Out of the Shadow of a Giant

by John Gribbin

  New to Pepys, and to Hooke, although we now know that the English clergyman and astronomer Jeremiah Horrocks had speculated along the same lines – that comets follow closed orbits around the Sun – three decades earlier. It happens that Hooke was wrong about this particular comet: it was not the same one that was seen in 1618, and it did not return in 1711. But the improving telescopic technology of the time was starting to show astronomers that comets did not move in straight lines, but followed curved paths through space; this was the beginning of the idea that led Halley, before too long, to make the prediction of the return of the comet that now bears his name. The significance for Hooke’s story is that it seems that, by the mid-1660s at the latest, he was already thinking about the possibility that comets (and therefore the planets) were under the influence of some kind of force, reaching out to them across space from the Sun itself. He realised that comets are part of the Sun’s family, not something weird or magical. This was among the insights that led him to carry out several experiments to investigate the nature of gravity, which we describe later. It is worth getting slightly ahead of our story, however, to highlight one of Hooke’s most important insights (perhaps the most important), which (like so many of his ideas) has been misattributed for hundreds of years.

  Going back into the mists of time, it had been assumed by natural philosophers that the ‘natural’ motion of objects such as planets unaffected by friction or other forces was circular. This had to be so, they reasoned, because circles are perfect, and only perfection could be at work in the heavens. They interpreted the seemingly irregular motion of planets in terms of epicycles, where the planets were constrained to move in small perfect circles around points which themselves moved in perfect circles around the Earth, or the Sun. When, only a short time before Hooke was born, Galileo carried out experiments involving balls rolling down inclined planes, he found that the balls rolled off the end of the ramp horizontally – literally, towards the horizon – and he realised that if there were no friction they would keep rolling for ever. But he knew that the Earth was round, so to him ‘horizontal’ motion meant always moving towards an always receding horizon, in a circle around the Earth. It was Hooke who realised, partly from his studies of comets, that any object that is not acted upon by an external force will keep moving in a straight line. Does that sound familiar? It should. It is something we all learn in school, where it is called ‘Newton’s First Law’ of motion. But it was Hooke who came up with it, and who (as we shall see) explained it to Newton.

  On 21 March 1666, when nobody outside Cambridge and few people inside Cambridge had heard of Isaac Newton, Hooke gave a lecture to the Royal about gravity, where he presented some of these ideas. He described several experiments involving his study of gravity, which he stated was ‘one of the most universal active principles in the world’ and set out his ambition to determine:

  whether this gravitating or attractive power be inherent in the parts of the earth [and] whether it be magnetical, electrical, or of some other nature distant from either

  as well as ‘to what distance the gravitating power of the earth acts’.

  On 23 May that year he presented his big idea to another meeting of the Royal, and in a paper entitled ‘Inflexion of a Direct Motion into a Curve by a Supervening Attractive Principle’. In that lecture (and many times afterward) Hooke used a long pendulum, with the bob moving in a circle, or (crucially, in terms of understanding the motion of the planets) an ellipse, not just to and fro; this demonstrated the nature of orbital motion, which, he pointed out, required a force (in this case, supplied via the string of the pendulum) to keep the bob ‘in orbit’. By attaching a secondary, shorter string, with its own bob, partway down the pendulum he could also demonstrate the motion of a ‘moon’ around a ‘planet’. The idea he presented to the Fellows (which really was ‘a very new opinion’) was that the natural motion of a planet is in a straight line – a tangent to its orbit – and that it is deflected from this tangential path by a force of attraction stemming from the centre of the planetary system – that is, a force emanating from the Sun. As he explained to the Fellows:

  I have often wondered why the planets should move about the Sun according to Copernicus’s supposition, being not included inn any solid orbsfn6 … nor tied to it, as their centre, by any visible strings.

  He stressed that ‘all bodies, that have but one single impulse’ ought to move in straight lines, and inferred that there must be another ‘impulse’ acting on the planets. If that impulse were a force of attraction from the Sun then:

  all the phenomena of the planets seem possible to be explained by the common principle of mechanic motions [and] the phenomena of the comets as well as of the planets may be solved.

  These two ideas, ‘Newton’s’ first law and the force of attraction between the Sun and planets (an inward, or centripetal, force), are the keys to the ‘Newtonian’ revolution in science that took place two decades later. It might have happened sooner, and had a different name, if Hooke’s attention had not been diverted by dramatic developments in England in 1665 and 1666. Conveniently for us, however, he had summed up what he described as his ‘first endeavours’ in a book published just before those changes took place.

  Micrographia, Hooke’s great book, was written and published on the instructions of the Royal Society as a deliberate attempt to promote the Society and its aims. Hooke has been described as a ‘reluctant author’,fn7 and almost all of his published work resulted from his contractual obligations, primarily to the Royal Society and to a slightly lesser extent to John Cutler and in connection with his role as a Gresham Professor. But the background to Micrographia predates Hooke’s appointment as Curator of Experiments.

  At the beginning of the 1660s, Christopher Wren was supposed to be preparing a book of microscopical observations for presentation to the King, who had seen some of his drawings of microscopic objects and been impressed by them, but the newly appointed Savilian Professor of Astronomy found that he had too much on his plate, and passed this task on to Hooke, who took over the work in September 1661. The design and manufacture of optical instruments – telescopes and microscopes – was improving dramatically at this time, and although Hooke was involved in developing some of the ideas that went into these instruments, he relied on expert craftsmen, notably Richard Reeve, for the tools of his trade. As he put it in his book: ‘all my ambition is that I may serve to the great Philosophers of this Age, as the makers and grinders of my Glasses did to me’.

  By the end of 1662, Hooke was presenting some of his microscopic studies to the Royal. The first of these observations, presented in December that year, dealt with the patterns of ice crystals seen in ‘frozen urine, frozen water, and snow’. The Fellows were sufficiently impressed that at the Council meeting of 25 March 1663 Hooke was ‘solicited to prosecute his microscopical observations, in order to publish them’. In the months that followed, Hooke made many specific observations at the behest of individual Fellows, as well as following up his own interests. The Council kept a keen eye on the progress of the work, with the book intended to provide an example of the experimental method, which was at the heart of their philosophy, and which they explicitly took from Bacon. In the book, Hooke emphasises the need ‘to begin to build anew upon a sure Foundation of Experiments’, and explicitly cites the ‘Noble and Learned’ Bacon as an inspiration. The book was partially intended as propaganda for the Society itself and for the new way of studying the world. It succeeded dramatically on both counts, thanks to Hooke’s known genius as a scientist and his perhaps unexpected skill as a writer. But it only got into print after some heart-searching by the Council, which has been detailed by John Harwood.fn8

  Hooke had more or less enough material for his book by March 1664, a year after he had formally been instructed to carry out the work. By then, the Royal had chosen a printer and discussed such details as the official Royal Society imprimatur to go in the front of the book. This emphasised in the cle
arest way that it was a Royal Society book, stating that:

  By the Council of the Royal Society of London for Improving of Natural knowledge.

  Ordered, That the Book, written by Robert Hooke, M.A. Fellow of this Society, Entitled, Micrographia, or some Physiological Descriptions of Minute Bodies, made by Magnifying Glasses, with Observations and Inquiries thereupon, Be printed by John Martyn and James Allestry, Printers to the said Society

  Novem. 23.

  1664. Brouncker. P.R.S

  But 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 Observ
ation 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 in the constituent parts of it, both as to their density and rarity, and as to their divers mutations and positions one to another.

  By Density and Rarity, I understand a property of a transparent body that does either more or less refract a Ray of light.

  And

  The redness of the Sun, Moon and Stars, will be found to be caused by the inflection of the rays within the Atmosphere … it is not merely the colour of the Air interpos’d.

  In other words, the colour is inherent in the original white light and is not some kind of pollution, or corruption, caused by the passage of light through the intervening medium – another discovery later attributed to Newton.

  The second great insight in Micrographia comes in Observation 16, where Hooke presents his ideas on combustion. The microscopic justification for including these ideas comes from his studies of charcoal and burnt vegetables, but the experiments from which his most impressive insights are drawn do not really involve the microscope at all. These included his observations of the way flames went out when a lit candle was shut in a sealed chamber, how small animals collapsed and died after a certain time in such a chamber, the gruesome vivisection of a dog, and the experiments with candles and living things involving the air pump. Having already, in Observation 9, asserted that heat is ‘a motion of the internal parts’ of a substance (also mentioned in Observations 7 and 8), he now draws a clear distinction between heat and combustion. ‘This Hypothesis,’ he says, ‘I have endeavoured to raise from an Infinite of Observations and Experiments, the process of which would be much too long to be here inserted.’ But as he tells us, the idea ‘has not, that I know of, been publish’d or hinted, nay, not so much as thought of, by any.’ He was right.