The object of this work has been to present, in a clear, logical, and if possible attractive form, the fundamental facts connected with our perception of colour, so far as they are at present known, or concern the general or artistic reader. For the explanation of these facts, the theory of Thomas Young, as modified and set forth by Helmholtz and Maxwell, has been consistently adhered to. The whole class of musical theories, as well as that of Field, have been discarded, for reasons that are set forth in the text. Turning now from the more purely scientific to the aesthetic side of the subject, I will add that it has been my endeavour also, to present in a simple and comprehensible manner the underlying facts upon which the artistic use of colour necessarily depends. The possession of these facts will not enable people to become artists; but it may to some extent prevent ordinary persons, critics, and even painters, from talking and writing about colour in a loose, inaccurate, and not always rational manner. More than this is true: a real knowledge of elementary facts often serves to warn students of the presence of difficulties that are almost insurmountable, or, when they are already in trouble, points out to them its probable nature; in short, a certain amount of rudimentary information tends to save useless labour. Those persons, therefore, who are really interested in this subject are urged to repeat for themselves the various experiments indicated in the text. Many of the earliest books, particularly those dating back to the 1900s and before, are now extremely scarce and increasingly expensive. We are republishing many of these classic works in affordable, high quality, modern editions, using the original text and artwork.