PLATEAU, Joseph Antoine Ferdinand, was born at Brussels in 1801, and died in 1883 at Ghent, where he had been professor of physics from 1835. He was a pupil and friend of Quetelet, who had much influence on the early part of his career. The more original investigations of Plateau refer chiefly to portions of one or other of two branches of science—physiological optics and molecular forces. His doctoral thesis (Liège, 1829) had for its subject “Impressions produced by Light on the Organs of Vision”; and it was succeeded by numerous memoirs, some of much value, on the persistence of visual impressions, subjective impressions of colour, irradiation, &c. Among other results of his studies was the invention of the philosophical toy known as the “thaumatrope.” We owe to him also something much more important, the process of studying the motion of a vibrating body by looking at it through equidistant radial slits in a revolving disk. In 1829 he imprudently gazed at the midday sun for 20 seconds, with the view of studying the after effects. The result was blindness for some days, succeeded by a temporary recovery; but for the next fourteen years his sight gradually deteriorated, and in 1843 he became permanently blind. This calamity did not interrupt his scientific activity. Aided by his wife and son, and afterwards by his son-in-law Van der Mensbrugghe, he continued to the end of his life his researches on vision,—directing the course of the experiments which they made for him, and interpreting the bearing of the results. He also published a valuable analytical catalogue of all the more important memoirs which had been written, from the earliest times to the end of the 18th century, on his favourite theme of subjective visual phenomena. But even more extraordinary ...