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.

Top 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.

• 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.

• 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.

• 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.

Verified Quotes

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.

• 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.

• 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.

• 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.