ascertain the duration of the impression produced by a glowing coal in the following manner. He attached it to the circumference of a wheel, the velocity of which was gradually increased until the apparent trace of the object formed a complete circle, and then measured the duration of a revolution, which was obviously that of the impression. To ascertain the duration of a revolution it is sufficient merely to know the number of revolutions described in a given time. Recently more refined experiments of the same kind have been made by Professors Plateau and Wheatstone."—S. F.
NOTE C.—Par. 50.
Every treatise on the harmonious combination of colours contains the diagram of the chromatic circle more or less elaborately constructed. These diagrams, if intended to exhibit the contrasts produced by the action and re-action of the retina, have one common defect. The opposite colours are made equal in intensity; whereas the complemental colour pictured on the retina is always less vivid, and always darker or lighter than the original colour. This variety undoubtedly accords more with harmonious effects in painting.