Newton's first work as Lucasian Professor was on optics. Every scientist since Aristotle had believed light to be a simple entity, but Newton, through his experience when building telescopes, believed otherwise: it is often found that the observed images have colored rings around them (in fact, he devised the reflecting telescope, Fig. 4.10, to minimize this effect). His crucial experiment showing that white light is composite consisted in taking beam of white light and passing it through a prism; the result is a wide beam displaying a spectrum of colors. If this wide beam is made to pass through a second prism, the output is again a narrow beam of white light. If, however, only one color is allowed to pass (using a screen), the beam after the second prism has this one color again. Newton concluded that white light is really a mixture of many different types of colored rays, and that these colored rays are not composed of more basic entities (see Fig. 4.11).