Page:Popular Science Monthly Volume 21.djvu/52

44 Copy the picture, of exact size, by scratching through the varnish, and then blacken the lines with ink. Hold the transparent plate at the distance of a foot from your eyes, and through it look at a point about five feet away. Very little motion of the plate is needed to get this point exactly aligned with each of the dots within the circles by looking with each eye in succession. Look at the point now with both eyes, and you will see, suspended in the air, probably just beyond the plate, apparently a solid cone of glass pointing toward you, the very fac-simile of our glass cone from which the pictures were taken.

Copy the picture also on paper or card-board, of exact size, but with the part marked R transferred to the left, and that marked L to the right. Hold up the point of a pencil about half-way between your eyes and the card. In a moment the proper position is found, where it is aligned with R for the right eye and with L for the left. Open both eyes and converge them upon the pencil-point. A little cone pointing toward you is suspended in the air just beyond the pencil, which may now be withdrawn. Move your head from side to side: the cone moves with you. It is brilliantly lustrous, sharp in outline, and much smaller than that previously seen. Two companion circles, one on each side, are left behind on the card, and are larger than the base of the suspended cone, but a little smaller than the circles originally were. Their appearance is due to images of R and L which fall upon retinal parts that in normal vision could not be simultaneously impressed by



an external single body. The sensations produced by them are hence not suggestive of singleness, and each is therefore referred separately outward in the direction from which the rays producing them have come. Such side-images are perceived also when the glass plate is employed. Try the same experiment now with the picture on the page; the miniature cone leaps off the paper into the air, but this time it is hollow, for its vertex is pointed to the place from which it seems to have sprung.