What has been said about unity of line applies obviously to the outlines bounding the masses, so that we need not say anything further on that subject. The particular quality of which something should be said, is the unity that is given to a picture by means of a well-arranged and rhythmically considered scheme of tone values.

The modifications in the relative tone values of objects seen under different aspects of light and atmosphere are infinite and ever varying; and this is quite a special study in itself. Nature is the great teacher here, her tone arrangements always possessing unity. How kind to the eye is her attempt to cover the ugliness of our great towns in an envelope of atmosphere, giving the most wonderful tone symphonies; thus using man's desecration of her air by smoke to cover up his other desecration of her country-side, a manufacturing town. This study of values is a distinguishing feature of modern art.

But schemes taken from nature are not the only harmonious ones. The older masters were content with one or two well-tried arrangements of tone in their pictures, which were often not at all true to natural appearances but nevertheless harmonious. The chief instance of this is the low-toned sky. The painting of flesh higher in tone than the sky was almost universal at many periods of art, and in portraits is still often seen. Yet it is only in strong sunlight that this is ever so in nature, as you can easily see by holding your hand up against a sky background. The possible exception to this rule is a dark storm-cloud, in which case your hand would have to be strongly lit by some bright light in another part of the sky to appear light against it.

This high tone of the sky is a considerable difficulty when one wishes the interest centred on the figures. The eye instinctively goes to the light masses in a picture, and if these masses are sky, the figures lose some importance. The fashion of lowering its tone has much to be said for it on the score of the added interest it gives to the figures. But it is apt to bring a heavy stuffy look into the atmosphere, and is only really admissible in frankly conventional treatment, in which one has not been led to expect implicit truth to natural effect. If truth to natural appearances is carried far in the figures, the same truth will be expected in the background; but if only certain truths are selected in the figures, and the treatment does not approach the naturalistic, much more liberty can be taken with the background without loss of verisimilitude.

But there is a unity about nature's tone arrangements that it is very difficult to improve upon; and it is usually advisable, if you can, to base the scheme of tone in your picture on a good study of values from nature.

Such effects as twilight, moonlight, or even sunlight were seldom attempted by the older painters, at any rate in their figure subjects. All the lovely tone arrangements that nature presents in these more unusual aspects are a new study, and offer 202unlimited new material to the artist. Many artists are content to use this simply for itself, the beauty of a rare tone effect being sufficient with the simplest accessories to make a picture. But in figure composition, what new and wonderful things can be imagined in which some rare aspect of nature's tone-music is combined with a fine figure design.

These values are not easily perceived with accuracy, although their influence may be felt by many. A true eye for the accurate perception of subtle tone arrangements is a thing you should study very diligently to acquire. How then is this to be done? It is very difficult, if not impossible, to teach anybody to see. Little more can be said than has already been written about this subject in the chapter on variety in mass. Every mass has to be considered in relation to an imagined tone scale, taking black for your darkest and white for your highest light as we have seen. A black glass, by reducing the light, enables you to observe these relationships more accurately; the dazzling quality of strong light making it difficult to judge them. But this should only be used to correct one's eye, and the comparison should be made between nature seen in the glass and your work seen also in the glass. To look in a black glass and then compare what you saw with your work looked at direct is not a fair comparison, and will result in low-toned work with little brilliancy.

Now, to represent this scale of tones in painting we have white paint as our highest and black paint as our lowest notes. It is never advisable to play either of these extremes, although you may go very near to them. That is to say, there should never be pure white or pure black masses in a picture. There is a kind of screaminess set up when one goes the whole gamut of tone, that gives a look of unrestraint and weakness; somewhat like the feeling experienced when a vocalist sings his or her very highest or very lowest note. In a good singer one always feels he could have gone still higher or still lower, as the case may be, and this gives an added power to the impression of his singing. And in art, likewise, it is always advisable to keep something of this reserve power. Also, the highest lights in nature are never without colour, and this will lower the tone; neither are the deepest darks colourless, and this will raise their tone. But perhaps this is dogmatising, and it may be that beautiful work is to be done with all the extremes you can "clap on," though I think it very unlikely.

In all the quieter aspects of lighting this range from black to white paint is sufficient. But where strong, brilliantly lit effects are wanted, something has to be sacrificed, if this look of brilliancy is to be made telling.