Out of the Shadow of a Giant: How Newton Stood on the Shoulders of Hooke and Halley. John Gribbin
in the interval from March 1664 to November 1664, the contents of the book had been carefully vetted and discussed by selected Fellows. This caused them some disquiet, because – strictly speaking, exceeding his brief – Hooke did not restrict himself to presenting the observations that he had made with the microscope, but also offered theoretical explanations for why things might be the way they are. He also professes a mechanistic view of Nature, pointing out in the Preface that the reason why we may hope to use mechanical techniques – experimental science – to reveal the workings of the world is that the world operates on the same principles as a machine:
We may perhaps be inabled to discern all the secret workings of Nature, almost in the same manner as we do those that are the productions of Art [artifice], and are manag’d by Wheels, and Engines, and Springs, that were devised by humane Wit.
All of this elevated Hooke’s perceived status to that of a natural philosopher, rather than a ‘mere’ mechanical experimenter. But if his ideas were wrong, the Royal did not want to be seen to endorse them. Ultimately, the Council decided to allow Hooke’s speculations to appear in the book, but only if it was made clear that they were his alone, and not the official view of the Society. They ordered:
That the president be desired to sign a licence for the printing of Mr. HOOKE’S microscopical book: And, That Mr. HOOKE give notice in the dedication of that work to the society, that though they have licensed it, yet they own no theory, nor will be thought to do so: and that the several hypotheses and theories laid down by him therein, are not delivered as certainties, but as conjectures; and that he intends not at all to obtrude or expose them to the world as the opinion of the society.
Hooke complied, and one result of all this is that we can be sure the book is all his own work, enhancing his reputation even more. And he wrote in English, in the first person, making his ideas widely acceptable. The book was the first scientific best-seller. Samuel Pepys saw the sheets being prepared when he happened to visit the bookbinders on other business, and promptly ordered a copy of the book. He received it on 20 January 1665, and the next evening ‘sat up till 2 a-clock in my chamber, reading of Mr. Hooke’s Microscopicall Observations, the most ingenious book that ever I read in my life’.fn9 A couple of weeks later, Pepys was himself admitted as a Fellow of the Royal Society, and noted in his diary the luminaries present at the meeting. ‘Above all,’ he tells us, ‘Mr Boyle today was at the meeting, and above him Mr Hooke, who is the most, and promises the least, of any man in the world that I ever saw.’ In other words, in spite of Hooke’s unprepossessing appearance, Pepys rated him above Boyle as a scientist. Clearly, this was at least partly thanks to the impression made by Micrographia.
To us, the speculations that gave the Royal cold feet are more significant than the illustrations that were the original raison d’être for the book, astonishing though they were at the time, and still are, considering the difficulties Hooke had to cope with. Remember, for example, that the only light sources he had were the Sun, candles and simple oil lamps. In a standard setup, light from an oil lamp was focused first through a globe containing a transparent solution of brine, and then through a lens on to the specimen he wanted to study. Straining his eyes to concentrate on the image, he then had to draw what he saw with meticulous precision. Micrographiafn10 contains sixty illustrated ‘observations’, fifty-seven of them microscopic and three astronomical, made with the aid of a telescope. In a demonstration of his skill as a communicator and his methodical way of working as a scientist, Hooke begins with observation ‘of the Point of a sharp small Needle’. ‘As in geometry,’ he writes, ‘the most natural way of beginning is from a Mathematical point.’ He goes on to describe, with illustrations,fn11 how even the smoothest, sharpest needle looks rough and rounded under the microscope, and he makes a digression to describe the appearance of full stops, both printed and handwritten, which were abundantly ‘disfigur’d’ even when they appeared perfectly round to the human eye. And he is not averse to a pun, saying after a digression ‘But to come again to the point …’ The style is easy and accessible even to modern eyes, and the illustrations still stunning. Although in modern times some critics have suggested Hooke could not possibly have seen the detail he claimed, Brian J. Ford, an expert in the history of microscopy, found that by using similar instruments and making careful adjustments of light and focus he could indeed reach the level of detail reported by Hooke. We shall not, however, describe each of the sixty observations in detail. Instead, we shall follow the example of Hooke’s biographer Margaret ‘Espinasse in picking out four key topics that helped to revolutionise seventeenth-century science.
The first highlight is Hooke’s work on light and optics, which is doubly important because it would lead to an intense disagreement with Newton, and one of the most misunderstood comments in the history of science (see Chapter Four). Observation 9 of the Micrographia deals with ‘the colours observable in Muscovy glass, and other thin bodies’. This ‘glass’ is a mineral that is ‘transparent to a great thickness’, but is made up from many thin layers discernible under the microscope. Hooke was intrigued by the way this material converted white light into a rainbow pattern of colours, and discovered microscopic flaws in the layers of the material: ‘with the Microscope I could perceive, that these Colours were ranged in rings that incompassed the white speck or flaw.’ Newton, of course, is today remembered as the man who discovered that white light could be split into rainbow colours, and these rings are known, of course, as ‘Newton’s rings’. Hooke explained the phenomenon as a result of the combination (we would now say interference) of light reflected from the upper and lower surfaces of the thin layers, and described how the effect was only produced if the layers were thinner than a critical thickness; his explanation was based on the idea that light is a form of wave, in his words ‘a very short vibrating motion’, but incorrectly suggested that red and blue are the primary colours from which others are derived by ‘dilutions’.
Even here, though, Hooke’s reasoning was sound, given the state of knowledge at the time, and based on an experiment that clearly intrigued the young Isaac Newton. Hooke allowed a narrow beam of sunlight to enter the top of a conical flask filled with water, striking the surface of the water at an angle. He saw how the beam of light was spread out as it entered the water, producing a band of colour with red (he called it scarlet) on one side and blue on the other, with other fainter colours in between. It was this that led him to infer that white light is a mixture of colours (which is correct) and that red and blue are the primary colours, which are mixed together in different amounts to produce different colours (which was wrong, but not stupid). This experiment, described in Observation 9, is what pointed Newton towards his experiments with prisms, for which he is credited for the discovery that white light is a mixture of colours.
But the breadth of Hooke’s interests and the depth of his theorising (the things that worried the Council of the Royal) can be seen in his summing up at the end of the Observation:
I think these I have newly given are capable of explicating all the Phenomena of colours, not only of those appearing in the Prisme, Water-drop or Rainbow, and in laminated or plated bodies, whether in thick or thin, whether transparent, or seemingly opacous.
The whole Observation amounts to what we would now call a scientific paper, and as ‘Espinasse points out it is ‘a progression of precise observation, masterly analysis and induction, and speculation’.
In Observation 58, one of the three astronomical observations, Hooke returns to optics to discuss the phenomenon of refraction, starting out from the by then well-known telescopic observation that ‘the Sun and Moon neer the Horizon, are disfigur’d (losing that exactly-smooth terminating circular limb, which they are observ’d to have when situated near the Zenith)’. After discussing several other phenomena, notably ‘that both fix’d Stars and Planets, the neerer they appear to the Horizon, the more red and dull they look, and the more they are observ’d to twinkle’, he concludes:
First, that a medium, whose parts are unequally dense, and mov’d by various motions and transpositions as to one another, will produce all these visible effects upon the Rays of light, without any other coefficient cause.
Secondly, that there is in the Air or Atmosphere, such a variety