Fig. 77.
Take a square, one of those shown in Fig. 77 and give it a neutral colour, let this colour be called “null,” and be such that it makes no appreciable difference to any colour with which it mixed. If there is no such real colour let us imagine such a colour, and assign to it the properties of the number zero, which makes no difference in any number to which it is added.
Above this square place a red square. Thus we symbolise the going up by adding red to null.
Away from this null square place a yellow square, and represent going away by adding yellow to null.
Fig. 78.
To complete the figure we need a fourth square. Colour this orange, which is a mixture of red and yellow, and so appropriately represents a going in a direction compounded of up and away. We have thus a colour scheme which will serve to name the set of squares drawn. We have two axes of colours—red and yellow—and they may occupy as in the figure the direction up and away, or they may be turned about; in any case they enable us to name the four squares drawn in their relation to one another.
Now take, in Fig. 78, nine squares, and suppose that at the end of the going in any direction the colour started with repeats itself.
We obtain a square named as shown.
Let us now, in [fig. 79], suppose the number of squares to be increased, keeping still to the principle of colouring already used.