Isaac Newton is well known for his work on light and the spectrum, topics that formed a major part of his book Opticks, published in 1704. One of the main consequences of this work was Newton's development of the reflecting telescope.

The earliest telescopes, such as those used by Galileo, consisted of glass lenses mounted in a tube. Newton discovered that when light (from a star, for example) passed through a lens the different colours were refracted by differing amounts. This meant that the components of white light were brought to a focus at different places and the image of a star would then appear to be surrounded by a spectrum of colours. This effect is called chromatic aberration, and was not easily rectified using the technology available in Newton's time.

To solve the problem, Newton designed a telescope that used mirrors, rather than lenses, to bring the light to a focus. It is interesting to note that Newton did not invent the idea of a reflecting telescope: the honour for this goes to the Scottish mathematician James Gregory, who designed such an instrument in the early 1660s. Newton's design was simpler than Gregory's and the basic idea is shown
in figure 1.

Light from the object being viewed is collected by the concave primary mirror and reflected to a smaller, secondary plane mirror (sometimes called the "flat"). The flat is inclined at 45 degrees to the axis of the telescope and reflects light to an eye lens which forms an image.

Newton communicated the details of his telescope to the Royal Society in 1670, but it did not become widely known until the publication of Opticks more than thirty years later. The photograph below shows Newton's original telescope: