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150 THE ADVANCE OF PHOTOGRAPHY
seem to be absolutely alike, when looked at through the instrument form one picture, which appears no longer plane but solid.
The two pictures which are seemingly alike are, in fact, different. If we look at a cube with the right eye, we see rather more of the right side ; if we look with the left eye, we see something more of the left side, taking for granted that the head is not moved. The pictures of the right and of the left eyes are combined with each other and give the impression of solidity.
If we close one eye, the impression of solidity is far weaker ; the objects appear plane or flat. This may not be readily credited, because men do not often seek an explanation of what they see, but look at objects much too hastily. But it can be easily ascertained that such is the fact if a bottle be placed before a wall, or in front of an upright book. If we then look at them with both eyes, we readily perceive the distance of the bottle from the wall or book, but directly we close one eye the bottle and the book appear to be almost contiguous, and it is only by moving the head on one side that we clearly distinguish the distance between them.
Accordingly, the use of both eyes is necessary for a perception of solidity. It is only in this manner that we come to the conviction that space has not only height and breadth, but also depth. One-eyed persons only receive this impression by turning the head on one side. If objects are very remote, the difference between the views which the right eye and the left eye have of them is very inconsiderable ; and, accordingly, such remote objects appear flat and without solidity, and it is only when we change our position and observe them from different sides that we become acquainted with their solidity. This is therefore purely an affair of experience. Every person will recognize a distant house as a solid object, because we know from experience that a house is solid ; but that we