Newton's reflector

• 1. Introduction

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  Newton's reflector, built by Isaac Newton in 1668, began as a proof of a theory rather than a quest for stargazing power. Newton had convinced himself that every refracting telescope ever made suffered from an inescapable flaw: light passing through a lens splits into its component colours, blurring the image at the edges. Opticians called this chromatic aberration, and Newton believed no lens-maker could ever fully escape it. So instead of improving lenses, he abandoned them entirely. The telescope he constructed used a curved metal mirror as its objective, sidestepping the dispersion problem completely. What emerged from his workshop was a small instrument, barely six inches long, that would become the prototype for one of the most widely used telescope designs in history. Who built it alongside him? How did it reach the attention of King Charles II? And why does the telescope the Royal Society displays today carry an inscription that is almost certainly wrong?

• 2. Mirror Over Lens

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  Newton's choice of materials set his instrument apart from anything built before. He mixed a custom alloy of six parts copper to two parts tin, an early form of what metallurgists would come to call speculum metal. He then devised his own methods for shaping and grinding the mirror, and he may have been the first person to use a pitch lap to polish an optical surface to the required smoothness. He deliberately chose a spherical curve for the mirror rather than the mathematically superior parabola, because he had already satisfied himself that chromatic aberration, not spherical aberration, was the dominant fault in the refracting telescopes of his day. A secondary flat mirror, angled at 45 degrees and placed near the focus of the primary, deflected the light sideways through a hole in the tube wall, where an eyepiece magnified the image. This diagonal secondary became the hallmark of what later generations called the Newtonian telescope. Newton built not just the mirrors but the tube, the mount, and every fitting himself.

• 3. What Newton Wrote

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  Newton left a detailed description of his second telescope in his own words, and that account is specific enough to reconstruct the instrument almost exactly. The primary mirror was 2 inches across and roughly one-third of an inch thick, thick enough to resist flexing. It was ground to fit a sphere 25 inches in diameter, giving it a focal length of 6.25 inches. Rather than letting all the mirror's surface contribute to the image, Newton placed a disk with a small hole between his eye and the eyepiece, stopping the effective aperture down to 1.3 inches. The eyepiece was a plano-convex lens with a focal length of around 4.5 millimetres, which produced a magnification he measured at 35 times. He compared the little reflector against a refracting telescope four feet long and found he could read text at a greater distance with his own instrument. He noted that objects appeared somewhat darker in the reflector, attributing the dimness partly to light lost by reflection from the metal and partly to excessive magnification. His own verdict: magnifying 30 or 25 times would have shown objects more brightly. He also recorded making two mirrors, polishing both, comparing them, and grinding the inferior one again to try to improve it.

• 4. Three Telescopes, One Name

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  There were actually three telescopes, not one, and the confusion between them runs deep. The first, built in 1668, was a prototype Newton showed only to a small group of friends in Cambridge. Its mirror was probably about 1.3 inches in diameter and the tube around 6 inches long. Newton rarely mentioned it in later years, so his second telescope is the one that most historical accounts treat as his first. The second instrument, completed in 1671, carried a 2-inch mirror and a focal length somewhere between 6.25 and 6.3 inches. Newton presented this second telescope to the Royal Society in December 1671, and by 1731 it had deteriorated until nothing remained but its two metal mirrors. Those mirrors have since disappeared entirely. The third telescope was built in 1671-1672 in collaboration with Newton's chamber-fellow at Trinity College, John Wickins. Newton himself noted that Wickins did a better job of figuring the mirror than he had managed on the second telescope. This third instrument is the one that appeared later in the London shop of instrument makers Heath and Wing, alongside a piece that had belonged to Edmond Halley, suggesting it had passed from Newton to Halley and then into trade. Heath and Wing restored it and presented it to the Royal Society in 1766.

• 5. A King, a Society, and a Plaque

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  Isaac Barrow, Newton's friend and predecessor as Lucasian Professor at Cambridge, brought the second telescope to the attention of the Royal Society at the end of 1671. The Fellows were impressed enough to demonstrate it before King Charles II in January 1672. Contemporary accounts described the instrument as about 7 inches long and 2.25 inches in diameter, with the image viewed through a hole in the side of the tube roughly the size of a large pin's head. The tube was a single piece, and focusing was achieved by sliding the mirror up from the bottom rather than by adjusting the eyepiece. It sat on a ball-and-socket mount. The Royal Society kept this second telescope until, by 1731, only the two metal mirrors survived, and those eventually vanished from their records. The telescope the Society now holds is certainly the third, the one made with John Wickins, not the one shown to the king. At some later date a plaque was attached to it bearing an inscription claiming it was Newton's first telescope, made in 1671. Both claims on that plaque are wrong. The mirror in the current instrument contains arsenic, an improvement Newton himself proposed after finding the silver-containing second mirror too soft. The second mirror's silver content is what distinguishes it from the third; the third's arsenic is proof of a later revision.

• 6. Legacy in Aperture

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  Newton's first instrument was barely 6 inches long, but the design it introduced grew rapidly. By the end of the 18th century, the largest reflecting telescope had reached nearly 50 inches of aperture, while the largest achromatic lens objective available to astronomers measured no more than about 5 inches. That comparison captures the design's structural advantage: mirrors can be made far larger than practical lenses because they need only one precisely shaped surface rather than two, and they can be supported from behind rather than clamped at their edges. Newton himself demonstrated the telescope was capable of detecting the four Galilean moons of Jupiter and resolving the crescent phase of Venus. Replicas have been built at intervals to mark the design's significance: F.L. Agate made one for the Science Museum in London in 1924, two more were produced in the 1960s from the original instrument, with one going to the Queen and another to the European Northern Observatory at La Palma, and a further replica was made in 1984 for the chief designer of the William Herschel Telescope, eventually finding its way into the collection of the National Maritime Museum.