The best way to train one's sense of colour is to dabble in landscape painting. At first, one feels that there must be a personal Devil, but with luck the colours begin to clear, showing that the tones of night and the deep sea are based on indigo, while those of the day are blue, red and yellow variously mixed. The blend of blue and red is violet; the mixture of blue and yellow gives us green; and if we want an orange we use red and yellow. The blending of all seven is sunlight in theory, but makes mud in practice. {12} In nature there are permanent colours like those of the night, and transient hues like those of the sunrise or sunset. So the blue of the sky and yellow of the earth make the green of living plants which seems to be permanent until, in decay, the blue turns out to be transient, and passes away leaving the herbage yellow. It is odd that the natural food of the horse is dried herbage from which the blue has faded. And so it is with man. We may eat green salads, containing transient blue, but the permanent colours of our food are free from blue, and based on red and yellow. Neither horse nor man would fatten on blue food. [Sidenote: Magic of sunlight] Sunlight shining through blue glass will stop the growth of plants. The various actions of coloured light upon the human body are being studied in many hospitals. [Sidenote: Climate and colour] The blue indigo and violet, or actinic rays, appear to have a special mission in burning bad microbes, such as the germs of disease. A green forest, for example, despite the permanent yellow in its colour, is said to be partly transparent to these rays which kill germs lurking in the soil. The flesh of men and beasts is red and yellow, save only for the blue tinge of blood from which the oxygen has {13} been exhausted. Yet even despite its colouring, the tissue of the flesh is partly transparent, so that actinic rays may kill bacilli, and sunshine is used as a medicine for the sick. But the rays which begin by killing germs may be strong enough in time to burn the living tissues. For that reason man and the greater animals are armoured by red and yellow liquid paints in the layers of skin, which vary in strength and volume with the degree of sunlight in each climate, from pale hues in cloudy districts of low sun to an intense black in the tropics. Stocks native to forest shelter such as men, elephants and pigs are guarded only with skin body colour. Those exposed to direct light--horses, cattle and sheep, have also a coat of hair as a second armour against the actinic rays, and this also varies in colour with the strength of sunshine, from white in the regions of snow to the golden dun of lions and tigers, the dun and bay of horses and the black of many species in regions of strong light.