In fact, numerous observations made during several months, on the view of coloured objects, which were verified by my pupils and others, much accustomed in their profession to judge of colours and to appreciate the least differences between them, have been collected and described as well-known facts. Upon reflecting on the mutual relations of these facts, and in seeking the principle of which they were the results, I was led to the discovery of that which I have named the “Law of Simultaneous Contrast of Colours.” Thus this work is the fruit of the method _à posteriori_: facts are observed, defined, described, then generalized in a simple expression, which has all the characters of a law of nature. This law, once demonstrated, becomes a means, _à priori_, of assorting coloured objects so as to obtain their best possible effect, according to the taste of the person who arranges them; it becomes also a means of estimating whether the eyes are well organized for seeing and judging of colours; and whether painters have copied exactly the colours of known objects. In reviewing the Law of Contrast with regard to its application, and in submitting to experiment all the laws which appear to me to result from it, I have been led to extend it to the arts of tapestry, to the various sorts of painting and printing, to illuminating, horticulture, &c. But, in order to anticipate the judgments of some readers, upon the value of the opinions I have advanced (2nd part, 2nd division) respecting the Gobelins and Beauvais tapestry and the Savonnerie carpets, I must state, that, being an entire stranger to the inspection and direction of the works which are executed in the Royal manufactories, as well as to the choice of models, my views and opinions should be regarded only as those of an individual who has had frequent occasion to see and examine various artistic productions, on the preparation of which he could exercise no influence, the duties which attach me to the Gobelins being exclusively those of Director of its Dye Works. THE LAWS OF Harmony and Contrast of Colours. INTRODUCTION. 1. A ray of solar light is composed of an indeterminate number of variously coloured rays, which are distributed into groups, termed _red rays, orange rays, yellow rays, green rays, blue rays, indigo rays, violet rays_. 2. But all the rays comprised in the same group, the red for example, are not identical in colour; on the contrary, they may be considered as differing more or less among themselves, although we recognise the impression they produce separately, as comprised in that which we ascribe to red light.