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, composition, etc.; and the practical means, processes, and materials through which and with which these are worked out. it is obvious that one cannot become a good painter without the ability to know what is good painting, and to prefer it to bad painting. therefore, i have taken space to cover, in some sort, the whole ground, as the best way to help the student towards becoming a good painter. if, also, the student of pictures should find in this book what will help him to appreciate more truly and more critically, i shall be gratified.