English: Various methods have been adopted for studying the effect of mixing colours. This may be done in a simple way by having a slit in the form of the letter V (see EB1911 Vision - Form of Double Slit for Partial Superposition of Two Spectra.jpg), of which the two portions ab and bc form a right angle; behind this slit is placed a vertical prism, and two spectra are obtained, as seen in this figure, where bfea is the spectrum of the slit ab, and cefd that of the slit cd; the coloured spectra are contained in the triangle gef, and by arrangement, the effects of mixture of any two simple colours may be observed.