On the bottom of the vessel, (see Plate,) A1BD, fig. 6, place three pieces of money, as a half-crown, a shilling, and a sixpence; the first at E, the second at F, and the third at G. Then let a person be placed with his eye at H, so that he can see no farther into the vessel than I; and tell him, that by pouring water into the vessel, you will make him see three different pieces of money, which he may observe are not poured in with the water.

For this purpose, desire him to keep himself steady in the same position, and, pouring the water in gently, that the pieces of money may not be moved out of their places, when it comes up to K, the piece G will become visible to him; when it comes up to L, he will see the two pieces G and F; and when it rises to M, all the three pieces will become visible: the cause of which is owing to the refraction of the rays of light, in their passage through the water; for while the vessel is empty, the ray HI will proceed in a straight line; but in proportion as it is filled with water, the ray will be bent into the several directions NG, OF, PE, and by these means the pieces are rendered visible.

A curious Experiment, of nearly the same kind as the last, called Optical Augmentation.

Take a large drinking-glass, of a conical figure, and having put a s lilling into it, fill the glass about half full with waterthen place a plate on the top of it, and turn it quickly over, so that the water may not get out. This being done, look through the glass, and you will now perceive a piece of money of the size of half-a-crown; and somewhat higher up, another piece of the size of a shilling. But if the glass be entirely filled with water, the large piece at the bottom only wil be visible.

This phenomenon is occasioned by your seeing the piece through the conical surface of the water, at the side of the glass, and through the flat surface at the top of the water, at the same time; for the conical surface dilates the rays, and makes the piece appear larger, while the flat surface only refracts them, and occasions the piece to be seen higher up in the glass, but still of its natural sire.