The masses that go to make up a picture have variety in their shape, their tone values, their edges, in texture or quality, and in gradation. Quite a formidable list, but each of these particulars has some rhythmic quality of its own about which it will be necessary to say a word.

Variety of Shape.

As to variety of shape, many things that were said about lines apply equally to the spaces enclosed by them. It is impossible to write of the rhythmic possibilities that the infinite variety of shapes possessed by natural objects contain, except to point out how necessary the study of nature is for this. Variety of shape is one of the most difficult things to invent, and one of the commonest things in nature. However imaginative your conception, and no matter how far you may carry your design, working from imagination, there will come a time when studies from nature will be necessary if your work is to have the variety that will give life and interest. Try and draw from imagination a row of elm trees of about the same height and distance apart, and get the variety of nature into them; and you will see how difficult it is to invent. On examining your work you will probably discover two or three pet forms repeated, or there may be only one. Or try and draw some cumulus clouds from imagination, several groups of them across a sky, and you will find how often again you have repeated unconsciously the same forms. How tired one gets of the pet cloud or tree of a painter who does not often consult nature in his pictures. Nature is the great storehouse of variety; even a piece of coal will suggest more interesting rock-forms than you can invent. And it is fascinating to watch the infinite variety of graceful forms assumed by the curling smoke from a cigarette, full of suggestions for beautiful line arrangements. If this variety of form in your work is allowed to become excessive it will overpower the unity of your conception. It is in the larger unity of your composition that the imaginative faculty will be wanted, and variety in your forms should always be subordinated to this idea.

Nature does not so readily suggest a scheme of unity, for the simple reason that the first condition of your picture, the four bounding lines, does not exist in nature. You may get infinite suggestions for arrangements, and should always be on the look out for them, but your imagination will have to relate them to the rigorous conditions of your four bounding lines, and nature does not help you much here. But when variety in the forms is wanted, she is pre-eminent, and it is never advisable to waste inventive power where it is so unnecessary.

But although nature does not readily suggest a design fitting the conditions of a panel her tendency is always towards unity of arrangement. If you take a bunch of flowers or leaves and haphazard stuff them into a vase of water, you will probably get a very chaotic arrangement. But if you leave it for some time and let nature have 187a chance you will find that the leaves and flowers have arranged themselves much more harmoniously. And if you cut down one of a group of trees, what a harsh discordant gap is usually left; but in time nature will, by throwing a bough here and filling up a gap there, as far as possible rectify matters and bring all into unity again. I am prepared to be told this has nothing to do with beauty but is only the result of nature's attempts to seek for light and air. But whatever be the physical cause, the fact is the same, that nature's laws tend to pictorial unity of arrangement.

Variety of Tone Values

It will be as well to try and explain what is meant by tone values. All the masses or tones (for the terms are often used interchangeably) that go to the making of a visual impression can be considered in relation to an imagined scale from white, to represent the lightest, to black, to represent the darkest tones. This scale of values does not refer to light and shade only, but light and shade, colour, and the whole visual impression are considered as one mosaic of masses of different degrees of darkness or lightness. A dark object in strong light may be lighter than a white object in shadow, or the reverse: it will depend on the amount of reflected light. Colour only matters in so far as it affects the position of the mass in this imagined scale of black and white. The correct observation of these tone values is a most important matter, and one of no little difficulty.