Free The Faber Book of Science by John Carey

not go sideways or upwards, but constantly to the earth’s centre? Assuredly, the reason is, that the earth draws it. There must be a drawing power in matter: and the sum of the drawing power in the matter of the earth must be in the earth’s centre, not in any side of the earth. Therefore does this apple fall perpendicularly, or towards the centre. If matter thus draws matter, it must be in proportion of its quantity. Therefore the apple draws the earth, as well as the earth draws the apple. That there is a power, like that we here call gravity, which extends itself through the universe.
    And thus by degrees he began to apply this property of gravitation to the motion of the earth and of the heavenly bodies, to consider their distances, their magnitudes and their periodical revolutions; to find out that this property, conjointly with a progressive motion impressed on them at the beginning, perfectly solved their circular courses; keptthe planets from falling upon one another, or dropping all together into one centre; and thus he unfolded the universe. This was the birth of those amazing discoveries, whereby he built philosophy on a solid foundation, to the astonishment of all Europe.
    Asked at an earlier stage in his life how he had discovered the law of universal gravitation, Newton had replied ‘By thinking on it continually’ – a remark that supplements, but does not contradict, Stukeley’s apple story.
    Newton’s law, set out in the Principia (1687), states that every particle of matter in the universe attracts every other particle with a force that varies according to its mass and to the inverse square of the distance between them. This remained the accepted explanation of gravity until it was superseded by Einstein’s theory of general relativity in 1915 (see p. 267, below).
    Newton’s other great scientific work was the Optics, not published till 1704, but based on experiments he made as a young man at Cambridge to discover the nature of light:
    In a very dark chamber, at a round hole, about one third part of an inch broad, made in the shut [shutter] of a window, I placed a glass prism, whereby the beam of the sun’s light, which came in at that hole, might be refracted upwards towards the opposite wall of the chamber, and there form a coloured image of the sun …
    So began Newton’s account of his experiments with prisms, which led him to the discovery that ordinary white light is really a mixture of rays of every variety of colour. He found, too, that the ray of each colour bends at a certain definite angle on passing through the prism – red being the least bendable, followed by ‘orange, yellow, green, blue, indigo, deep violet’. The richness of his response to colour is evident in his experimental accounts, as here where he is explaining that a ray of a single (or ‘homogeneal’) colour, shining upon objects, makes them all appear of that colour:
    All white, grey, red, yellow, green, blue, violet bodies, as paper, ashes, red lead, orpiment, indigo bice [dark blue], gold, silver, copper, grass, blue flowers, violets, bubbles of water tinged with various colours, peacock’s feathers, the tincture of lignum nephriticum [a wood imported from Spain, the blue infusions of which were used for kidney-disease], and suchlike, in red homogeneal light appeared totally red, in blue light totally blue, in green light totally green, and so of other colours. In the homogeneal light of any colour they allappeared totally of that same colour, with this only difference, that some of them reflected that light more strongly, others more faintly. I never yet found any body, which by reflecting homogeneal light could sensibly change its colour.
    From all which it is manifest that if the sun’s light consisted of but one sort of rays, there would be but one colour in the whole world.
    Newton’s friend Edmond Halley (observer of ‘Halley’s Comet’) had engaged in underwater operations off the Sussex coast in a