CASTEL, LOUIS BERTRAND, a learned mathematician, was born at Montpellier in 1688, and entered the order of the Jesuits in 1703. At first he was a student of literature, but afterwards devoted himself entirely to mathematics and natural philosophy. He wrote a Traité de la Pénitence Universelle, Paris, 1724; Mathématique Universelle; Le plan du Mathématique abrégé, and other works. But the work which attracted most attention at the time was his Optique des Couleurs, or treatise on the melody of colours, a subject which he endeavoured to illustrate by a Clavessin Oculaire, or ocular harpsichord. The treatise and the illustration were, however, quickly forgotten. He also published a critical account of the system of Sir Isaac Newton in French. Castel died in 1757.