Newton Builds His Reflecting Telescope Summary

  • Last updated on November 10, 2022

Newton was the first person to produce a reflecting telescope that worked as well as a refracting telescope. Presenting this telescope to the Royal Society brought Newton to the attention of the international scientific community, and he quickly used the society as a forum in which to present his new theory of light and colors.

Summary of Event

Sir Isaac Newton came from an inauspicious rural background and was largely unknown until he made his first reflecting telescope while at Cambridge University. He began showing it, with justifiable pride, in 1669. Eventually, word of the device reached members of the Royal Society Royal Society of London—a society begun in 1660 as a group of scientists and others who met weekly to discuss items of interest. A member of the society, Newton’s mentor and former mathematics professor Isaac Barrow, Barrow, Isaac presented Newton’s second telescope to the Society late in December, 1671. Delighted with the telescope, the Society requested more information. Newton obliged them and also took the opportunity to present them with his “New Theory on Light and Color” in February, 1672. Suitably impressed, they elected Newton a member of the Royal Society and thereby brought the young mathematics professor into contact with the leading scientists of the day. [kw]Newton Builds His Reflecting Telescope (late Dec., 1671) [kw]Telescope, Newton Builds His Reflecting (late Dec., 1671) [kw]Reflecting Telescope, Newton Builds His (late Dec., 1671) Science and technology;Late Dec., 1671: Newton Builds His Reflecting Telescope[2440] Physics;Late Dec., 1671: Newton Builds His Reflecting Telescope[2440] Inventions;Late Dec., 1671: Newton Builds His Reflecting Telescope[2440] England;Late Dec., 1671: Newton Builds His Reflecting Telescope[2440] Telescope;reflecting Newton, Sir Isaac;reflecting telescope

The first known reflecting telescope was made by the Italian Jesuit Nicolas Zucchi in about 1616, but it was Newton who made the first reflecting telescope that rivaled refracting telescopes (which use lenses instead of mirrors). Newton had always been good at constructing things. Even as a boy, he filled his room with tools and various mechanical clocks and other devices he had made.

The key to Newton’s telescope was its “speculum,” or metal mirror. After experimenting with several alloys from which to construct the mirror, he settled on using copper, tin, and a little silver. Since that seemed to tarnish too quickly, when he made his next telescope, he substituted arsenic for the silver. The materials had to be carefully melted, combined, poured out to cool, ground to the proper shape, and painstakingly polished. The resulting speculum was 1.3 inches (3.3 centimeters) across. He fastened it in the bottom of a 6-inch (15.25-centimeter) pasteboard tube and mounted a much smaller flat mirror at the top of the tube. The flat mirror was tilted so that it reflected the image from the main mirror out through an eyepiece lens mounted in the side of the telescope. The telescope was focused by turning a large metal screw to move the main mirror. It magnified about 40 times, and Newton boasted that he could use it to read the Royal Society’s printed journal from 100 feet (30 meters) away!

At the time, it was widely believed that white light was pure and simple and that it became colored as it was degraded by passing through glass. If a white light ray strikes the surface of a prism at an angle so that different parts of the ray travel through different amounts of glass, it was supposed that those parts of the incident ray would degrade by different amounts and therefore emerge as various colors. Newton began a series of experiments in 1664 to determine if this were so, and it was the results of these experiments that he presented to the Royal Society as his theory of light and color.

Although Newton ground and polished his own lenses, it is believed that he purchased his prisms at Cambridge’s midsummer fair. To use these prisms, he darkened his room and allowed sunlight to enter only through a small hole in the window shutter. The sunbeam made a round, bright spot—an image of the Sun—on the far wall. Next, he placed a prism in the sunbeam and watched the bright spot spread into rainbow colors. Newton described its shape as an oblong with straight sides and curved top and bottom. He named this rainbow-colored oblong the “spectrum,” from a Latin word for ghost; thus, the rainbow is the ghost of light. Newton was surprised to find the spectrum five times longer than it was wide. Its width was about the same as the sun’s original image and was what was expected given the angular size of the sun, the size of the hole in the shutter, and the distance to the wall, but there was no simple geometric explanation for the length of the spectrum.

Newton next made a small hole in a board and placed this board behind the prism so that only light of a single color could reach the wall. Then he placed a second prism between the board and the wall. The path of the colored beam was bent by the second prism, but the color was not further changed. Since red light, for example, passed through the second prism without further changing color, the prisms clearly were not “degrading” the light. Another explanation for color change had to be found. Newton removed the board and placed the second prism near the first so that the full spectrum fell on it. As he inverted the second prism so that it was upside-down compared with the first prism, the spectrum on the wall collapsed back into the bright circular image of the sun. That is, passing the light through more glass did not “degrade” it into more colors, but converted it back into white light.

After many more experiments to prove that these results were not caused by imperfections in the prisms, the size or shape of the hole in the shutters, the fact that the Sun’s rays are not quite parallel (he made the same observations using the planet Venus), or any other aspect of the experiment, Newton became convinced that white light is compound and that the rainbow colors are primary. That is, the eye’s response to seeing a mixture of the rainbow colors all coming from the same spot is to see white. To emphasize this, he painted the sectors of a disk various colors, and then showed that it looked white while it was spinning rapidly.

Robert Hooke, Hooke, Robert a fellow of the Royal Society, believed that light was a wave motion in an all-pervading substance called the aether, but Newton believed that a light ray consisted of a stream of “corpuscles” (small particles). He argued that if light were a wave, an object would not cast a sharp shadow, since waves would bend around the object into the shadow region. In either case, when a ray of light passes through a prism, its path is bent, or “refracted,” as it passes from air into glass, and again as it passes from glass back into air. Newton described the various colors of light as having different “refrangibilites”; that is, different capacities to be refracted.

Newton’s theory of light and color did not depend upon knowing whether light consisted of particles or waves, but in his paper he unwisely included the claim that after reviewing his experiments perhaps everyone would agree that light was a particle phenomenon. To Newton’s consternation, Hooke immediately claimed that Newton had proven no such thing and went on to argue about that while ignoring Newton’s experimental findings. Although Newton quickly agreed that the claim had not been proven and stated that either the wave or the particle model could support his findings, Hooke would not be satisfied. Their disagreement eventually became so heated that Newton refused to publish his major work on optics until after Hooke died. Physics;optics


All of the large telescopes today, including the Hubble Space Telescope, are direct descendants of Newton’s reflecting telescope. Without them, we would know relatively little about the universe beyond Earth’s atmosphere. To put Newton’s telescope in perspective, it took about fifty years for technology to advance enough consistently to make significantly better reflecting telescopes than his.

René Descartes Descartes, René calculated that the surface of a lens should not be spherical in shape but hyperbolic, in order to focus a point source of light into a single sharp image point. The fuzziness of an image formed by a spherically surfaced lens is called spherical aberration. Newton found that chromatic aberration, the inability of a lens to focus all of the colors simultaneously, was a thousand times worse than spherical aberration. Since chromatic aberration is inherent in the properties of light and glass, Newton switched from trying to make a perfect lens to building a reflecting telescope (which does not suffer from chromatic aberration). In actual fact, Newton suspected chromatic aberration could be overcome by combining lenses with different refracting powers, but he found the reflecting telescope more appealing.

Perhaps more important than any of Newton’s devices or discoveries was the way in which he did science. Between them, Galileo (who died the year Newton was born), Christiaan Huygens Huygens, Christiaan (Newton’s contemporary), and Newton himself made purely philosophical science unacceptable. They established as a widely accepted criterion the modern view that good science should be tied to both good experimental evidence and mathematical models.

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Berlinski, David. Newton’s Gift: How Sir Isaac Newton Unlocked the System of the World. New York: Simon & Schuster, 2002. A biography that concentrates on Newton, the man, not on his science.
  • citation-type="booksimple"

    xlink:type="simple">Westfall, Richard S. Never at Rest: A Biography of Isaac Newton. New York: Cambridge University Press, 1980. One of the classic biographies of Newton, complete and comprehensive.
  • citation-type="booksimple"

    xlink:type="simple">White, Michael. Isaac Newton: The Last Sorcerer. Reading, Mass.: Perseus Press, 1997. A biography of Newton as both scientist and alchemist.
Related Articles in <i>Great Lives from History: The Seventeenth Century</i>

René Descartes; Galileo; Robert Hooke; Christiaan Huygens; Sir Isaac Newton. Telescope;reflecting Newton, Sir Isaac;reflecting telescope

Categories: History