371. At a certain calico-printer’s a recipe for printing green had always succeeded up to a certain period, when it began to give bad results. They were lost in conjectures upon the cause, when a person, who at the Gobelins had followed my researches on contrast, recognised that the green of which they complained, being printed on a ground of blue, inclined to yellow through the influence of orange, the complementary of the ground. She therefore advised that the proportion of blue in the colouring composition should be increased in order to correct the effect of contrast. The recipe, modified according to this suggestion, gave the beautiful green which they had obtained formerly. 372. Thus every recipe for colours to be applied upon a ground of another colour, must be modified conformably to the effect which the ground will produce. It is this great facility in correcting the ill effect of certain contrasts which explains why they so often succeed without being able to account for it. Here, notwithstanding their colour, the eye judges them to be colourless, or of the tint complementary to that of the ground. These appearances have been the subject of questions frequently addressed to me by the manufacturers of printed stuffs, and by drapers: they are due to the _law of simultaneous contrast of colours_. In fact, when the patterns appear white, the ground acts by contrast of tone (9); if they appear coloured (and this appearance generally succeeds to that where they appear white), the ground then acts by contrast of colour (13). The manufacturer of printed stuffs therefore will not seek to attribute the cause of these phenomena to the chemical actions in his operations. 373. Ignorance of the law of contrast has, among drapers and manufacturers, been the subject of many disputes, which I have been happy to settle amicably, by demonstrating to the parties that they had no possible cause for litigation in the cases they submitted to me. I will relate some of these, to prevent similar disputes. Certain drapers gave to a calico-printer some cloths of single colours, red, violet, and blue, upon which they wished black figures to be printed. They complained that upon the _red_ cloths he had put _green_ patterns; upon the _violet_, the figures appeared _greenish-yellow_; upon the _blue_, they were _orange-brown_ or _copper_-coloured—instead of the _black_ which had been ordered. To convince them that they had no ground for complaint, it sufficed to have recourse to the following proofs:— 1. I surrounded the patterns with white paper, so as to conceal the ground; the designs then appeared black. 2. I placed some cuttings of black cloth upon stuffs coloured red, violet, and blue; the cuttings appeared like the printed designs, _i. e._, of the colour complementary to the ground, although the same cuttings, when placed upon a white ground, were of a beautiful black.