How a reflecting telescope works, 1704.
© Science Museum / Science & Society Picture Library
Plate from 'Opticks' (1704) by Isaac Newton (1642-1727), showing how a reflecting telescope works (fig 29), how white light is dispersed into its constituent colours by a prism (fig 24) and other optical principles. 'Opticks' is the book in which Newton explored the properties of light, making a number of pioneering discoveries. For instance, he found that it was imposible to focus an image distinctly using a single lens, which led him to build a reflecting telescope in 1668.