When the experiment is not nicely made, the wafers will only seem to approach to each other, or, perhaps, the edges will seem to overlap.

This, we think, proves the position very clearly; for there is no unnatural medium looked through, and the external light is only cut off from distracting the sight. Moreover, when the tubes are dark, they will appear only as one.

Experiment 2.-Take two very straight thin slips of wood, about a foot in length, of different colours, as yellow and blue; place them on a level table at the height of the eyes, and at the distance of the eyes from each other; look attentively with each eye along each slip of wood, and only one slip will be seen, apparently occupying the middle space, at an equal distance from each, and, as in the former experiment of the two colours, green and yellow transparently intermingled.

This is even a less artificial proof than in the case of the wafers; for there is no exclusion of the light, and no foreign medium of sight employed. When the experiment is imperfectly performed, the slips of wood will only appear as one, at the ends farthest from the eyes.

Experiment 3.-Two silk strings may be fixed on a table, in the same way as the slips of wood, and they will, as in the former case, appear only as one, in a line with the nose, exactly perpendicular to the centre of the eye-brows.

Experiment 4.-The last experiment may be varied, by taking three silk threads, of different colours, as blue, yellow, and red, and fixing them by a pin on a level board, the height of the eye; then bring two of them to the edge of the board, one to each eye, and the third in the middle space, so as to bisect the angle formed at the pin; look steadily at the two threads opposite to the eyes, and they will appear as one, and their colours transparently intermingled in the lines where the middle thread really is. The middle thread, also, will not appear in its place, but will seem as two threads in the real place of the two outer threads, with colours, also, intermingled.

Experiment 5.-Take two pins, or any two things equally small and alike, and fix a third in a board, at the height of the eye. Hold one of the pins before the right eye, so as to cover the pin in the board, and, without moving the head, hold the other pin before the left eye, so as to cover the pin in the board. Look, now, steadily at all the three pins, and they will appear as one. This experiment requires nicety of management, but it is very satisfactory, as a proof of the position.

These experiments are more than sufficient to satisfy any candid observer, as to objects not appearing exactly where they are, in respect of the right and the left.

Experiment 6.-Take a card, or piece of paste

board, large enough to cover both eyes, and drill in it a small hole oposite to each eye, thus :

Look through these holes at a candle, or any luminous object, and, instead of seeing two holes in the card, as you might suppose, you will see, when the card is close to the eyes, only one hole, near the middle of the card, thus:

When the card is held at a short distance from the eyes, you will see three holes, thus:

Now, the conclusion cannot be resisted, that as the candle appears actually behind the middle of the card, where there is no hole, that it is seen greatly nearer to the eye than its true position. The experiments with cards may be varied, thus:

Experiment 7.-With only one hole in a card, look at two candles placed on a table, at the distance of the eyes from each other. Instead of one hole, you will see two, and the two candles. Look steadily at the hole, and only in a confused manner at the candles, and you will see the hole single as it really is,

but you will see only one candle compounded of the two, as you may prove by shading them with coloured glass.

Experiment 8.-Except an object be on a level with the eyes, it will not appear perpendicular, although it is so. The proof of this is confirmed, by suspending a hair or a thread perpendicular to the horizon, and looking stedfastly at a point in it, at the height of the eye, at the same time glancing the eye up and down. The hair will appear double in every part, except the point at the height of the eye.

Experiment 9.-Analogous to this is the experiment of lying down on the back, under a high wall or tower, and looking backwards and upwards to the summit, when it will seem to bend over the spectator, as if about to fall upon him.

PERSPECTIVE.

These experiments prove satisfactorily, as we think, that most, if not all, the things which we see around us appear to be where they really are not. Observations, which are familiar to every one, confirm the same singular conclusion. For example, in standing at the end of a long street, or of a double row of trees, the two most distant extremities seem to meet, or, at all events, to be greatly nearer than where we are standing. A tree, also, which is near us, appears much higher than a lofty mountain at a distance. The distant parts, likewise, of a long wall,

appear to a spectator, at one end of it, to curve towards him. In short, all the principles which constitute the science of Perspective are so many proofs of the position.

VISIBLE MOTION.

There are similar deceptions of sight, if we may call them so, with respect to the motion and rest of objects, several instances occurring, according to circumstances, in which objects at rest seem to be in motion, and the contrary.

Experiment 10.-Look with the right eye, the left being closed, at any remote object, such as a distant spire, through a small hole in a card. After steadfastly looking in this manner for some time, look suddenly at the hole, and the spire will appear to move from left to right. If you use the left eye, it will appear to move from right to left. Again, look suddenly to the spire, and it will seem to move back to the place it appeared in at the beginning of the experiment.

On a similar principle it is, that the moon appears to move along the heavens in an opposite direction to the clouds, which are driven by the winds over her apparent disk; and that the banks of a river seem to move in an opposite direction to those who are sailing on the river. It seems to be, because the muscles of the eye move it, or at least make the effort to move it, in the direction of the apparent