In a similar manner by dividing the spectrum into any two portions whatever—as, for example, by the complicated stencil shown in [Fig. 9]—we can obtain an indefinite number of pairs of complementary colours.
Fig. 9.—Stencil Card for Complementary Colours.
But it is by no means indispensable that both or either of a pair of complementary colours should be compound. To prove this, two strips of card with narrow vertical openings A and B are prepared as shown in [Fig. 10]. The cards are placed one above the other and can be slipped in a horizontal direction, so that the narrow openings can be brought into any desired part of the spectrum which is indicated in outline by the dotted oblong.
Fig. 10.—Slide for mixing any two Spectral Colours.
Bring the opening A of the upper card into the yellow of the spectrum and the opening B of the lower card into the blue. The bright patch formed upon the screen will then be illuminated by simple blue and yellow rays; yet it will be white—not green, as it would be if Brewster’s theory were correct. If upon the first trial the white should not be absolutely pure, it can easily be made so by partially covering either A or B—the first if the white is yellowish, the second if it is bluish. Simple spectral blue and yellow are therefore no less truly complementary colours than are the compound hues formed when the spectrum is divided into two parts.
It is noticeable, however, that the white light resulting from the combination of blue and yellow, though it cannot be distinguished by the eye from ordinary white light, is yet possessed of very different properties. Most coloured objects when illuminated by it have their hues greatly altered; a piece of ribbon, for example, which in common light is bright red, will appear when held in the blue-yellow light to be of a dark slate colour, almost black.