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. CHAPTER X THE NERVOUS SYSTEM AND SIMPLE NERVOUS ACTIONS