Our power of estimating the direction of sound is much less accurate than that of estimating the direction of light; and, further, our knowledge as to the way in which this perception of direction occurs is much more limited in the case of sound. We judge with the greatest accuracy the direction of a ray of light, and we know that we do so almost entirely by the aid of the muscular sense. How do we judge the direction of a sound?

Before attempting a reply to this question it will clear that matter up, to a certain extent, by giving a statement of a few facts which have been found out by experiment, and are well attested by various writers:—

(1) Individuals who are deaf of one ear have little power of estimating the direction of sound.

(2) Individuals who are blind of both eyes generally show a power of judging the direction of sound far superior to those gifted with sight. This is particularly true if the eye affection dates from childhood.

(3) The power of estimating the direction of sound depends upon its pitch and its quality. A pure tone of low pitch is very difficult to localise; with the higher notes this becomes easier, and with a compound sound it is also easy.

(4) The accuracy with which we estimate the direction of a sound is greatest when the sound is in the line of axis of the two ears, and least when it is in the median plane of the body.