518 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS SUBJECTIVE COLOUR PHENOMENA Having sketched some of those branches of colour science which might possibly be of use in cosmetics, it is only fair to balance the account by mentioning a few colour phenomena which, for the present, lie outside the realm of colour measurement. Some are subtle, some are startling, they nearly all occur when small patches of colour are viewed in close association with larger areas which contrast strongly in colour or luminance. One of the difficulties in the way of putting forward a universal theory of these effects is that some are apparently contradictory. The first effect to be noted is the so-called "small-field tritanopia", or, in simpler terms, a relative insensitivity to blue in a small patch when directly fixated as compared with a much larger patch of the same physical 2 o quality («ø versus or over, subtended at the eye). This effect is not of great interest in patterns of surface colours. Of much wider occurrence is the contrast effect in which the appearance of a small patch may be greatly modified by being in close association with a larger patch. For example, a small patch of grey will look lighter if sur- rounded by black, darker if surrounded by white. The same applies to small coloured patches, which show, in some cases, a slight change in hue as well. If the large surrounds are coloured, but there are no great differences in relative brightness, then modifications in colour of the small patches become noticeable (Fig. 14). These effects may all be regarded as aspects of adaptation controlled by the large fields: the small patches have to fall into line with the larger. For instance, a middle grey is light compared with a black and will appear so when the eye's vision is dominated by black on the other hand, if the domination is by white, the grey will appear dark. The colour effects from simple combinations of large and small fields may be explained along the same lines. There is, however, a special group of effects in which narrow lines of one colour are closely spaced on a general field of another colour. Black lines darken the general background colour, white lines lighten it. Coloured lines drag the background colour towards their own colour. This phenomenon, known as the yon Bezold effect (Fig. 15), is apparently in contradiction to the contrast effects. It is possible that it may be explained by scattering in the retina, in the eye media in front of the retina, or from part to part of the retina, whereas the contrast effects are more likely to be motivated at higher levels in the brain. The importance of the higher levels in the per-

Figttre 14 The effect of colour contrast ou colour appearance. The green areas are identical physically, but "repelled" by the colours of the surrounds. Figure 1.5 The you Bezold effect of thin liuesofcolour. The greeuareas are identical physically, but are "attracted" by the colours of the lines. Facing page 518