The Copernican theory opened the way for an intellectual upheaval that came to be known as the Scientific Revolution, whose principal figures were Tycho Brahe (1546–1601), Johannes Kepler (1571–1630), Galileo Galilei (1564–1642) and Isaac Newton (1642–1727), though physicians, alchemists, botanists, philologists and historians all played important roles. Without all of these players, the shift would not have been so far-reaching and so deep.
The heliocentric theory of Copernicus had very few believers at first, though it gained some support when it was used as the basis for new, though not necessarily better, astronomical tables. The first of these were the Prutenic Tables, published in 1541 by Erasmus Reinhold (1511–53), who in the introduction praises Copernicus but is silent about his heliocentric theory.
The Prutenic Tables were the first complete planetary tables prepared in Europe for three centuries. They were demonstrably superior to the older tables, which were now out of date, and so they were used by most astronomers, lending legitimacy to the Copernican theory even when those who used them did not acknowledge the sun-centred cosmology of Copernicus. As the English astronomer Thomas Blundeville wrote in the preface to an astronomy text in 1594: ‘Copernicus...affirmeth that the earth turneth about and that the sun standeth still in the midst of the heavens, by help of which false supposition he hath made truer demonstrations of the motions and revolutions of the celestial spheres, than ever were made before.’
Meanwhile astronomy was being revolutionised by the Danish astronomer Tycho Brahe, who in the last quarter of the seventeenth century made systematic observations of significantly greater accuracy than any ever done in the past, all just before the invention of the telescope.
Tycho made his first important observation in August 1563, when he noted a conjunction of Saturn and Jupiter. He found that the Alfonsine Tables were a month off in predicting the date of the conjunction, and that the Prutenic Tables were several days in error. This convinced Tycho that new tables were needed, and that they should be based upon more accurate, precise and systematic observations, which he would make with instruments of his own design in his own observatory.
The first of Tycho’s observatories was at Augsburg in Germany, where he lived in the years 1569–71. The instruments that he designed and built for his observatory included a great quadrant with a radius of some 19 feet for measuring the altitude of celestial bodies. He also constructed a huge sextant with a radius of 14 feet for measuring angular separations, as well as a celestial globe 10 feet in diameter on which to mark the positions of the stars in the celestial map that he began to create.
Tycho returned to Denmark in 1571, and on 11 November of the following year he began observing a nova, or new star, that suddenly appeared in the constellation Cassiopeia, exceeding even the planet Venus in its brilliance.
Tycho’s measurements indicated that the nova was well beyond the sphere of Saturn, and the fact that its position did not change showed that it was not a comet. This was clear evidence of a change taking place in the celestial region, where, according to Aristotle’s doctrine, everything was perfect and immutable.
The nova eventually began to fade, its colour changing from white to yellow and then red, finally disappearing from view in March 1574. By then Tycho had written a brief tract entitled De nova stella (The New Star), which was published at Copenhagen in May 1573. The treatise impressed King Frederick II of Denmark, who gave Tycho an annuity along with the small offshore island of Hveen, in the Oresund Strait north of Copenhagen, the revenues of which would enable him to build and equip an observatory. Tycho settled on Hveen in 1576, calling the observatory Uraniborg, meaning ‘City of the Heavens’. That same year Tycho and his assistants began a series of observations of unprecedented accuracy and precision that would continue for the next two decades, laying the foundations for what would prove to be the new astronomy.
A spectacular comet appeared in 1577 and Tycho made detailed observations that led him to conclude that it was farther away than the moon, in fact even beyond the sphere of Mercury, and that it was in move around the sun among the outer planets. This contradicted the Aristotelian doctrine that comets were meteorological phenomena occurring below the sphere of the moon. He was thus led to reject Aristotle’s concept of the homocentric crystalline spheres, and he concluded that the planets were moving independently through space.
Despite his admiration for Copernicus, Tycho rejected the heliocentric theory, both on physical grounds and on the absence of stellar parallax. Faced with the growing debate between the Copernican and Ptolemaic theories, Tycho was led to propose his own planetary model, with Mercury and Venus revolving around the sun, which together with the other planets and the moon orbited around the stationary earth. Tycho believed that his model combined the best features of both the Ptolemaic and Copernican theories, since it kept the earth stationary and explained why Mercury and Venus were never very far from the sun.
Tycho’s patron Frederick II died in 1588 and was succeeded by his son Christian IV, who was then eleven years old. When Christian came of age, in 1596, he informed Tycho that he would no longer support his astronomical research. Tycho was thus forced to abandon Uraniborg, taking with him all of his astronomical instruments and records, hoping to find a new royal patron.
Tycho moved first to Copenhagen and then in turn to Rostock and Wandsburg Castle, outside Hamburg. He remained for two years at Wandsburg Castle, where in 1598 he published his Astronomiae instauratae mechanica, a description of all his astronomical instruments. He sent copies of his treatise to all of the wealthy and powerful people who might be interested in supporting his further researches. He appending his star catalogue to the copy he presented to the Emperor Rudolph II, who agreed to support Tycho’s work, appointing him as the court astronomer.
Thus in 1600 Tycho moved to Prague, where he set up his instruments and created a new observatory at Benatky Castle, several miles north-east of the city. Soon afterwards he hired the young German mathematician Johannes Kepler, who had sent him an interesting treatise on astronomy, the Mysterium Cosmographicum, based on the Copernican theory. In the introduction to this book Kepler writes of his excitement on discovering the work of Copernicus, which he described as ‘a still unexhausted treasure of truly divine insight into the magnificent order of the whole world and of all bodies’.
Kepler sent copies of his treatise to a number of scientists, including Galileo. In his letter of acknowledgement, dated 4 August 1597, Galileo congratulated Kepler for having had the courage, which he himself lacked, of publishing a work supporting the Copernican theory.
Kepler wrote back to Galileo on 13 October 1597, encouraging him to continue supporting the Copernican theory. ‘Have faith, Galilii, and come forward!’ he wrote. ‘If my guess is right, there are but few of the prominent mathematicians of Europe who would wish to secede from us: such is the power of truth.’
Kepler finally arrived in Prague with his family early in 1600, beginning a brief but extraordinarily fruitful collaboration with Tycho. When Kepler began work at Prague he had hopes that he could take Tycho’s data and use it directly to check his own planetary theory. But he was disappointed to find that most of Tycho’s data was still in the form of raw observations, which first had to be subjected to mathematical analysis. Moreover Tycho was extremely possessive of his data and would not reveal any more of it than Kepler needed for his work.
Tycho assigned Kepler the task of analysing the orbit of Mars, which up to that time had been the responsibility of his assistant Longomontanus, who had just resigned. Mars and Mercury are the only visible planets with eccentricities large enough to make their orbits significantly different from perfect circles. But Mercury is so close to the sun that it is difficult to observe, leaving Mars as the ideal planet for checking a mathematical theory, which is why Kepler was so enthusiastic at being able to analyse its orbit.
Early in the autumn of 1601 Tycho brought Kepler to the imperial court and introduced him to Emperor Rudolph. Tycho then proposed to the emperor that he and Kepler compile a new set of astronomical tabulations to be called the Rudolfine Tables, which Rudolph agreed to subsidise.
Soon afterwards Tycho fell ill, and after suffering in agony for eleven days he died on 24 October 1601. On his deathbed he made Kepler promise that the Rudolfine Tables would be completed, and he expressed his hopes that it would be based on the Tychonic planetary model. As Kepler later wrote of Tycho’s final conversation with him: ‘although he knew I was of the Copernican persuasion, he asked me to present all my demonstrations in conformity with his hypothesis.’
Two days after Tycho’s death Emperor Rudolph appointed Kepler as court mathematician and head of the observatory in Prague. Kepler thereupon resumed his work on Mars, now with unrestricted access to all of Tycho’s data. At first he tried the traditional Ptolemaic methods – epicycle, eccentric and equant – but no matter how he varied the parameters the calculated positions of the planet disagreed with Tycho’s observations by up to eight minutes of arc. His faith in the accuracy of Tycho’s data led him to conclude that the Ptolemaic theory of epicycles, which had been used by Copernicus, would have to be replaced by a completely new theory.
After eight years of intense effort Kepler was finally led to what are now known as his first two laws of planetary motion. The first law is that the planets travel in elliptical orbits, with the sun at one of the two focal points of the ellipse. The second law states that a radius vector drawn from the sun to a planet sweeps out equal areas in equal times, so that when the planet is close to the sun it moves rapidly and when far away it goes slowly. These two laws, which appeared in Kepler’s Astronomia nova (The New Astronomy), published in 1609, became the basis for his subsequent work on the Rudolfine Tables. Kepler’s first two laws of planetary motion eliminated the need for the epicycles, eccentrics and deferents that had been used by astronomers from Ptolemy to Copernicus.
Meanwhile the whole science of astronomy had been profoundly changed by the invention of the telescope. The earliest telescope seems to have appeared in 1604, when a Dutch optician named Zacharias Janssen constructed one from a specimen belonging to an unknown Italian, after which he sold some of them at fairs in northern Europe. After hearing of the telescope, Galileo constructed one in his workshop in 1609, after which he offered it to the Doge of Venice for use in war and navigation. After improving on his original design, he began using his telescope to observe the heavens, and in March 1610 he published his discoveries in a little book called Siderius nuncius (The Starry Messenger).
The book begins with his observations of the moon, which he found to look very much like the earth, with mountains, valleys and what he thought were seas. Seen in the telescope, the planets were pale illuminated discs, whereas the stars remained brilliant points of light. The Milky Way proved to consist of numerous stars, not a nebula reflecting the light of the sun, as some had thought, nor an atmospheric phenomenon, as Aristotle had concluded. He counted more than ninety stars in Orion’s belt, where only nine are visible to the naked eye. He discovered four moons orbiting around Jupiter, a solar system in miniature, which he used as an additional argument in favour of the Copernican theory. He called the Jovian moons the ‘Medicean Stars’ in honour of Cosimo de Medici, the Grand Duke of Tuscany. Cosimo responded by making Galileo his court philosopher and appointing him to the chair of mathematics at the University of Pisa. Galileo had no obligation to teach at the University of Pisa or even to reside in the city, and so after his appointment, in September 1610, he departed to take up residence in Florence.
Galileo sent a copy of the Siderius nuncius to Kepler, who received it on 8 April 1610. During the next eleven days Kepler composed his response in a little work called Dissertatio cum Nuncio sidereal (Answer to the Sidereal Messenger), in which he expressed his enthusiastic approval of Galileo’s discoveries and reminded readers of his own work on optical astronomy, as well as speculating on the possibility of inhabitants on the moon and arguing against an infinite universe.
Kepler borrowed a telescope from the Elector Ernest of Cologne at the end of August 1610, and for the next ten days he used it to observe the heavens, particularly Jupiter and its moons. His excitement over the possibilities of the new instrument was such that he spent the next two months making an exhaustive study of the passage of light through lenses, which he published later in 1610 under the title Dioptrice, which became one of the foundation stones of the new science of optics.
The death of Rudolph II early 1612 forced Kepler to leave Prague and take up the post of district mathematician at Linz, where he remained for the next fourteen years. One of his official duties was a study of chronology, part of a programme of calendar reform instituted by the Archduke Ferdinand II, son of the late emperor Rudolph.
During the period that Kepler lived in Linz he continued his calculations on the Rudolfine Tables and published two other major works, the first of which was the Harmonice Mundi (Harmony of the World), which appeared in 1619. The most important part of the Harmonice Mundi is the relationship now known as Kepler’s Third Law of Planetary Motion, which he discovered on 15 May 1618, and presents in Book V. The law states that for each of the planets the square of the period of its orbital motion is proportional to the cube of its distance from the sun (or, strictly speaking, the semi-major axis of its elliptical orbit).
There had been speculations about the relation between the periods of planetary orbits and their radii since the times of Pythagoras, Plato and Aristotle, and Kepler was very excited that he had at last, following in the footsteps of Ptolemy, found the mathematical law ‘necessary for the contemplation of celestial harmonies’.
In 1626 Kepler was forced to leave Linz and move to Ulm, where he published the Rudolfine Tables in September 1627, dedicating them to the Archduke Ferdinand II. The new tables were far more accurate than any in the past, and they remained in use for more than a century. Kepler used his tables to predict that Mercury and Venus would make transits across the disk of the sun in 1631.
The transit of Venus was not observed in Europe because it took place at night. The transit of Mercury was observed by Pierre Gassendi in Paris on 7 November 1631, representing a triumph for Kepler’s astronomy, for his prediction was in error by only 10 minutes of arc as compared to 5 degrees for tables based on Ptolemy’s model. But Kepler did not live to see his theories vindicated, for he passed away on 15 November 1630.
Meanwhile Galileo had been active in advancing the cause of Copernicanism against the accepted cosmology of Aristotle, which in its reinterpretation by St Thomas Aquinas formed part of the philosophical basis for Roman Catholic theology. At the beginning of March 1616 the Holy Office of the Inquisition in Rome placed the works of Copernicus and all other writings that supported it on the Index, the list of books that Catholics were forbidden to read, including those of Kepler. The decree held that believing the sun to be the immovable centre of the world is ‘foolish and absurd, philosophically false and formally heretical’. Pope Paul V instructed Cardinal Bellarmine to censure Galileo, admonishing him not to hold or defend Copernican doctrines any longer. On 3 March Bellarmine reported that Galileo had acquiesced to the Pope’s warning, and that ended the matter for the time being.
After his censure Galileo returned to his villa at Arcetri outside Florence, where for the next seven years he remained silent. But then in 1623, after the death of Paul V, Galileo took hope when he learned that his friend Maffeo Cardinal Barbarini had succeeded as Pope Urban VIII. Heartened by his friend’s election, Galileo immediately proceeded to publish a treatise entitled Il Saggiatore (The Assayer), which appeared later that year, dedicated to Urban VIII.
Il Saggiatore was favourably received in the Vatican, and Galileo went to Rome in the spring of 1623 and had six audiences with the Pope. Urban praised the book, but he refused to rescind the 1616 edict against the Copernican theory, though he said that if it had been up to him the ban would not have been imposed. Galileo did receive Urban’s permission to discuss Copernicanism in a book, but only if the Aristotelian-Ptolemaic model was given equal and impartial attention.
Encouraged by his conversations with Urban, Galileo spent the next six years writing a book called the Dialogue Concerning the Chief World Systems, Ptolemaic and Copernican, which was completed in 1630 and finally published in February 1632. The book is divided into four days of conversations between three friends: Salviati the Copernican, Sagredo the intelligent sceptic who had been converted to Copernicanism, and Simplicio the Aristotelian.
The arguments for Copernicanism were very persuasive and poor Simplicio, the Aristotelian, is defeated at every turn. Simplicio’s closing remark represents Galileo’s attempt to reserve judgment in the debate, where he says that ‘it would still be excessive boldness for anyone to limit and restrict the Divine power and wisdom to some particular fancy of his own’. This statement apparently was almost a direct quote of what Pope Urban had said to Galileo in 1623. When Urban read the Dialogue he remembered these words and was deeply offended, feeling that Galileo had made a fool of him and taken advantage of their friendship to violate the 1616 edict against teaching Copernicanism. The Florentine ambassador Francesco Niccolini reported that after discussing the Dialogue with Urban, the Pope broke out in great anger and fairly shouted, ‘Your Galileo has ventured to meddle with things that he ought not, and with the most grave and dangerous subjects that can be stirred up these days.’
Urban directed the Holy Office to consider the affair and summoned Galileo to Rome. Galileo arrived in Rome in February 1633, but his trial before the court of the Inquisition did not begin until April. There he was accused of having ignored the 1616 edict of the Holy Office not to teach Copernicanism. The court deliberated until June before giving its verdict, and in the interim Galileo was confined in the palace of the Florentine ambassador. He was then brought once again to the Holy Office, where he was persuaded to acknowledge that he had gone too far in his support of the Copernican ‘heresy’, which he now abjured. He was thereupon sentenced to indefinite imprisonment and his Dialogue placed on the Index. The sentence of imprisonment was immediately commuted to allow him to be confined in one of the Roman residences of the Medici family, after which he was moved to Siena and then, in April 1634, allowed to return to his villa at Arcetri.
After he returned home Galileo took up again the researches he had abandoned a quarter of a century earlier, principally the study of motion. This gave rise to the last and greatest of his works, Discourses and Mechanical Demonstrations Concerning Two New Sciences, of Mechanics and of Motions, completed in 1636, when Galileo was seventy-two and suffering from failing eyesight. Since publication in Italy was out of the question because of the papal ban on Galileo’s works, his manuscript was smuggled to Leyden, where the Discourses was published in 1638, by which time he was completely blind.
Galileo died at Arcetri on 8 January 1642, thirty-eight days before what would have been his seventy-eighth birthday. The Grand Duke of Tuscany sought to erect a monument in his memory, but he was advised not to do so for fear of giving offence to the Holy Office, since the Pope had said that Galileo ‘had altogether given rise to the greatest scandal throughout Christendom’.
The Scientific Revolution climaxed with the work of Newton, who was born on 25 December 1642, the same year that Galileo had died. His humble background delayed his formal education, but he was finally admitted to Cambridge, where he was enrolled at Trinity College in June 1661. At Cambridge Newton was introduced to both Aristotelian science and cosmology as well as the new physics, astronomy and mathematics that had been developed in western Europe. In 1663 he began studying under Isaac Barrow (1630–77), the newly-appointed Lucasian professor of mathematics and natural philosophy. Barrow edited the works of Euclid, Archimedes and Apollonius, and published his own works on geometry and optics, with the assistance of Newton.
By Newton’s own testimony he began his researches in mathematics and physics late in 1664, shortly before an outbreak of plague closed the university at Cambridge and forced him to return home. During the next two years, his anni mirabilis, he says that he discovered his laws of universal gravitation and motion as well as the concepts of centripetal force and acceleration. He applied these laws to compute the centripetal acceleration at the earth’s surface caused by its diurnal rotation, finding that it was less than the acceleration due to gravity by a factor of 250, thus settling the old question of why objects are not flung off the planet by its rotation. He computed the centripetal force necessary to keep the moon in orbit, comparing it to the acceleration due to gravity at the earth’s surface, and found that they were inversely proportional to the squares of their distances from the centre of the earth. Then, using Kepler’s third law of planetary motion together with the law of centripetal acceleration, he verified the inverse square law of gravitation for the solar system. At the same time he laid the foundations for the calculus and formulated his theory for the dispersion of white light into its component colours. ‘All this was in the two plague years 1665 and 1666,’ he wrote, ‘for in those years I was in the prime of my age for invention & minded Mathematicks & Philosophy more than at any time since.’
When the plague subsided Newton returned to Cambridge in the spring of 1667. Two years later he succeeded Barrow as Lucasian professor of mathematics and natural philosophy, a position he was to hold for nearly thirty years.
During the first few years after he took up his professorship Newton devoted much of his time to research in optics and mathematics. He continued his experiments on light, examining its refraction in prisms and thin glass plates as well as working out the details of his theory of colours. He also carried on with his chemical experiments, where, like many of his contemporaries, he was still influenced by the old notions of alchemy.
Newton’s silence allowed Robert Hooke to claim that he was the first to discover the inverse square law of gravitational force. In November 1662 Hooke had been appointed as the first Curator of Experiments at the newly-founded Royal Society in London, a position he held until his death in 1704, making many important discoveries in mechanics, optics, astronomy, technology, chemistry and geology.
Meanwhile Newton continued his researches on light, and he succeeded in making a reflecting telescope that was a significant improvement on any of the refractors then in use. News of his invention leaked out and he was urged to exhibit it at the Royal Society in London, which was just then beginning to hold its formal weekly meetings. The exhibit was so successful that Newton was proposed for membership in the Royal Society, and on 11 January 1672 he was elected as a Fellow.
As part of his obligations as a Fellow, Newton wrote a paper on his optical experiments, which he submitted on 28 February 1672, to be read at a meeting of the Society. The paper, subsequently published in the Philosophical Transactions of the Royal Society, described his discovery that sunlight is composed of a continuous spectrum of colours, which can be dispersed by passing light through a refracting medium such as a glass prism. He found that the ‘rays which make blue are refracted more than the red’, and he concluded that sunlight is a mixture of light rays, some of which are refracted more than others. Furthermore, once sunlight is dispersed into its component colours it cannot be further decomposed. This meant that the colours seen on refraction are inherent in the light itself and are not imparted to it by the refracting medium.
The paper was widely criticised by some of Newton’s contemporaries, it did not confirm or deny any general philosophy of nature, while others insisted that his experimental findings were false, since they themselves could not find the phenomena that he had reported. Newton replied patiently to each of these criticisms in turn, but after a time he began to regret ever having presented his work in public. To make matters worse, Hooke began to claim that Newton’s telescope was far inferior to one that he himself had made.
For these and other reasons Newton, early in 1673, offered his resignation to the Royal Society. The Secretary, Henry Oldenburg, refused to accept his resignation and persuaded him to remain. Then in 1676, after a public attack by Hooke, Newton broke off almost all association with the Royal Society. That same year Hooke became Secretary of the Society and wrote a conciliatory letter in which he expressed his admiration for Newton. Referring to Newton’s theory of colours, Hooke said that he was ‘extremely well pleased to see those notions promoted and improved which I long since began, but had not time to compleat’.
Newton replied in an equally conciliatory tone, referring to Descartes’ work on optics. ‘What Descartes did was a good step. You have added much several ways, and especially in taking the colours of thin plates into philosophical consideration. If I have seen further than Descartes, it is by standing on the shoulders of Giants.’
But despite these friendly sentiments, the two were never completely reconciled, and Newton maintained his silence. Nevertheless they continued to communicate with one another, a correspondence that was to lead again and again to controversy, the bitterest dispute arising from Hooke’s claim that he had discovered the inverse square law of gravitation before Newton.
By 1684 others besides Hooke and Newton were convinced that the gravitational force was responsible for holding the planets in their orbits, and that this force varied with the inverse square of their distance from the sun. Among them were the astronomer Edmund Halley (1656–1742), a good friend of Newton’s and a fellow-member of the Royal Society. Halley made a special trip to Cambridge in August 1684 to ask Newton ‘what he thought the Curve would be that would be described by the Planets supposing the force of attraction toward the Sun to be reciprocal to the square of their distance from it’. Newton replied immediately that it would be an ellipse, but he could not find the calculation, which he had done seven or eight years before. And so he was forced to rework the problem, after which he sent the solution to Halley that November.
By then Newton’s interest in the problem had revived, and he developed enough material to give a course of nine lectures in the autumn term at Cambridge, under the title of De Motu Corporum (The Motion of Bodies). When Halley read the manuscript of De Motu he realised its immense importance, and he obtained Newton’s promise to send it to the Royal Society for publication. Newton began preparing the manuscript for publication in December 1684, and sent the first book of the work to the Royal Society on 28 April 1686.
On 22 May Halley wrote to Newton saying that the Society had entrusted him with the responsibility for having the manuscript printed. But he added that Hooke, having read the manuscript, claimed that it was he who had discovered the inverse square nature of the gravitational force and thought that Newton should acknowledge this in the preface. Newton was very much disturbed by this, and in his reply to Halley he went to great lengths to show that he had discovered the inverse square law of gravitation and that Hooke had not contributed anything of consequence.
The first edition of Newton’s work was published in midsummer 1687 at the expense of Halley, since the Royal Society had found itself financially unable to fund it. Newton entitled his work Philosophicae Naturalis Principia Mathematica (The Mathematical Principles of Natural Philosophy), referred to more simply as the Principia. In the introductory section of the Principia Newton states his three laws of motion and his law of universal gravitation:
Law 1: Every body perseveres in its state of being at rest, or of moving uniformly forward, except insofar as it is compelled to change its state of motion by forces impressed...Law 2: A change of motion is proportional to the motive force impressed and takes place along the straight line in which that force is impressed...Law 3: To every action there is always an opposite and equal reaction; in other words, the action of two bodies upon each other are always equal, and always opposite in direction.
Then in Book I he analyses both terrestrial and celestial motion to establish his law of universal gravitation, which states that the gravitational force between any two bodies in the universe depends on the product of their masses and the inverse square of the distance between them. The rest of the Principia is a systematic application of the law of gravitation and the three laws of motion to explain phenomena ranging from the tides and the motion of projectiles and those of the celestial bodies to the precession of the equinoxes, a synthesis of the new physics and astronomy.
A second edition of the Principia was published in 1713 and a third in 1726, in both cases with a preface written by Newton. Meanwhile Newton had in 1704 published his researches on light, much of which had been done early in his career. Unlike the Principia, which was in Latin, the first edition of his new work was in English, entitled Opticks, or a Treatise of the Reflexions, Refractions, Inflexions and Colours of Light. The first Latin edition appeared in 1706, and subsequent English editions appeared in 1717, 1721 and 1730; the last, which came out three years after Newton’s death, bore a note stating that it was ‘corrected by the author’s own hand, and left before his death, with his bookseller’.
In the introduction to the Opticks Newton reveals the purpose he had in mind when composing his work. ‘My design in this Book,’ he writes, ‘is not to explain the Properties of Light by Hypotheses, but to propose and prove them by Reason and Experiment.’
The topics dealt with in Book I include the laws of reflection and refraction, the formation of images, and the dispersion of light into its component colours by a glass prism. Other topics include the properties of lenses and Newton’s reflecting telescope; the optics of human vision; the theory of the rainbow; and an exhaustive study of colour. Newton’s proof of the law of refraction is based on the erroneous notion that light travels more rapidly in glass than in air, an error stemming from his that light was corpuscular in nature.
Newton’s corpuscular view of light came from his acceptance of the atomic theory. He writes of his admiration for ‘the oldest and most celebrated Philosophers of Greece...who made a Vacuum, and Atoms, and the Gravity of Atoms, the first Principles of their Philosophy’. But in Book II, in the section entitled ‘Observations concerning the Reflexions, Refractions, and Colours of thin transparent bodies,’ Newton presents the first evidence for the wavelike nature of light.
In Book II Newton also comments on the work of the Danish astronomer Olaus Roemer (1644–1710), who in 1676 measured the velocity of light by observing the time delays in successive eclipses of the Jovian moon Io as Jupiter receded from the earth. Newton’s estimation of the velocity of light was more accurate than that of Roemer, who computed that light would take eleven minutes to travel from the sun to the earth, as compared to the correct value of eight minutes and twenty seconds. Newton concluded that ‘Light is propagated from luminous Bodies in time, and spends about seven or eight Minutes of an Hour in passing from the Sun to the Earth.’
In Book III the opening section deals with Newton’s experiments on diffraction, the bending of light when it passes from one medium to another. The remainder of the book consists of a number of hypotheses, not only on light, but on a wide variety of topics in physics and philosophy. The first edition of the Opticks had 16 of these Queries, the second 23, the third and fourth 31. It would seem that Newton, in the twilight of his career, was bringing out into the open some of his previously undisclosed speculations, his heritage for those who would follow him in the study of nature.
Newton died in London on 20 March 1727, four days after presiding over a meeting of the Royal Society, of which he had been President since 1703. His body lay in state until 4 April, when he was buried with great pomp in Westminster Abbey. Voltaire, writing of Newton’s funeral, noted that ‘He lived honoured by his compatriots and was buried like a king who had done good to his subjects.’