PIIANTASCOPE. A curious instru ment invented by Prof. John Locke, which will illustrate, in a manner never before accomplished, "single vision by each eye." It is very simple, and has neither lenses, prisms, nor reflectors. It consists of a flat hoard base, about nine by eleven inches, with two upright rods, one at each end, a horizontal strip con necting the upper ends of the uprights, and a screen or diaphragm, nearly as largo as the base, interposed between the top strip and the tubular base, this screen being adjustable to any intermediate height. The top strip has a slit one fourth of an inch wide, and about three inches long from left to right. The ob server places his eyes over this slit, look ing downward. The movable screen has also a slit of the same length, but about an inch wide. If there are two identical pictures of a flower, about one inch in di ameter, placed the one to the left and the other to the right of the centre of the ta bular base, or board forming the support, and about two and a half or three inches apart from centre to centre. A flower pot or vase is painted on the upper screen, at the centre of it as regards right and left, and with its top even with the lower' edge of the open slit. By looking down ward through the upper slit, and direct ing both eyes steadily to a mark, a quasi stem, in the flower pot or vase—instantly a flower similar to one of those on the lower screen, but of half the size, will ap pear growing out of the vase, and in the open slit of the moveable screen. On directing the attention through the upper screen to the base, this phantom flower disappears, and only the two pictures on each side of the place of the phantom re main. The phantom itself consists of the
two images painted on the base, optically superimposed on each other. If one of these images be red and the other blue, thephantom will be purple. If two iden tical figures of persons be placed at the proper positions on the lower screen, and the upper screen be gradually slid up from its lowest point, the eye being di rected to the index, each image will at first be doubled, and will gradually re cede, there being of course four in view until the two contiguous coincide, when three only are seen. This is the proper point where the middle or double image is the phantom seen in the air. If the screen be raised higher, then the middle images pass by each other, and again four are seen receding more and more as the screen is raised.
As all this is the effect of crossing the axes of the eyes, it follows that a person with only one perfect eye cannot make the experiments. They depend on binoc ular vision.
All these effects depend on the princi ple that one of the two primitive pictures is seen by one eye, and the other by the other eye, and that the axes are so con verged by looking at the index or mark on the upper screen that those separate images fall on the points in the eye, which produce single vision. To a person who has perfect voluntary control over the axes of his eyes, the upper screen and in dex are unnecessary. Such an observer can at any time look two contiguous per sons into one, or superimpose the image of one upon the image of the other.