If we wish to denote the upper yellow line BD, fig. 81, we can speak of it as the yellow γ line, meaning the yellow line which is separated from the primary yellow line by the red movement. In a similar way each of the other squares has null points, red and yellow lines. Although the yellow square is all yellow, its line CD, for instance, can be referred to as its red line. This nomenclature can be extended. If the eight cubes drawn, in fig. 82, are put close together, as on the right hand of the diagram, they form a cube, and in them, as thus arranged, a going up is represented by adding red to the zero, or null colour, a going away by adding yellow, a going to the right by adding white. White is used as a colour, as a pigment, which produces a colour change in the pigments with which it is mixed. From whatever cube of the lower set we start, a motion up brings us to a cube showing a change to red, thus light yellow becomes light yellow red, or light orange, which is called ochre. And going to the right from the null on the left we have a change involving the introduction of white, while the yellow change runs from front to back. There are three colour axes—the red, the white, the yellow—and these run in the position the cubes occupy in the drawing—up, to the right, away—but they could be turned about to occupy any positions in space. [Illustration: Fig. 82.] [Illustration: Fig. 83. The three layers.] We can conveniently represent a block of cubes by three sets of squares, representing each the base of a cube. Thus the block, fig. 83, can be represented by the layers on the right. Here, as in the case of the plane, the initial colours repeat themselves at the end of the series. Proceeding now to increase the number of the cubes we obtain fig. 84, in which the initial letters of the colours are given instead of their full names. Here we see that there are four null cubes as before, but the series which spring from the initial corner will tend to become lines of cubes, as also the sets of cubes parallel to them, starting from other corners. Thus, from the initial null springs a line of red cubes, a line of white cubes, and a line of yellow cubes. If the number of the cubes is largely increased, and the size of the whole cube is diminished, we get a cube with null points, and the edges coloured with these three colours. [Illustration: Fig. 84.] The light yellow cubes increase in two ways, forming ultimately a sheet of cubes, and the same is true of the orange and pink sets. Hence, ultimately the cube thus formed would have red, white, and yellow lines surrounding pink, orange, and light yellow faces. The ochre cubes increase in three ways, and hence ultimately the whole interior of the cube would be coloured ochre. We have thus a nomenclature for the points, lines, faces, and solid content of a cube, and it can be named as exhibited in fig. 85. [Illustration: Fig. 85.]