No. XI.—AN ILLUSION WITH COINS
If you place four coins in the positions shown at the top of this diagram, and attempt, or challenge some one to attempt, without any measuring, to move the single coin down in a straight line until the spaces from C to D on either side exactly equal the distance from A to B—
It must drop as far as is shown here, which seems to the unaided eye to be too far.
This excellent illusion can be shown as an after-dinner trick with four napkin-rings.
No. XII.—THE FICKLE BARREL
Here is another excellent optical illusion. Look attentively at the diagram below, and notice in which direction you apparently look into it, as though it were an open cask.
Now shake the paper, or move it slightly, and you will find, more often than not, that you seem to see into it in quite the opposite direction.