Against the wall of a room fix three small pieces of paper, about the size of a sixpence; let them be about half a foot asunder, and the height of the eye; stand about a yard distant, and, keeping both eyes steadfastly fixed on the centre piece, all three pieces are visible. Now shut the right eye (keeping the left still on the centre), and the piece which is opposite to the left eye disappears; or close the left eye, and the right piece cannot be seen: so that if either eye be shut, the paper opposite its fellow becomes invisible; plainly proving, that some objects opposite the left eye are viewed by the right, and vice versâ, with the left eye closed, and the right piece consequently invisible; remove the right eye from the centre, and carry it to the piece on the left; the right piece now becomes visible, but the centre disappears; and so on alternately, the three pieces not being visible at the same time, as when both eyes are open, showing one of the uses of having two eyes.
VARIATION.
Another method of trying this experiment is by holding both the thumbs together at a little distance from, and at the height of the eyes: shut the left eye, and keep the right steadfastly fixed on the left thumb-nail; move the right thumb gently away in a horizontal direction, and at the distance of two or three inches, the top of the thumb disappears; but by carrying it a little further, it becomes visible again.
COLOURED GLASSES IN FOGS.
When there is a fog between two places, so that the one station can with difficulty be seen from the other, if the observer passes a coloured glass between his eye and the eye-piece of his telescope, the effect of the fog is very sensibly diminished, so that frequently signals from the other station can be very plainly perceived, when, without the coloured glass, the station itself could not be seen. The different colours do not at all produce this effect in the same degree. The red seems the most proper for the experiment. Those who have good sight prefer the dark red: those who are short-sighted like light red better.
JAPANESE MYSTIC MIRRORS.
The bronze looking-glasses of Japan have the curious property of showing in their reflection of a strong light on a screen, not only their own polished surface, but the figures on the back.
These figures are fashioned by striking them out; they are then ground down till the raised metal is levelled to the deepest indentation, when the pattern is stamped once more, the face again ground, and the operation repeated.
The alteration in the metal where this compression has visited it, is not perceptible even with a magnifying glass, but in reflection it is shown.