Newton's reflecting telescope. Diagrams showing how a reflecting telescope works, published by the English physicist, mathematician and astronomer Sir Isaac Newton (1643-1727). These diagrams are from Newton's famous work Opticks (1704). The bottom diagram shows rays of light entering a telescope (from left), being focused by a curved reflecting mirror (far right) onto a small mirror (lower left), and reflected by that small mirror into the eye of an observer (bottom left). The other diagrams show other optical principles, including dispersal of white light by a prism (figure 24, upper left). Newton invented the first practical reflecting telescope in 1668.