Kirsten Walsh writes…
In Ian Lawson’s recent post, he mentioned Hooke’s work on colours in thin films. In this post, I’ll look at how Newton used his hypotheses on light to build on Hooke’s work in some interesting and important ways.
In his optical work of the early 1670s, while Newton prefers theories to hypotheses, he thinks that hypotheses are acceptable, even useful, for two purposes:
- To ‘illustrate’ (i.e. provide an intuitively plausible explanation of) the theory; and
- To ‘suggest’ experiments.
However, he insists that hypotheses should always be removed from the final version of the theory. Recall Newton’s claim from his 1672 paper: “I shall not mingle conjectures with certainties”.
In December 1675, Newton wrote his paper, “An hypothesis explaining the Properties of Light”. Here, he published his hypotheses on the nature of light for the first time. To summarise them briefly:
- There is an ‘aethereal medium’;
- Aether vibrates, carrying sounds, smells and light;
- Aether penetrates and passes through the pores of solid substances;
- Light is neither the aether itself, nor the vibrations, but a substance that is propagated from ‘lucid’ bodies and travels through the aether;
- Light warms the aether and the aether refracts the light; and
- The rays (or bodies) of which light consists differ from one another physically.
In this paper, Newton claims that he is only discussing these hypotheses for the purposes of ‘illuminating’ his theory. Moreover, he does not assert that these hypotheses are true, and emphatically does not use them to support his theory. For example, when he discusses hypothesis (4), Newton is careful not to push too forcefully for any particular account of light. He says one might suppose light to be “an aggregate of various peripatetic qualities”, or “unimaginably small and swift” corpuscles of various sizes, or “any other corporeal emanation or impulse or motion of any other medium diffused through the body of the aether”:
- Onely whatever Light be, I would suppose, it consists of Successive rayes differing from one another in contingent circumstances, as bignes, forme or vigour… And further I would suppose it divers from the vibrations of the aether.
In this paper, there is a notable emphasis on experiment. For example, when Newton discusses hypothesis (1), he gives an account of a new electrical experiment which seems to support his claim. And when he discusses hypothesis (3), he discusses the implications for Boyle’s tadpole experiments. But the most important experiments in this paper are his investigations on the colours that appear between two glass surfaces.
Alan Shapiro notes that Newton began these investigations while he was reading Hooke’s Micrographia. But his experiments and mathematical descriptions quickly developed into something well beyond the scope of Hooke’s investigations. Hooke described the colours that appear when two thin sheets of glass are placed one on top of the other. When he made the thin film of air between the two sheets thicker or thinner by pressing the two sheets together with greater or lesser force, the colours changed. He observed that different colours appeared at different thicknesses, but he was unable to quantify this observation as he was unable to measure accurately the thickness of the film at any given point. Newton had the idea of placing a convex lens on top of a flat sheet of glass. This enabled him to easily calculate the thickness of the film of air, and the colours appeared as a set of concentric coloured circles centred at the point of contact between the two surfaces. These concentric circles are now known as ‘Newton’s Rings’.
Next Newton considered his hypotheses. According to hypothesis (2) the vibrations of the aether vary in size, according to hypothesis (3) aether passes through the pores of solid substances, and according to hypothesis (6) rays of different colours will cause aethereal vibrations of different sizes. If these hypotheses were correct, he argued, then light of a particular colour would be reflected either when the length of the vibration, or some multiple of the length of the vibration, matched the thickness of the film, and transmitted otherwise. So he predicted that:
- if the Glasses in this posture be looked upon, there ought to appear at A [the centre], the contact of the Glasses, a black spott, & about that many concentric circles of light & darknesse, the squares of whose semidiameters are to sense in arithmetical progression.
Newton’s “Hypothesis” paper provides a good example of his method of hypotheses. He remains carefully detached from his own hypothesis, using it only to ‘illustrate’ his theory and to suggest further experiments. Newton was also careful to keep his hypotheses well separate from his theory; the paper ends with a series of ‘Observations’ that contain no reference to his hypotheses at all!