We can commonly tell whether an object is solid, readily enough with one eye, but still better with two eyes, and sometimes only by using both.  If we look at a square piece of ivory with one eye alone, we cannot tell whether it is a scale of veneer, or the side of a cube, or the base of a pyramid, or the end of a prism.  But if we now open the other eye, we shall see one or more of its sides, if it have any, and then know it to be a solid, and what kind of a solid.

We see something with the second eye which we did not see with the first; in other words, the two eyes see different pictures of the same thing, for the obvious reason that they look from points two or three inches apart.  By means of these two different views of an object, the mind, as it were, feels round it and gets an idea of its solidity.  We clasp an object with our eyes, as with our arms, or with our hands, or with our thumb and finger, and then we know it to be something more than a surface.  This, of course, is an illustration of the fact, rather than an explanation of its mechanism.

Though, as we have seen, the two eyes look on two different pictures, we perceive but one picture.  The two have run together and become blended in a third, which shows us everything we see in each.  But, in order that they should so run together, both the eye and the brain must be in a natural state.  Push one eye a little inward with the forefinger, and the image is doubled, or at least confused.  Only certain parts of the two retinae work harmoniously together, and you have disturbed their natural relations.  Again, take two or three glasses more than temperance permits, and you see double; the eyes are right enough, probably, but the brain is in trouble, and does not report their telegraphic messages correctly.  These exceptions illustrate the every-day truth, that, when we are in right condition, our two eyes see two somewhat different pictures, which our perception combines to form one picture, representing objects in all their dimensions, and not merely as surfaces.

Now, if we can get two artificial pictures of any given object, one as we should see it with the right eye, the other as we should see it with the left eye, and then, looking at the right picture, and that only, with the right eye, and at the left picture, and that only, with the left eye, contrive some way of making these pictures run together as we have seen our two views of a natural object do, we shall get the sense of solidity that natural objects give us.  The arrangement which effects it will be a stereoscope, according to our definition of that instrument.  How shall we attain these two ends?