Out of the Shadow of a Giant: How Newton Stood on the Shoulders of Hooke and Halley

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’,

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.

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 clearest 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’.

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. Micrographia

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,

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 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.

One of the key series of experiments that he hints at here was carried out as demonstrations at the Royal in January and February 1665. In a beautiful example of the scientific method at work, he showed first that gunpowder would still burn in the absence of air, and then that neither of two of the three ingredients of gunpowder, charcoal and sulphur, would burn on their own in the absence of air. But each of them could be reignited by adding the third ingredient, which he knew as saltpetre but which we call potassium nitrate. As Hooke says in Micrographia, it is clear from these experiments that combustion involves ‘a substance inherent, and mixt with the Air, that is like, if not the very same, with that which is fixt in Salt-peter.’ That substance is, of course, oxygen; the chemical formula for potasssium nitrate is KNO

.

Hooke’s idea is that something in the air is essential to combustion, which takes place when that something combines with something in the burning object. ‘There is no such thing as an Element of Fire’, he asserts, dismissing the idea that had held sway since the time of Ancient Greece. A flame ‘is nothing else but a mixture of Air and volatile sulphureous parts of dissoluble or combustible bodies, which are acting upon each other whilst they ascend, that is, flame seems to be a mixture of Air, and the combustible volatile parts of any body’. Further, the component of air that is essential for combustion is also, Hooke tells us, essential for life. In Observation 22, almost as an aside, he mentions that there is a ‘property in the Air which it loses in the Lungs, by being breath’d’. In being so close to the discovery of oxygen, Hooke was nearly a century and a half ahead of his time; right up until the end of the eighteenth century, the phlogiston theory of combustion (which said, flying in the face of experiments like those Hooke carried out with the air pump, that burning substances released phlogiston, rather than absorbing something from the air) held sway, and Hooke’s ideas were forgotten. In 1803, chemist John Robison wrote:

I do not know of a more unaccountable thing in the history of science, than the total oblivion of this theory of Dr. Hooke, so clearly expressed, and so likely to catch attention.

But it did catch the attention of one person, the serial plagiarist Isaac Newton. In an appendix to his book on optics, hurried into print immediately after Hooke’s death (see postscript to Chapter Seven), Newton presented a suite of ideas about combustion that chemist Clara de Milt has described, with admirable academic restraint, as ‘very, very much like those of Hooke’. As Private Eye might put it, could they by any chance be related?

The third great insight presented in Micrographia comes in Observation 17: Of Petrify’d wood, and other Petrify’d bodies. The petrified objects he refers to are what we now call fossils. Before Hooke, it was widely thought that these were, in his words, ‘Stones form’d by some extraordinary Plastick virtue latent in the earth’. In other words, that these were just curious stones that happened to resemble the forms of living things. But he dismissed this notion, and stated unequivocally (‘I cannot but think’) that they were ‘the Shells of certain Shel-fishes, which, either by some Deluge, Inundation, Earthquake, or some other such means, came to be thrown to that place’. ‘That place’, he was well aware, was high up in a mountain, or on the cliffs that he had walked as a boy on the Isle of Wight. So how did such things as wood and shells become petrified, or fossilised? Hooke’s description of the process could almost come from the pages of a modern textbook of geology:

this petrify’d Wood having lain in some place where it was well soak’d with petrifying water (that is, such water as is well impregnated with stony and earthy particles) did by degrees separate, either by straining and filtration, or perhaps, by precipitation, cohesion or coagulation, abundance of stony particles from the permeating water, which stony particles, being by means of the fluid vehicle convey’d, not onely into the Microscopical pores, and so perfectly stoping them up, but also into the pores or interstitia, which may, perhaps, be even in the texture or Schematisme of that part of the Wood, which, through the Microscope, appears most solid.

And as for shells, they must have been:

fill’d with some kind of Mudd or Clay, or petrifying Water, or some other substance, which in tract of time has been settled together and hardened in those shelly moulds.

Hooke clearly understood two things: that there were geological processes that transformed once-living things into ‘petrified’ rock, and that there were geological processes that transformed the structure of the Earth’s crust. Implicit in this was the understanding that the timescales involved (‘tract of time’) were far greater than the ‘official’ chronology of a few thousand years derived from the Bible.

Hooke even begins to hint at the kind of investigations that would lead to the idea of evolution:

It were therefore very desirable, that a good collection of such kind of Figur’d stones were collected; and as many particulars, circumstances, and informations collected with them as could be obtained, that from such a History of Observations well rang’d, examin’d and digested, the true original or production of all those kinds of stones might be perfectly and surely known.

Soon after the publication of Micrographia, a Dane, Niels Steensen (who used the Latinised version his name and is remembered as Steno), publicised very similar ideas, and suggested that different rock strata, containing fossils such as sharks’ teeth, had been laid down under water, far from the present-day seas, at different times during Earth’s history by a succession of floods. Coincidence? Hooke didn’t think so. He had developed these ideas further in his Cutlerian Lectures which we discuss later. Henry Oldenburg, the Secretary of the Royal Society and someone who often rubbed Hooke up the wrong way, was in correspondence with scientists across Europe as part of his job. Steno published his ideas in 1669, in Latin. Oldenburg promptly made the Royal aware of the book, and arranged for it to be translated into English, which helped to ensure that Steno became remembered as the inventor, or discoverer, of these ideas. Hooke was not exactly pleased and tried unsuccessfully to get recognition that he at the very least had the idea first. When it was suggested that he had borrowed his ideas from Steno, rather than the other way around, he was moved to write a letter, read to a meeting of the Royal on 27 April 1687, in which he said:

I must now add in my own vindication that I did long since prove Steno had much of his treatise from my Lectures, which some time before that I had read [in Gresham College] which Lectures Mr Old. Borrowed and transcribed and by Divers circumstances I found he had transmitted the substance of if not the very Lectures themselves [to Steno]. And he did as good as own it, and upon my challenging him with it he did in two of his transactions publish that I had Read A great part of that Doctrine & hypothesis in my Lectures in Gresham Colledge Some time before Mr Steno had published his Booke.

There is no reason to doubt Hooke’s version of affairs, and there is no doubt at all that his work preceded that of Steno, whether or not Steno got word of it via Oldenburg. Steno, by the way, never gave a clue one way or the other: he disappeared from the scientific scene after writing his book. He became a Catholic priest in 1675, and was ordained as a bishop in 1677, inflicting on his body such a harsh regime of fasting and self-denial that he died in 1686, at the age of forty-eight.

Hooke’s more extensive ideas about earthquakes, Earth history and geology will be covered in Chapter Nine. Now, we still have a fourth great insight from Micrographia to discuss, although here we diverge from ‘Espinasse’s assessment of which of the ideas Hooke presented there were most significant. She picks out his discovery of the structures he named cells (after the rooms occurred by monks in a monastery) in thin slices of cork (Observation 18). As Hooke puts it, no ‘Writer or Person’ had ‘made any mention of them before this’. But although the name was taken up and used by later biologists, it was in a slightly different context. The ‘pores’, as he also called them, that Hooke had found are not living cells, but non-living structures left over from the growth of the plant. The first person to see and study live cells under a microscope was Hooke’s Dutch contemporary Antoni van Leeuwenhoek. In 1674 he described an algae, Spirogyra, and other organisms that moved of their own volition; he named them animalcules (‘little animals’). In this area, Hooke’s work was important, but not as important as the work of van Leeuwenhoek and others. In our estimation, his astronomical Observations were of far greater importance.

Hooke was a serious and highly respected astronomer. On 9 May 1664, using a twelve-foot-long refracting telescope, he had discovered the Great Red Spot of Jupiter, and used it to measure the rotation of the giant planet. Contemporary (and now more famous) astronomers such as the Italian Giovanni Cassini picked up on the discovery, and referred to the phenomenon as ‘Hooke’s Spot’. But it was observations of something much closer to home that led Hooke to important insights that appeared in Observation 60: Of the Moon. This is a short contribution that to a casual glance looks like a mere filler. That couldn’t be more wrong.

Observation 60 provides a nice example of the scientific mind – Hooke’s scientific mind – at work: making observations, devising hypotheses, testing them by experiment and further observation, and drawing general conclusions from specific cases. Remember that this was less than sixty years after Galileo, with the aid of one of the first astronomical telescopes, discovered that the Moon is not a perfect sphere but pockmarked with craters and scarred by mountain ranges. Hooke was intrigued by the nature of these craters, and puzzled over their origin. He described them as ‘almost like a dish, some bigger, some less, some shallower, some deeper, that is, they seem to be a hollow Hemisphere, incompassed with a round rising bank, as if the substance in the middle had been digg’d up, and thrown on either side’. Which establishes, as if we did not already know, that he was a good and accurate observer.