In addition to the kinds of information which have been described thus far, that the distance senses bring in, there is another kind, fully as important as any in our actual use of our senses; that is information as to the “direction from us” of the object or objects which are arousing the sense. We can get this through all the distance senses, but much more perfectly in the case of sight than in the others. We locate the direction of objects that we smell by turning the head this way and that, sniffing meanwhile, and noting the position in which the odor is caught most clearly. Animals with a keen sense of smell, like dogs, can locate directions very accurately by this means. In the case of hearing the method is to turn the head until the sound is equally loud in both ears. We would expect that a person who was hard of hearing in one ear would never be able to locate sounds by this method; but, as a matter of fact, such persons unconsciously allow for the difference in hearing in the two ears, and so can judge the direction of sounds about as well as any of us. Animals, like horses or rabbits, that have very movable outer ears, undoubtedly can locate sounds much more accurately than we can. Our outer ears are of almost no use in hearing; persons who have had the misfortune to lose them hear practically as well as anyone.
We locate directions with the sense of sight with perfect accuracy, because unless the image of the object we are looking at falls on the center of the retina it is not seen clearly. The only way to make the image fall just there is to look directly at the object. The muscle sense in the eye muscles is extremely delicate, so that if the eyes are rolled at all in looking at anything we know it and can judge, also, how much they are turned from the straight position. In this way we are able to tell exactly the direction from us of any object we can see.
We can judge the distance of a near object very accurately by noting the degree to which the two eyes have to be turned in in order to see it clearly with both. We are quite unconscious of this means of making the judgment; all we know is that we can tell. It is easy to prove that it depends on the two eyes by closing one and trying to make movements that depend on accurate knowledge of distance. A good example is threading a needle sideways. With both eyes open this can be done fairly easily, but with one shut it cannot be done at all, except by chance. Objects so far away that the eyes are not turned in perceptibly in looking at them are judged as to distance wholly on the basis of their size. It is clear that the actual size of any image on the retina will depend in part on how large the object is, and in part on how far away it is. If an object that we know to be large casts a small image on the retina, we conclude that it is far away. It follows that unless there are some familiar objects in view, judgments of distance are not at all trustworthy. A good illustration is in looking up a bare hillside, and trying to estimate the distance to the top. If, while this estimation is being made, a man or horse suddenly comes into view at the top, the man or horse will nearly always appear unexpectedly large, showing that the top of the hill is not actually as far away as it was judged to be.
The possession of two eyes instead of one is an advantage to us in another way, in addition to helping in the estimation of near distances. This is in making objects appear solid, or in other words, in helping the estimation of depth. When we look about us we have no difficulty in realizing that some objects are near and others far, and that the objects themselves have some parts that are nearer to us than other parts. A great many things assist us in this realization. First and foremost comes that which is known in art as perspective, namely the tendency of distant objects or distant parts of objects to appear smaller than those that are near. This can best be illustrated in the case of parallel lines extending away from the eye, as when one stands on a straight railroad track and looks along it. Although we know perfectly well that the rails are the same distance apart all along, if we were to believe our eyesight implicitly we should think that they came gradually together. It is on account of this matter of perspective that drawings of solid objects must show supposedly parallel sides nearer together at the far end than at the near. Besides perspective there are the shadows to be taken into account. Only solid objects cast shadows, so if we see a shadow apparently cast by any object we naturally conclude that it is solid. Both perspective and shadows can be and are used by artists in making drawings and paintings look real. In fact, they have almost no other means of doing this. As we all know, even the cleverest paintings do not give an impression of depth equal to that which comes from actually looking at solid objects. This is because of help we get from the two eyes in the latter case. The reason for the difference is that when we look at a solid object with both eyes the view we get with one eye is not exactly the same as with the other; we see a little farther around on the left side with the left eye, and on the right side with the right eye. The combined view with the two eyes gives us an impression of solidity that cannot possibly be had when the view with the two eyes is exactly the same, as when we look at a picture. The only way in which the impression of depth can really be gotten in a picture is by using the familiar method of the stereoscope, where two pictures are taken simultaneously by two cameras, placed a little farther apart than the two eyes; and then the two are looked at together through a special pair of prisms.
By means of the three distance senses, smell, hearing, and sight, we are informed pretty completely as to what is around us. All three give an idea as to the direction from us of objects; although sight does this better than either of the others. Sight, also, lets us know accurately as to the distance away of objects, provided they are fairly near. Smell and hearing, as well as sight, may give us some idea as to the distance of far objects, but only when we are dealing with familiar sensations. A very faint smell, or faint sound, means a distant object provided we know enough about the source to know that if the object were near the sensations would be keener. Our judgment of distant objects by sight is better than this, but not by any means perfect. When we contrast the distance senses with the contact senses we see at once that the great advantage coming from the possession of distance senses is in the time that is permitted for action. In the chapter on contact senses we emphasized the fact that the response to them must be immediate; there is no time to pick and choose. When we can learn of the presence of objects before they reach us, and something of their direction and distance as well, we can usually take time to select the most fitting course of action. We do not have to jump into a mud puddle to escape an automobile if we see it soon enough. We shall learn in a later chapter how this opportunity of choice is wrapped up with our development into highly intelligent animals.