589. I entered upon this study, not having spontaneously chosen it, but because it appeared to me indispensable before pretending to establish a sound judgment on the beauty of the dyer’s colours. As soon as I felt the necessity for this study, in my capacity of Director of the Dyeing Department of the Royal Manufactories, my first care was to discover whether I saw colours as the generality of persons see them. I was soon perfectly convinced that I did, and not till then did I venture to make my researches the subjects of public lectures. These have been repeated before the students of the Polytechnic School. Certain questions addressed to my auditors to satisfy me that they saw the things I put before their eyes as I saw them myself, have, in the majority of cases, always proved them to be so, and yet my demonstrations were given in the reception-hall at the Gobelins,—a place ill adapted for the exhibition of the phenomena of contrast to a large audience. Certain observations by myself, tested by a great number of persons in my laboratory, and afterwards publicly exhibited, form the subject of this book; all who repeat my experiments will discover whether my opinion is well-founded, or whether an opinion is correct which pretends that the sight of colours is not capable of giving a general positive result. Because some individuals have organs of sight so imperfect that they cannot distinguish green from red, or blue from grey, &c., must we write our treatises on optics without mentioning either red, green, or blue, and cast away these colours from the palette? Assuredly human nature is too limited to allow of our making such a sacrifice of our common organization to the infirmity of an individual. 590. In order to comprehend clearly how experiment and observation, after having disentangled the causes which exercise a determinate influence upon the sight of colours, led me to adopt the opinion that these phenomena are perfectly defined by the law of contrast and the conclusions therefrom, doubtless it will suffice to consider how 1. Our former ignorance respecting the different states of the eye, which, in seeing colours, give rise to the phenomena of simultaneous, successive, and mixed contrasts. And our former ignorance respecting the definite influence that the direct or diffused light of the sun exercises, according to its intensity, upon the colours of bodies, have led to the establishment of an opinion contrary to my own;—that is to say, the opinion that _the same colour appears so diversely to different persons, and even to the same person, that nothing general or precise can be deduced from the sight of coloured objects, with regard to their respective colours_. 2. To consider how the following have passively contributed to belief in this opinion. The limited number of ideas we have generally about the modifications of coloured bodies, by their mutual mixtures; or in other terms, upon the colours resulting from these mixtures.