If we produce a compound hue by mixing together the colours of any portion of the spectrum, and a second compound hue by mixing the remainder of the spectrum, it must be evident that these two hues are necessarily complementary, for when they are united they contain together all the elements of the entire spectrum, and therefore appear as white. This may be illustrated with the aid of the colour-patch apparatus. Place at H (Fig. 3) a cardboard stencil of the form shown in Fig. 8, and focus upon it a little spectrum, the principal hues of which are indicated by the letters R O Y G B V (red, orange, yellow, green, blue, violet). The two oblong apertures in the card should be of exactly the same height, and the card so placed that one aperture may admit rays extending from the red end of the spectrum to about the middle of the green, while the other admits rays from the remainder of the spectrum. If now the lower aperture be covered, only the red, orange, yellow, and part of the green rays will pass through the stencil, and these being combined by the lens K (Fig. 3) will form upon the screen a bright patch, the colour of which will be yellow. If the upper aperture be covered, and the rest of the green, together with the blue and violet rays, allowed to pass through the other, the colour of the patch will become blue; and if both apertures be uncovered at the same time, rays from the whole length of the spectrum will pass through the stencil, and the patch will, of course, turn white. The yellow and the blue which were compounded from the two portions of the spectrum are, therefore, in accordance with the definition, complementary colours. 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. [Illustration: _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. [Illustration: _Fig. 10.--Slide for mixing any two Spectral Colours._]