A four-dimensional cube-like solid if transparent and looked at with one eye would appear something like this. But it is obviously impossible to depict a four-dimensional figure on a two-dimensional surface like this page.--From “The Fourth Dimension Simply Explained,” Munn and Company, N. Y.] Since our two eyes present us slightly different pictures of an object we infer from these its size, shape and distance, but this is guesswork. Still we have a pretty clear idea of a cube although we have never seen it in its solidity. But the attempt to visualize the hypercube, the four-dimensional figure corresponding to the cube, strains our imagination to the breaking point. Some mathematicians endowed with constructive imaginations of high power claim to have got by long hard thinking some sort of a shadowy and fleeting perception of it, but their visions--if they are not imaginary--do not help out us ordinary folks. But if we cannot imagine--that is, image--the hypercube we know all about it, even its name. It is called the “tesseract,” and it is bounded by eight cubes just as the cube is bounded by six squares and the square by four lines. The tesseract has 24 square faces, 32 edges and 16 rightangular corners. TIME AS THE FOURTH DIMENSION Although we find it hard to conceive of a fourth dimension in space we have no such difficulty in case the fourth dimension is time. In fact, we use this idea all the while and could not get along without it. To fix the position of any event requires four dimensions. For instance, a man is shot. Where? At the corner of 7th Avenue and 42d Street, New York. This fixes the place by two coördinates crossing at right angles in a plane. But was it above or below this, on the twentieth floor of the Times Building or in the Subway? Knowing this fixes the third dimension, but we have still to fix its position in a fourth dimension, time. Was it today or last week and what hour? If then we find out all four we can distinguish this shooting from any that may have occurred in other places at the same time or at other times in the same place.