ABSTRACT
—In the first place it is necessary to premise that the term purple has, at various periods in the world’s history, been applied to various colours more or less red. Strictly speaking, it should be restricted to the pure secondary colour of the prismatic spectrum composed of pure red and pure blue, in which blue is in greater proportion than it is in violet, which may be considered as composed of equal quantities of pure prismatic blue and red. The action of light develops the three primary colours in the following order:- yellow, blue, red. Between these one finds green resulting from the mixture of yellow with blue, and violet resulting from mixture of blue with red. In making the experiment slowly — that is, in diffused light — the succession of colours may be distinctly observed. But while the yellow disappears when the action of light is prolonged, the blue always remains in notable quantity.
