§ 9. The Effects of Contrast.—A man passing a streetlamp in moonlight casts two shadows. That which is cut off from the light of the lamp and only illuminated by the moon, appears blue. Now, moonlight is white or nearly so. The blue appearance of the shadow is due to contrast with the yellow illumination thrown by the lamp on the surrounding field of view. The excitement of the retina by the yellow light indirectly affects that portion of the retina or of the central nervous matter which is not directly excited by it. The influence thus exerted by the yellow light produces an effect similar to that which would be produced by a blue light acting directly. Now blue is complementary to yellow. The general law of contrast is that a colour in any part of the field of view tends to tinge adjoining parts with its complementary colour. The effect is greatest when a large field of uniform colour acts on a small one. A small spot of grey on a relatively extensive field of blue, appears distinctly yellowish. If a small spot of red be substituted for the grey, it will combine its own colour with the contrast colour. It will appear yellowish red or reddish yellow. The effect of contrast is most marked at the meetingpoint of the two colours. It is interfered with by lines of demarcation separating them, such as a pencilmark drawn round the red spot on the blue field. It is also interfered with by differences in the texture of the coloured surfaces. For these reasons, it comes out most clearly when contours are obliterated, and differences of texture reduced to a minimum. The most favourable conditions are obtained in the case of coloured shadows, or by projecting the light from coloured glasses on a wall, or by means of coloured disks in rapid rotation with the colours in concentric zones. A simple method is to place a small piece of paper on a larger sheet, and to cover both with a sheet of tissue paper. The tissue paper obliterates contours and conceals difference of texture. The contrast effect is of course in general stronger in proportion as the direct excitation of the part of the retina affected by it is weaker; thus grey is better to experiment with than white. The influence of contrast is also operative between black and white. The same grey will appear darker on a white background, and lighter on a black background. If contrasted colours are complementary to each other, the contrast renders them more saturated.