which the suggested colours appear in the neighbourhood of others: we allude to coloured shadows. To arrive at these we first turn our attention to shadows that are colourless or negative.
63.
A shadow cast by the sun, in its full brightness, on a white surface, gives us no impression of colour; it appears black, or, if a contrary light (here assumed to differ only in degree) can act upon it, it is only weaker, half-lighted, grey.
64.
Two conditions are necessary for the existence of coloured shadows: first, that the principal light tinge the white surface with some hue; secondly, that a contrary light illumine to a certain extent the cast shadow.
65.
Let a short, lighted candle be placed at twilight on a sheet of white paper.