122. Up to this point I have avoided the consideration of Tones and
Tone-Relations. I have spoken of possible changes of tone in dots and
in lines; changes of value, of color, of color-intensity; but it is not
in dots nor in lines that these changes call for particular attention.
Our interest has been in the positions, measures, shapes, and
attitudes of dots and lines, and in the possibilities of arrangement
and composition. When it comes to the consideration of areas and
area-systems, however, the subject of tone-relations becomes one of the
greatest interest, because areas are defined and distinguished, not
only by their outlines, but quite as much by differences of tone; that
is to say, by tone-contrasts.