specially worthy of the painter's attention. One practical difficulty is that the colour appears in a darkened room, and can therefore only be compared with other tints produced in a similar way, and not with local colour of objects placed near the screen. But a special arrangement for comparison of the local colour of particular objects can be made by throwing detached beams of white light from an arc-lamp upon them. The two narrow bands or strips of pure white light usually projected upon the screen at each side of the colour-field also give a standard of comparison between the colours upon it and absolute white, and enable the depth and intensity of low tones to be better appreciated. If no such bands of light are used, low-toned colours, by reason of their being surrounded by darkness, are apt to appear too luminous, and their full strength cannot be realized. Somewhat the same conditions exist in stained glass windows in a dark church, and it was probably in order to enable the beauty of the sombre colours which the great stained glass designers of the Middle Ages often used that they frequently sur-
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