James Gregory

James Gregory 1638-75, Scottish mathematician. He invented a reflecting telescope (1661), which he described in his Optica promota (1663). In 1668 he became professor of mathematics at the Univ. of St. Andrews and, in 1674, professor of mathematics at the Univ. of Edinburgh. He originated a photometric mode of measuring the distance of stars and wrote Geometriae pars universalis (1668) and Exercitationes geometricae (1668).

Gregory, James (1638–75)Scottishmathematician. His Optica promota (1663) describes the first design of telescope (the Gregorian reflector) to feature two mirrors as a means of circumventing chromatic aberration. In 1668 he described the inverse-square law for the brightness of stars, and used it to estimate the distance of Sirius by assuming Sirius and the Sun to be of comparable luminosity (his result was far smaller than the true value). His nephew, the Scottish mathematician David Gregory (1661–1708), suggested that chromatic aberration could be overcome by using a lens made of two elements of different composition; the idea, published in 1695, may have come from I. Newton.