Towards the conclusion of the previous episode in this series where I discussed Isaac Newton’s early years, I mentioned that from 1664 to the early 1670s, he engaged in one of the most remarkable periods of self-education ever recorded. That duration was detailed in a post that dispelled the myth surrounding the so-called Annus mirabilis. A significant portion of this time was allocated to the experimental examination of light, particularly what Newton refers to as “the phenomenon of colour.” Prior to Newton, it was commonly believed that colour emerged from the alteration of white light by external factors, becoming altered when it passed through a prism or lens, for instance. His findings demonstrated, as we are all aware today, that white light itself is actually made up of numerous colours, each possessing a distinct refractive index, with the spectrum being created by the refraction of white light. The rainbow is formed by sunlight being refracted through droplets of rain. These revelations marked Newton’s initial public forays as a natural philosopher, the first of which was a notable success, and the second teetered on the edge of disaster.
Before delving into these events, we must update our understanding of Newton’s advancement at the University of Cambridge. In 1669, Isaac Barrow (1630–1677) stepped down from his role as Lucasian Professor of Mathematics and recommended the then twenty-six-year-old Newton as his successor; a suggestion that was embraced by the college officials, and the young Isaac was formally appointed.
This straightforward historical occurrence raises several concerns. Firstly, by 1669, Newton had produced absolutely no publications; he was a blank slate, yet he receives a professorship? His exceptional mathematical prowess had evidently become known to Barrow, who, after all, put forth the recommendation, likely with support from others, suggesting he was the ideal candidate for the position, thus his appointment. I previously composed an entire blog post titled, “Only 26 and Already a Professor,” in which I analyzed the seemingly remarkable reality of a twenty-six-year-old unknown being appointed to what is now considered the most esteemed mathematics chair in the world.
In actuality, during its initial decades, the Lucasian chair was far from prestigious. Its rise to prominence would commence with Newton’s later endeavors and subsequently be bolstered by the list of renowned mathematicians and physicists who occupied the position after him. In those formative years, it was, in fact, largely inconsequential, and within the still predominantly Aristotelian university framework, it garnered little interest. This lack of appeal to Barrow’s self-image is why he resigned. Very few students attended Newton’s lectures, if any, and he often lectured to an empty hall, cutting his presentations short to return to his chambers. However, the position did offer a salary, allowing Newton to dedicate himself to his rigorous research.
In the late 1660s, Newton’s primary focus was his studies in optics, and he began to make appearances beyond the university realm on both practical and theoretical fronts.
His research uncovered that light consisted of a spectrum of colours, each with a varying refractive index. This had significant implications for lenses and telescopes. The images produced by seventeenth-century telescopes were decidedly not sharp; they were blurred with coloured edges. It was assumed that this issue arose from spherical aberration. A spherical lens does not converge all the rays that pass through it to a single point but over a small span, resulting in a spread-out image. This phenomenon was first identified by Ibn al-Haytham (965–c. 1040) in his Kitāb al-Manāẓir, which was later translated into Latin as De Aspectibus or Perspectiva and was widely known. The theoretical solutions were also established; lenses needed to be ground into different shapes—parabolic, hyperbolic—yet people lacked the technical expertise to realize this. It was understood that increasing the focal length of the objective lens mitigated spherical aberration, leading to the remarkable aerial telescopes of Christiaan Huygens (1629–1695) and Johannes Hevelius (1611–1687).
Newton recognized that lenses, which are essentially prisms, also experienced chromatic aberration, and that this played a much larger role in creating the blurred image than spherical aberration. Newton believed it would be impossible to create a lens devoid of chromatic aberration, a significant scientific miscalculation in his career, prompting him to embark on constructing a telescope that utilized a mirror to focus the incoming light rays instead of a lens—a reflector.
Newton was by no means the first to conceive the idea of employing a mirror instead of a lens to direct light rays. The reflector telescope has origins that trace back to Hero of Alexandria (1st century CE?), as documented in other sources. While Newton was still an undergraduate, the Scottish mathematician and astronomer James Gregory (1638–1675) had published a design for a