599. The study of the positive facts just reviewed, leads to a certainty in the view of colours which all may acquire who devote themselves to it. They will see how fruitful it is in applications, and that it is independent of every hypothesis, and that it would be impossible to obtain this result, if there did not commonly exist among men an average organization of the eye, which permits them to perceive in similar circumstances the same modifications, but with varied intensity of perception. 600. Having noticed the series of principles upon which my book is founded, I next consider these facts under the three following relations:— 1. The certainty they give in judging of the colour of an object. 2. The certainty they give to our judgment in the various arts which address the eye by coloured materials. 3. The union they establish between the principles common to many arts, which speak to the eye as it were various languages, in employing different materials. 4. The influence that the disposition of the spectator’s mind may have upon his judgment of a work of art. SECTION I. ON THE CONNEXION OF THE LAW OF SIMULTANEOUS CONTRAST OF COLOURS WITH THE JUDGMENT WE FORM UPON ALL COLOURED BODIES, UNDER THE RELATIONS OF THE RESPECTIVE BEAUTY OF PURITY OF THEIR COLOURS, AND OF THE EQUALITY OF THE DISTANCE OF THEIR TONES IF THESE BODIES BELONG TO THE SAME SCALE. 601. The most simple and general conclusion deduced from the law of contrast is certainly that which concerns the judgment we exercise, either by taste or profession, on a colour, whether presented by a coloured paper, a textile fabric, a glass, an enamel, a picture, &c. All those who have some experience in the matter consider one condition as essential to be fulfilled to avoid error, namely, that the colour concerning which we have to determine be compared with another colour analogous to it. _If we are ignorant of the law of contrast, the result of this comparison is not exact, whenever the objects compared are not identical._ I now proceed to demonstrate this by examples adapted to the application of the principle spoken of. Further, a more remote consequence of the law affords the means of knowing whether the tones of a scale of wool or silk are equidistant. ON THE COMPARISON OF TWO SAMPLES OF THE SAME COLOUR. 602. When we have to do with two patterns of any kind, which are related to the same colour, if there is no identity between the tints, we must take into account the contrast which exaggerates the difference; thus if the one be greenish-blue, it will make the other appear less green, or more indigo, or even more violet, than it really is; and, reciprocally, the first will appear greener than when viewed alone; the same with the reds, if one is more orange than the other, the latter will appear more purple, and the former more orange, than they really are.