the true external source is unchanged, the perception being 
involuntary while the conditions are adjusted at will. The 
judgment of distance by the ear is far more uncertain than by 
the eye, there being no other criterion than the degree of 
energy of the vibrations which give rise to sensation; but the 
perception of direction may be moditied by imposing special 
conditions, such as fatiguing one ear with agiven tone and 
then listening to the same with both ears. For a fixed posi- 
tion of the eye, the perception of direction may be modified at 
will by methods already described, or by pressing upon the 
eyeball, while that of distance is also subject to variable condi- 
tions. Although the binaural estimate of direction and dis- 
tance may be made less uncertain by properly adjusting the 
ponton of the head to the wave front, or, as in the case of the 
ower animals, by directing the two ears at will toward the 
source of sound, no one has attempted to apply geometry to 
the binaural localization of sounds. Its application to binocu- 
lar vision is now found to be wholly unreliable in the very 
department for which it was deemed most satisfactory, that of 
stereoscopic perspective. 
4". Errecr or ExpERIENCE IN VISION. 
with nearness of the point of fixation, and this is pictured upon 
the yellow spot of the retina. No one whose eyes are healthy 
has any consciousness cf possessing any retina except 1 rela- 
tion to external objects, or any tympanum except in relation to 
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