dimensions) and one time axis of reference, they are referred to a set of four interchangeable axes involving four homogeneous co-ordinates, three of space and one of time. Time, in other words, is used by them as though it were a dimension of space—the fourth dimension.
Professor N. A. Oumoff, in his address before the second Mendeleevsky Convention, dealt with this idea in the following clever way:
"The element of time is involved in all spatial measurements. We cannot define the geometrical form of a solid moving in relation to us, we are always defining its kinematical form. Therefore our spatial measurements are in reality proceeding not in a three-dimensional manifold, that is, having three dimensions of height, length, and width, but in a four-dimensional manifold. The first three dimensions we can represent by the divisions of a tape-measure upon which are marked feet, yards, or some other measure of length. The fourth dimension we shall represent by a moving-picture film upon which each point corresponds to a new phase of the world's phenomena. The distances between the points of this film are measured by a clock going 1indifferently with this or that velocity. The transition from one point to another of this film corresponds to our concept of the flow of time. This fourth dimension we therefore call time. The moving-picture film can replace the reel of any tape-measure, or the tape-measure can replace the film. The ingenious mathematician Minkowsky proved that all these four dimensions are equipotent and interchangeable."
If time is space, if it is the measure of a direction at right angles to every direction that we know, why is this so difficult to realize in experience?
It is because we cannot apprehend the higher space-world as we do this one (its three-dimensional section), in its entirety as one takes in a landscape at a glance, but only fragment by fragment, that is, successively. If you look at the landscape through the slit formed by the two hands, moving them circle-wise so that everything is seen piecemeal and successively, and if you utterly absorb yourself, limiting your mind, just as your vision is limited, you get finally the sense that the landscape is perpetually becoming; that what has just
