Isaac Newton: The Last Sorcerer. Michael White

such a stall that Newton purchased his prism.

      According to Conduitt, ‘In August 1665, Sir I. bought a prism at Stourbridge fair to try some experiments upon Descartes’s books of colours.’²² However, on this occasion Conduitt got his dates wrong and Newton actually acquired the prism on his visit to the fair in 1664, not 1665. Plague prevented the holding of the fair in 1665 – a fact documented in the diary of Alderman (later Mayor) Samuel Newton (no relation): ‘On the first of September, a proclamation was posted prohibiting Stourbridge fair on account of the great plague in London.’²³ It is also agreed by most authorities that, because of the plague, Newton had left Cambridge to return to Woolsthorpe before the beginning of August 1665, and was absent from the university for most of the next two years.²⁴

      The prevailing hypothesis of light at the time was that of Descartes. He believed that light was a ‘pressure’ transmitted through the transparent medium of the ether. Sight, he claimed, was due to this pressure impinging upon the optic nerve.

      Newton was acquainted with this hypothesis and had already made notes on the subject in his philosophical notebook. But it is likely that, in keeping with his support for atomism, by the summer of 1664 he was beginning to doubt the accuracy of Descartes’s explanation. He was already thinking that light might be corpuscular, and by imagining light to be particle-like he was more readily able to explain phenomena such as reflection, refraction, and optical and chromatic distortions. Writing to Henry Oldenburg, the Secretary of the Royal Society, some eight years later, Newton described his earliest experiments with the prism:

      I procured me a triangular glass-prism, to try therewith the celebrated Phenomena of Colours. And in order thereto having darkened my chamber, and made a small hole in my window-shuts, to let in a convenient quantity of the sun’s light, I placed my prism at its entrance, that it might be thereby refracted to the opposite wall. It was at first a very pleasing divertissement, to view the vivid and intense colours produced thereby …²⁵

      With the prism he was able to demonstrate how white light is composed of a range of component colours and how it can be split into the colours of the spectrum, with blue light, at one end of the spectrum, being bent (or refracted) more markedly than red light, at the other end. Furthermore, he was able to judge – correctly – that the colour of an object depends upon which part of the spectrum is absorbed by it and which part reflected. ‘Hence redness, yellowness etc.,’ he wrote, ‘are made in bodies by stopping the slowly moved rays without much hindering of the motion of the swifter rays, & blue, green & purple by diminishing the motion of the swifter rays & not of the slower.’²⁶

      In short, an object will look red if the other colours (what Newton refers to as ‘the slowly moved rays’) are absorbed by it more than is red light. The red will then be reflected back much more than the other colours. In the same way, an object will appear blue because it reflects blue more than the other colours (those that Newton called ‘the swifter rays’). The ability to absorb or reflect different parts of the spectrum depends on the nature of the object and produces the rich diversity of colour that we observe in the universe.^*

      Following this discovery, Newton copied out an extract from Descartes’s Dioptricks and wrote after it:

      Of Light

      Light cannot be by pressure for we should see in the night as well or better than in the day we should see a bright light above us because we are pressed downwards … there could be no refraction since the same matter cannot press 2 ways, the Sun could not be quite eclipsed, the Moon & planets would shine like suns. A man going or running would see in the night …²⁷

      Throughout the summer and autumn of 1664 Newton conducted further experiments and observed diffraction through feathers and different fabrics held up to the light: ‘A feather or a black riband put between my eye and the setting Sun makes glorious colours,’ he observed.²⁸ But then his fervour for experiment seemed to take him over. Within days he performed two experiments that left him almost totally blind.

      The first near-catastrophe was when he looked directly at the Sun for too long, with the intention of observing coloured rings and spots before the eyes – a practice he repeated over and over again. In a letter to his friend the political philosopher John Locke, written a quarter of a century later, in 1691, he describes the experience:

      I looked a very little while upon the Sun in a looking glass with my right eye and then turned my eyes into a dark corner of my chamber & winked to observe the impression made & the circles of colours which encompassed it & how they decayed by degrees & at last vanished … And now in a few hours’ time I had brought my eyes to such a pass that I could look upon no bright object … but I saw the Sun before me, so that I could neither write nor read but to recover the use of my eyes shut myself up in my chamber made dark for three days together & used all means to direct my imagination from the Sun.²⁹

      This mishap could be put down to Newton’s ignorance of the true danger involved, but what is the explanation for the following experiment?

      I took a bodkin [from the illustration accompanying this entry in the notebook, astonishingly, this appears to be a small dagger similar to an envelope knife], and put it between my eye and the bone as near to the backside of my eye as I could: & pressing my eye with the end of it (so as to make the curvature in my eye) there appeared several white, dark and coloured circles. Which circles were plainest when I continued to rub my eye with the point of the bodkin, but if I held my eye and the bodkin still though I continued to press my eye with it yet the circles would grow faint often disappear until I resumed them by moving my eye or the bodkin.³⁰

      Youthful enthusiasm and dedication are one thing, but most people would agree that sticking a blade into one’s own eye goes far beyond the call of duty. As a result, by nearly causing permanent blindness, he came close to destroying his scientific career almost before it had begun.

      In spite of these set-backs, Newton was learning rapidly from his experiments. The synthesis of Baconian method, innate talent and theoretical rigour was almost complete, but one crucial element was still missing.

      There is general disagreement regarding the timing and even the exact method by which Newton acquired the mathematical knowledge that transformed his approach. No mathematics appear in the Philosophical Notebook, but early in 1664, and before his optical experiments, he began to make mathematical notes in what he called the ‘Waste Book’, the barely used notebook of his stepfather, Barnabas Smith.³¹ By late that summer he was already familiar with the most complex mathematical ideas of the times, gleaned largely from major texts of the period, including John Wallis’s Arithmetica Infinitorum (1655) and Descartes’s Geometry.³²

      Until his entry into Cambridge University, Newton’s mathematical knowledge had been limited to simple arithmetic, perhaps some algebra and a little trigonometry. But it is a mark of his genius that during the course of only two years he taught himself advanced mathematics and developed the calculus.

      From letters and a collection of private papers which a few privileged disciples were allowed to rake through towards the end of his life, it is clear that Newton approached mathematics with the same autodidactic fervour he had shown as an adolescent pursuing his quest for knowledge at the Clarks’ home. But this time, in his enthusiasm, he skipped the fundamentals. The French mathematician Abraham Demoivre made the most intense study of Newton’s earliest mathematical work, and in a memorandum he wrote in 1727 he gives us an account