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Page:Popular Science Monthly Volume 65.djvu/234

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THE POPULAR SCIENCE MONTHLY.

definite order. It is thus by virtue of the eye's movements that there is secured the perception of the orderly spatial arrangement of the world revealed to our vision.

Further evidence of the important functions which the eye's movements fulfill in perception might be adduced, but it is not desirable for the purpose in hand to take up what is more complicated and debatable. What has been said may suffice to call attention to the great significance of the mobility of the sense organ.

Evidence not less striking might be brought forward in regard to the sense of touch; it might be shown that it is by the movement of the sense organ, say, the finger tips, that explorations of the body under investigation can be, in ordinary cases, best accomplished, and that it is by the producing and reversing of series of touch sensations that the spatial relations of tangible objects are clearly recognized.

The ear is immobile. Accordingly it is incapable of reflex movements for catching sounds, like those by which the eye is turned so quickly to meet the light coming from an object.

We find likewise that the perception of space by means of sound is in an extremely undeveloped form. Many have gone so far as to deny that sound has any spatial character. Yet surely this view can not be maintained. We locate sounds to the right or left, behind or in front; moreover, we distinguish sounds as differing in volume.

Yet it can readily be seen that the spatial characters of sound do not compare in precision and definiteness with those of the sensations of color and touch. What is the size of the thunder? The question at first seems absurd. Yet it can not be entirely absurd, for we speak of the peal as heaven-filling. The appearance of absurdity is due to the hopeless vagueness of the sound image in respect to extent.

If we analyze this vagueness we find that owing to the immobility of the ear we can not locate sounds with precision. All are familiar with the difficulty of telling, especially in strange surroundings, whence a sound comes, unless the eye gives its help. The ringing of the bell of an unseen bicycle may cause us the most painful perplexity till we can learn its source by sight. Psychological experiments show in detail how untrustworthy are our attempts to localize sounds. Not that they are entirely untrustworthy. It may be that sounds have a special quality according to the direction from which they come and the way in which they strike upon the external ear; and recognition of this quality may give help towards their localization. But at the best, we are not freed from manifold confusions and errors. Thus it is found by experiment that while the change of position of the sounding body may be soon noticed, the direction of the change may be thought to be the opposite of what it is in reality. Again, relatively loud sounds are located preferably in front of the head, even when their source is