James Gregory was a Scottish scientist and first Regius Professor of Mathematics at St Andrews who described the first practical reflecting telescope. He worked on using infinite convergent series to find the areas of the circle and hyperbola and was one of the founders (with Newton and Leibniz) of calculus.
The above watercolour shows James Gregory working in Upper Parliament Hall, which was his laboratory: on the floor is a north-south meridian line along which he aligned his telescope; to his left is one of the first pendulum clocks which was constructed especially for him.
James Gregory was born in the Manse of Drumoak, a small parish on Deeside, eleven miles from Aberdeen. He came from a family already noted for mathematics. Encouraged by his brother, who was himself a gifted mathematical scientist, James wrote his first book, entitled Optica Promota. A masterly account of mirrors and lenses, the book contains a description of the earliest reflecting telescope, known by his name, the Gregorian telescope, which brought him fame at the age of 24.
The telescope design attracted the attention of several people in the scientific establishment: Robert Hooke, the Oxford physicist who eventually built the telescope, Sir Robert Moray, polymath and founding member of the Royal Society and Isaac Newton, who was at work on a similar project of his own.
The Gregorian telescope was the first practical reflecting telescope and remained the standard observing instrument for a century and a half. However, the Gregorian telescope design is rarely used today, as other types of reflecting telescopes are known to be more efficient for standard applications.
In the Optica Promota he also described the method for using the transit of Venus to measure the distance of the Earth from the Sun, which was later advocated by Edmund Halley and adopted as the basis of the first effective measurement of the Astronomical Unit.
In 1667 he issued his Vera Circuli et Hyperbolae Quadratura, in which he showed how the areas of the circle and hyperbola could be obtained in the form of infinite convergent series. One of the reasons he is not better known as a mathematician is that, out of modesty, he often didn't publish a result when he heard that Newton was going to do so.
James Gregory discovered the diffraction grating by passing sunlight through a bird feather and observing the diffraction pattern produced. In particular he observed the splitting of sunlight into its component colours - this occurred a year after Newton had done the same with a prism and the phenomenon was still highly controversial.
A crater on the Moon is named after him.
A detailed biography of James Gregory is available on the MacTutor History of Mathematics website constructed by Edmund Robertson and John O'Connor at the University of St Andrews..