Let us assume that the three dimensions of space are visualized in the customary fashion, and let us substitute a color for the fourth dimension. Every physical object is liable to changes in color as well as in position. An object might, for example, be capable of going through all shades from red through violet to blue. A physical reaction between any two bodies is possible only if they are close to each other in space as well as in color. Bodies of different colors would penetrate each other witout interference ... If we lock a number of flies into a red glass globe, they may yet escape: they may change their color from red to blue and are then able to penetrate the red globe.