CASTEL, LOUIS BERTRAND (1688–1757), French mathematician, was born at Montpellier on the 11th of November 1688, and entered the order of the Jesuits in 1703. Having studied literature, he afterwards devoted himself entirely to mathematics and natural philosophy. He wrote several scientific works, that which attracted most attention at the time being his Optique des couleurs (1740), or treatise on the melody of colours. He endeavoured to illustrate the subject by a clavecin oculaire, or ocular harpsichord; but the treatise and the illustration were quickly forgotten. He also wrote Mathématique universelle (1728) and Traité de physique sur la pesanteur universelle des corps (1724). He also published a critical account of the system of Sir Isaac Newton in French in 1743.