This is a translation of work which first appeared in 1816 in Germany. Although Schopenhauer himself acknowledged that the treatise did not present any new and significant doctrines to his philosophy, he nonetheless considered it important enough to publish it again in revised form toward the end of his life, in 1854. As Professor Cartwright argues in his introduction, the book's philosophical value is to be found in the means it provides for increasing our understanding of Schopenhauer's philosophy, both in terms of its method and meaning. Not only does this book aim to offer insight into the younger Schopenhauer, it is also a significant document in the history of optics and colour theory.