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.

From this experiment, the use of having two eyes may be easily perceived; for he that has only one can never see three objects placed in this position; or all the parts of one object, of the same extent, without altering the situation of his eye.