as seen in fig. 18, in which bfea is the spectrum of the slit ab, and ccfd that of the slit cd; the coloured spectra arc contained in the triangle gcf, and, - by arrangement, the effects of mixture of any two simple colours may be observed. (6) By Method of Reflection. — Place a red wafer on b, in fig. 19, and a blue wafer on d, and Fig. Ia e y -Diagram of Double Spectrum partially superposed SO angle a small glass plate a as to transmit to the eye a Fig. 19. — Diagram showing Lambert's Method of mixing Sensations of Colour.

60° 45-5