| it soon, never trust me for the time | to come. |
A curious Experiment, which depends on an Optical Illusion.
On the bottom of the vessel, (see Plate,) AIBD, 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 shilling into it, fill the glass about half full with water; then 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 will 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 size.
Another curious Experiment, called Optical Subtraction.
Against the wainscot of a room fix three small pieces of paper, as A, B, C, fig. 7, (see Plate,) about a foot and a half or two feet asunder, at the height of your eye; and placing yourself directly before them, about five times the distance from them that the papers are from each other, shut one of your eyes and look at them with the other, and you will then see only two of those papers, suppose A and B; but altering the position of your eye, you will now see the third, and one of the first, suppose A; and by altering its position a second time, you will see B and C, but in neither case all three of them together.
The cause of this phenomenon is, that one of the three pencils of rays, which come from these objects, falls on the optic nerve at D, whereas, to produce distinct vision, it is necessary that the rays of light fall on some part of the retina E, F, G, H.