It is impossible to understand principles without some statement of theory; and a book in order to be practical must therefore be to some extent theoretical. I have been as concise and brief in the theoretical parts as clearness would permit of, and I trust they are not out of proportion to the practical parts.

Either to paint well, or to judge well of a painting, requires an understanding of the same things: namely, the theoretical standpoint of the painter; the technical problems of color, ...