Opticks

Summary

Opticks, or a Treatise of the Reflexions, Refractions, Inflexions and Colours of Light, is Isaac Newton's experimental account of light and colour, written in English and published in London in 1704. Newton opens by announcing that his design is not to explain the properties of light by hypotheses but to propose and prove them by reason and experiments, and the three books carry out that plan. Book I, Part I, lays down definitions and axioms of geometrical optics, including an account of how the eye forms an image on the retina and why old men need convex spectacles, then shows with prisms that sunlight consists of rays differing in refrangibility. In the sixth experiment, light passed through holes in two boards is refracted by a second prism, and each colour keeps its own degree of bending. Newton draws practical consequences: refracting telescopes cannot be perfected because colours focus at different points, so he describes his reflecting telescope. Part II argues that each homogeneal light has its own unchangeable colour, that rays are not themselves coloured but have a power to stir up a sensation of colour, and that white is a compound of all colours; it explains the rainbow and the permanent colours of natural bodies. Book II studies the colours of thin plates, soap bubbles and the rings seen between a lens and a flat glass, now called Newton's rings, measures their thicknesses, and introduces 'fits of easy reflexion and easy transmission' to account for the periodic pattern. Book III begins observations on the bending of light at the edges of bodies, reported earlier by Grimaldi, and breaks off into the Queries. These 31 questions, expanded across editions, range from heat, flame and vision to the nerves, a possible aethereal medium, double refraction in Iceland crystal, a defence of light as small particles, and the forces between particles in chemistry. Query 28 rejects explanations that fill space with a dense fluid and says natural philosophy must argue from phenomena toward a first cause that is not mechanical. Query 31 proposes that God in the beginning formed matter in hard particles, sets out the method of analysis and synthesis, and closes by hoping that perfected natural philosophy will enlarge moral philosophy. Newton had read parts of the work to the Royal Society in 1675 and delayed printing, he says, to avoid disputes. The library text is the fourth edition of 1730, printed from Newton's own corrections.

Plot or Argument

• Advertisements and opening design

  Newton explains that parts were written in 1675 and later, that he delayed publication to avoid disputes, and states that he will prove the properties of light by reason and experiments rather than hypotheses.

• Book I, Part I: rays of different refrangibility

  Definitions and eight axioms, including the eye as an optical instrument, lead to propositions proved by prism experiments that sunlight is composed of rays bent by different amounts; the consequences for refracting telescopes lead to a design for a reflecting one.

• Book I, Part II: the theory of colours

  Each homogeneal colour is shown to be immutable, white light is compounded of all colours, a colour circle predicts mixtures, and the theory explains prismatic colours, the rainbow and the colours of natural bodies.

• Book II: colours of thin plates

  Careful measurement of the rings and colours of soap bubbles, thin films of air and water, and thick polished plates leads Newton to the periodic 'fits' of easy reflexion and transmission and to a theory of the sizes of the particles of coloured bodies.

• Book III: inflexion of light

  Eleven observations on the fringes of shadows cast by hairs, knife-edges and slits extend Grimaldi's discovery that light bends near bodies; Newton says he was interrupted and leaves the inquiry to others.

• Queries 1–24: light, heat, vision and the aether

  Short questions ask whether bodies act on light at a distance, whether fire and flame are heated vapours, how the optic nerves carry images to the brain, and whether a rare aethereal medium might carry vibrations of light, sensation and voluntary motion.

• Queries 25–28: against the wave and plenum hypotheses

  Double refraction in Iceland crystal suggests rays have 'sides'; Newton argues that hypotheses of light as pressure or motion in a medium fail, and in Query 28 turns from mechanism to the first cause and the divine sensorium.

• Queries 29–31: particles, forces and method

  Light is proposed as small bodies; bodies and light may convert into each other; Query 31 surveys attractions in chemistry, cohesion and fermentation, describes God's creation of hard particles, states the method of analysis and synthesis, and ends with the enlargement of moral philosophy.

Lessons

• Prove by experiment, not hypothesis

  Newton begins by refusing to explain light by hypotheses and ends by describing analysis: experiments and observations, general conclusions drawn by induction, and no objections admitted except from experiments or certain truths. The Opticks is a long demonstration of that discipline, and the Rules of Reasoning in the Principia state the same commitments in compressed form.

  • What does Newton mean by a 'hypothesis', and why does he distrust them?
  • Newton says induction is 'no Demonstration' but still 'the best way of arguing which the Nature of Things admits of'. Is this a weakness or a strength of experimental science?
  • Are Newton's own Queries hypotheses in disguise, or something different?

• Colour is in the seeing as well as in the light

  Newton insists that rays are not, strictly, coloured; they have a disposition to stir up the sensation of a colour in the sensorium, as a vibrating string stirs up the sensation of sound. The distinction separates the physics of light from the experience of the perceiver.

  • Why does Newton compare colour to sound in a bell or a musical string?
  • If colours exist only as sensations, in what sense is the sky blue?

• Ask good questions at the edge of knowledge

  Book III stops short, and Newton turns his unfinished work into questions for those who come after. The Queries became one of the most fertile parts of the book, prompting later work on chemistry, electricity, heat and the nature of light.

  • Why might Newton have chosen to put his boldest ideas as questions rather than assertions?
  • How is a well-framed question a contribution to knowledge?

• Natural philosophy points beyond itself

  In Queries 28 and 31 Newton argues that the order of the planets and the contrivance of the eye and ear point to the wisdom of an intelligent Agent, and that knowledge of nature will enlarge our understanding of duty to God and neighbour. He also warns against treating the world as the body of God.

  • What evidence from nature does Newton offer for a wise Creator in Query 31?
  • Why does Newton think 'every true Step' in natural philosophy brings us nearer to the first cause?
  • How does Newton distinguish God from the world he made?

Context

Newton worked within the new experimental philosophy of the Royal Society, alongside Robert Boyle and Robert Hooke, and against the mechanical philosophy of Descartes, who explained light as pressure transmitted through a plenum. Christiaan Huygens had argued in 1690 that light is a wave in a pervasive medium. Newton's prism experiments, first reported in 1672, had drawn objections from Hooke and others, and the dispute made him reluctant to publish further on light.

Composition

By Newton's own account, parts were written for the Royal Society in 1675 and the rest some twelve years later, with Book III assembled from scattered papers. He published in 1704, the year after Hooke died. The first edition ended with 16 Queries and two mathematical tracts. Samuel Clarke's Latin translation, Optice (1706), added seven more, and the second English edition (1717–18) added eight on the aether and renumbered them to make 31. The fourth edition (1730), used here, was printed from Newton's corrected copy of the third.

Reception

Opticks was more readable than the Principia and spread Newtonian method across Europe, especially through Clarke's Latin version. Its language of God's 'sensorium' was challenged by Leibniz in his correspondence with Clarke (1715–16). The particle theory of light held the field through the eighteenth century until Thomas Young and Augustin Fresnel revived waves after 1800; Young drew directly on Newton's measurements of thin plates. The Queries influenced eighteenth-century chemistry and theories of heat and electricity.

Comparisons

• principia

  The Principia states Newton's four Rules of Reasoning in Philosophy and the General Scholium's 'hypotheses non fingo'; the Opticks shows those rules at work in a long experimental inquiry and gives the fuller statement of analysis and synthesis in Query 31.

• huygens-treatise-on-light

  Huygens explained reflection, refraction and double refraction by waves in an aether; Newton's Queries 25–29 answer such theories and propose light as small particles with 'sides'.

• discourse-on-method

  Descartes, whose Meteors accompanied the Discourse, also explained the rainbow and colours, but from a mechanical hypothesis about rotating particles; Newton's prisms test what Descartes reasoned.

• novum-organum

  Bacon called for induction from experiments and crucial instances; Newton's analysis by experiment and induction is often read as the realization of Bacon's programme, though Newton adds mathematical measurement.

• faraday-chemical-history-of-a-candle

  Faraday's lectures build a whole science from simple apparatus and repeatable demonstrations, as the Opticks does with prisms, lenses and thin films.

• dialogues-concerning-natural-religion

  Newton's argument from the contrivance of the eye and the uniformity of the planets is the kind of design argument that Hume's speakers Cleanthes and Philo debate later in the century.

Quotes

For Students

A reading companion for classical Christian high school students. See the student guide.

Reading Guide

Watch for

• The first sentence of Book I, where Newton sets proof by reason and experiments against explanation by hypotheses.
• The sixth experiment of Book I, Part I, in which light from a first prism passes through holes in two boards to a second prism, and each colour keeps its own refrangibility.
• Newton's careful statement in Book I, Part II, that rays are not themselves coloured, and his comparison with sound in a bell.
• The rings and colours of soap bubbles and thin plates in Book II, and how Newton turns them into measurements.
• Queries 28 and 31, where the discussion of light and matter opens onto the first cause, creation and the method of analysis and synthesis.

Key terms and names

Refrangibility

A ray's disposition to be bent out of its path when it passes from one transparent medium into another. Newton shows that rays of different colours have different refrangibility.

Homogeneal and heterogeneal light

Homogeneal light consists of rays of one refrangibility and colour; heterogeneal light, such as sunlight, is a mixture of many kinds.

Reflecting telescope

A telescope that gathers light with a curved mirror instead of a lens, avoiding the coloured fringes caused by the different refrangibility of rays.

Fits of easy reflexion and transmission

Newton's name for the alternating states a ray passes through, making it apt to be reflected at one thickness of a thin plate and transmitted at the next.

Inflexion

The bending of light as it passes near the edge of a body, now called diffraction; studied in Book III.

Sensorium

The place in the brain where sensations are received and perceived; Newton speaks of infinite space 'as it were' God's sensory.

Analysis and synthesis

Newton's method in Query 31: first reason from experiments to general causes by induction, then assume those causes as principles to explain and prove the phenomena.

Seminar Questions

1. Comprehension What does the experiment with two prisms and two boards show about the colours in sunlight?
2. Comprehension Why does Newton conclude that refracting telescopes cannot be perfected, and what does he propose instead?
3. Comprehension In Query 31, what are the two parts of the method Newton calls analysis and synthesis?
4. Interpretation Newton promises to avoid hypotheses but ends the book with dozens of bold questions. How do the Queries fit, or fail to fit, his stated method?
5. Interpretation Why does Newton think the eye, the ear and the uniform motions of the planets cannot be explained by 'blind Fate' or the mere laws of nature?
6. Interpretation Newton says God perceives things in infinite space 'as it were in his Sensory' but denies that the world is God's body. What is he trying to affirm, and what is he trying to avoid?
7. Big Question Newton hoped that perfecting natural philosophy would enlarge moral philosophy. Can knowledge of nature teach us our duty to God and to one another, and if so, how far?

Faith Connections

• Psalm 94:9 (Scripture)

  'He that planted the ear, shall he not hear? he that formed the eye, shall he not see?' Newton's question in Query 28, whether the eye was contrived without skill in optics, makes the same argument from the design of the senses to a knowing Creator.

• Romans 1:20 (Scripture)

  Paul says God's invisible power and divinity are clearly seen from the things that are made. Query 31's appeal to the 'Wisdom and Skill' visible in animals and planets follows this line of natural theology.

• Acts 17:27-28 (Scripture)

  Paul tells the Athenians that God is not far from any of us, for in him we live and move and have our being. Newton's picture of God as everywhere present to things themselves draws on this sense of divine nearness while denying that the world is part of God.

• Augustine, The Literal Meaning of Genesis I.19 (Church Fathers)

  Augustine warns Christians against making claims about the natural world that knowledgeable unbelievers can see are false. Newton's insistence on proving claims about light by experiment fits Augustine's call for careful knowledge of nature.

Logic and Rhetoric

Central argument: Experiments with prisms show that sunlight is a mixture of rays of different and unchangeable refrangibility and colour; since colours in bodies, rainbows and thin films can all be explained by this property without hypotheses about the nature of light, the experimental method of analysis and synthesis is the right way to study nature; and because that method, pursued to its end, leads from effects to causes and finally to a first cause that is not mechanical, natural philosophy also enlarges our knowledge of God and of moral duty.

Rhetorical device: rhetorical question (erotema). The Queries are written entirely as questions, such as 'Was the Eye contrived without Skill in Opticks, and the Ear without Knowledge of Sounds?' The form lets Newton put forward speculative claims while signalling that they await experimental proof.

Debate: Resolved: The study of nature leads the careful inquirer toward knowledge of a Creator.

Memory Work

My Design in this Book is not to explain the Properties of Light by Hypotheses, but to propose and prove them by Reason and Experiments

And though every true Step made in this Philosophy brings us not immediately to the Knowledge of the first Cause, yet it brings us nearer to it, and on that account is to be highly valued.

Writing Prompts

Essay: Newton opens by refusing to explain light by hypotheses and closes with 31 Queries. Using the two-prism experiment, Book II's thin plates and Queries 28 and 31, explain what Newton means by proving 'by Reason and Experiments', and argue whether the Queries are consistent with that method.

Comparison with Principia: The Principia's Rules of Reasoning and General Scholium and the Opticks' Query 31 both describe Newton's method. Compare the two statements and argue which gives the clearer account of how experimental science moves from observations to general laws.

Medical Lens

The Opticks is a physics treatise, but it contains a careful early-modern physiology of vision. Axiom VII describes the eye as a camera: cornea and crystalline humour focus a picture on the retina, which anatomists can see by removing the back coats of an excised eye, and the picture is carried along the fibres of the optic nerves to the brain. Newton explains presbyopia in old age as flattening of the eye corrected by convex spectacles, and myopia in the over-plump eye, and notes that jaundice tints vision. Query 15 proposes that the optic nerves partly cross where they meet, fibres from the right side of each eye going to the right side of the brain, to explain single vision with two eyes, an arrangement later confirmed anatomically. Query 16 describes pressure phosphenes and the persistence of visual impressions. Queries 23–24 speculate that vision and voluntary motion are carried by vibrations of an aethereal medium in the nerves. Newton's distinction between ray and sensation, and his passing remark that colours appear in dreams or to a madman without light, place perception in the sensorium, a theme of later philosophy of mind and psychiatry. These are period hypotheses, not modern neuroscience.

• Book I, Part I, Axiom VII

  The eye as an optical instrument; images on the retina conveyed by the optic nerves; presbyopia and convex spectacles; myopia; jaundice tinting vision.

  Anatomy Dissection Aging Senescence
• Book I, Part II, Prop. VII

  Colours seen in dreams and by a madman, or by pressing or striking the eye, set aside as arising from causes other than light.

  Dreams Nightmares Madness Psychosis Mind Body
• Book III, Query 15

  Proposal that fibres of the optic nerves partly cross where the nerves meet, so that half of each eye's picture goes to each side of the brain.

  Anatomy Dissection
• Book III, Queries 16 and 23–24

  Pressure phosphenes and persistence of vision; vision and animal motion speculatively explained by vibrations in the nerves excited by the will.

  Mind Body

Creative Prompts

• Classroom

  Reproduce Newton's two-prism experiment (Book I, Part I, Exp. 6) with a slit, two prisms and a white card, and record whether a single colour can be split again.

• Essay

  Newton says that rays are not coloured but stir up a sensation of colour. Write an essay on where colour 'lives': in the light, the object, or the mind.

• Dialogue

  Write a conversation between Newton and Huygens in 1704 on whether light is made of particles or waves, each using evidence from his own book.

Further Reading

• The Principia: Mathematical Principles of Natural Philosophy (trans. I. Bernard Cohen and Anne Whitman), Isaac Newton

• Never at Rest: A Biography of Isaac Newton, Richard S. Westfall

• The Leibniz-Clarke Correspondence (ed. H. G. Alexander), G. W. Leibniz and Samuel Clarke

• Franklin and Newton, I. Bernard Cohen