Abstract
Isaac Newton’s first optical paper (published in the Philosophical Transactions in February1672) was controversial: Newton argued for a new theory of light and colour when no one else thought the old one was inadequate, and he argued that his new theory was certainly true! A debate followed, in which Newton defended his claims against the objections of optical heavy weights, Robert Hooke, Christiaan Huygens, and Ignace-Gaston Pardies. One major sticking point between Newton and his critics concerned the number and division of colours. Newton argued that the number of different original colours was indefinite, but his critics objected to this inflated ontology. Each critic argued, for different reasons, that there were only two original colours. I examine Newton’s responses to these objections. I argue that they are revelatory of Newton’s unique methodology: a mathematico-experimental approach that eschewed ‘hypotheses’ in favour of ‘theories’. Nowadays, Newton’s first optical paper represents a landmark in the science of optics. Its exploitation of the correspondence between refraction and spectral colour provided a new approach to the study of light. And its views on the properties and nature of light, set a new agenda for the field.
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Notes
- 1.
Hooke and Huygens were wave theorists, and each would write a book concerning optics: Hooke wrote
Micrographia (1665) and Huygens wrote Traité de la lumière (1690).
- 2.
Commentators have found the style of Newton’s ‘New Theory’ enigmatic. Not only because it attempts to put forward some very sophisticated and novel scientific ideas in such a short paper (it’s only about 5000 words long!), but also because it combines an experimental focus with a quasi-geometrical approach to theorising. In fact, this paper is just the tip of the iceberg (Schaffer 1986: 84). Newton had been developing the ideas since 1665, and had presented them his Optical Lectures, delivered between 1670 and 1672 (Newton 1984).
- 3.
Henry Oldenburg, the founding Secretary of the Royal Society and the founding Editor of the Philosophical Transactions, wrote to Newton immediately to report on its reception: ‘I can assure you, Sir, that it there mett both with a singular attention and an uncommon applause, insomuch that after they had order’d me to returne you very solemne and ample thankes in their name (which herewith I doe most cheerfully) they voted unanimously, that if you contradicted it not, this discourse should without delay be printed, there being cause to apprehend that the ingenuous & surprising notion therein contain’d (for such they were taken to be) may easily be snatched from you […]’ (Newton 1959–1977: Vol. 1, 107)
- 4.
In fact, this is almost certainly a ‘rational reconstruction’ of the events that took place during the plague years (1665–1666). As we shall see, the careful construction of the experimental set-up belies the feigned casualness with which Newton describes the experiment. For discussion on this point, see (e.g. Whiteside 1966).
- 5.
More specifically, an elongated circle—curved at the ends and straight along the sides. In later work, Newton often depicted this shape in an idealised form, as a sequence of overlapping circles (e.g. Newton 1952: 65, fig. 23). Newton reported that the length of the image was five times longer than its breadth (Newton 1959–1977: Vol. 1, 92).
- 6.
The sine law, also called ‘Snell’s Law’ or the ‘Snell-Descartes law’, states that the ratio of the sines of the angles of incidence and refraction is a constant that depends on the medium through which the light passes.
- 7.
For an account of some of this history, see (Lindberg 1981)
- 8.
Moreover, Sabra notes, Newton’s assertion (that the image should have appeared circular) is the only indication of the experimental set-up in this paper. This lack of clarity may well have been the source of the criticisms of Pardies (Newton 1959–1977: Vol. 1, 131) and Linus (Newton 1959–1977: Vol. 1, 317–319), both of whom objected that the elongated image could be explained by the received theory of light.
- 9.
In this context, a ‘ray’ is just some smallest part of light.
- 10.
Pardies would claim that this variation was enough to account for the effect (Newton, 1959–1977: Vol. 1, 131).
- 11.
Refrangibility is the degree to which light can refract when passing from one medium into another, or a “predisposition, which every particular Ray hath to suffer a particular degree of Refraction” (Newton 1959–1977: Vol. 1, 96).
- 12.
- 13.
And indeed this was the case, until Chester Moore Hall succeeded in developing the achromatic lens, shortly after Newton’s death.
- 14.
It is a little misleading to refer to the modificationist view, since many different ones were proposed by, e.g. Aristotle, Descartes and Hooke. However, these all had one main feature in common: colour is the result of the modification of white light. For a discussion of the various versions of this view, see (Zemplén 2004).
- 15.
Here’s a fun fact: Newton was the first to use the term ‘spectrum’ to describe the coloured band into which a beam of light is decomposed by means of a prism (OED, December 2015).
- 16.
Bacon used the term ‘instantia crucis’ (i.e. a ‘crucial instance’) in the Novum Organum, but there is some confusion in the literature as to who first introduced the related term ‘experimentum crucis’. Peter Anstey and Michael Hunter have argued that, while the notion is often attributed to Hooke, it in fact should be attributed to Boyle, who introduced the notion in his Defence against Linus (1662) (Anstey and Hunter 2008: 112).
- 17.
- 18.
It is worth noting, however, that Bacon’s notion of instantia crucis appears to be broader than Newton’s notion of experimentum crucis: where the former refers to observations and experiments, the latter refers almost exclusively to experiments. In this sense, Newton’s experimentum crucis is similar to Boyle’s (Anstey and Hunter 2008: 112). For a discussion of the relationship between Newton’s experimentum crucis and Bacon’s instantia crucis, see (Hamou, Forthcoming).
- 19.
In fact, Hooke was one of the few scientists who was able to replicate Newton’s experiment.
- 20.
In his Opticks, published in 1704, Newton offers experimental support for this proposition.
- 21.
It is worth noting that, strictly speaking, Newton does not consider rays of light to be coloured. In the Opticks, he included a definition: “The homogeneal Light and Rays which appear red, or rather make Objects appear so, I call Rubrific or Red- making; those which make Objects appear yellow, green, blue and violet, I call Yellow-making, Green-making, Blue-making, Violet-making, and so of the rest. And if at any time I speak of Light and Rays as coloured or endued with Colours, I would be understood to speak not philosophically and properly, but grossly, and accordingly to such Conceptions as vulgar People in seeing all these Experiments would be apt to frame. For the Rays to speak properly are not coloured. In them there is nothing else than a certain power and disposition to stir up a Sensation of this or that Colour. For as Sound in a Bell or musical String or other sounding Body, is nothing but a trembling Motion, and in the Air nothing but that Motion propagated from the Object, and in the Sensorium ’tis a Sense of that Motion under the form of Sound; so Colours in the Object are nothing but a Disposition to reflect this or that sort of Rays more copiously than the rest; in the Rays they are nothing but their Dispositions to propagate this or that Motion into the Sensorium, and in the Sensorium they are Sensations of those Motions under the forms of Colours” (Newton 1952: 124–125).
- 22.
This shows how deeply entrenched modificationist intuitions were!
- 23.
It is not clear in what sense Newton’s seven-colour spectrum was new, since the notion of a seven-colour spectrum dates back at least to Aristotle. Establishing the originality of Newton’s spectrum is beyond the scope of this paper.
- 24.
But Newton was cautious. He said that although he had established that light is heterogeneous, “to determine more absolutely, what Light is, after what manner refracted, and by what modes or actions it produceth in our minds the Phantasms of Colours, is not so easie” (Newton 1959–1977: Vol. 1, 100). He said that he was not willing to speculate any further on these matters.
- 25.
I discuss Newton’s separation of hypotheses and theories below.
- 26.
Sabra has pointed out that this was barely intelligible to wave theorists (Sabra 1967: 280–282).
- 27.
Newton recognised this metaphysical commitment when he said: “Besides, who ever thought any quality to be a heterogeneous aggregate, such as Light is discovered to be” (Newton 1959–1977: Vol. 1, 100). In other words, substances can be combined in this way, but qualities cannot.
- 28.
In this letter to Huygens, Newton presented a new version of his theory of colours in a series of definitions and propositions.
- 29.
Newton sent his ‘Hypothesis’ to the Royal Society in December 1675, but manuscript evidence shows that the bulk of this paper was completed in 1672.
- 30.
Newton explained that he employed an assistant to make the judgements “partly because my owne eyes are not very criticall in distinguishing colours, partly because another, to whome I had not communicated my thoughts about this matter, could have nothing by his eyes to determin his fancy in makeing those marks” (Newton 1959–1977: Vol. 1, 376).
- 31.
A monochord is a musical stringed instrument wherein a single string is stretched over a sound box. The string is fixed at both ends, and one or more movable bridges are manipulated to demonstrate mathematical relationships between sounds. It was used as a scientific instrument to illustrate the mathematical properties of musical pitch.
- 32.
He continued to hold, however, that there were many degrees of colour: “the Spectrum pt. formed by the separated Rays […] appear tinged with this Series of Colours, violet, indigo, blue, green, yellow, orange, red, together with all their intermediate Degrees in a continual Succession perpetually varying. So that there appeared as many Degrees of Colours, as there were sorts of Rays differing in Refrangibility” (Newton 1952: 122).
- 33.
For a discussion of Newton’s theory of vision, see (Hamou 2014).
- 34.
This manuscript is found in a notebook kept by Newton during 1664–1666 (Cambridge University Library Add. Ms. 4000, ff. 137–143). I quote this passage from (Pesic 2006: 299–300). In an attempt at clarity, I have flouted convention by regularising Newton’s spelling and omitting his editing marks.
- 35.
Moreover, my speculation has other potential routes to testing (which I won’t explore here). For instance, if Newton thinks that ROYGBIV is a set of aesthetic categories, he will likely think the same of musical scales—and potentially take a similar angle on other distinctions of this kind. If more of Newton’s manuscripts are examined in this light, and a pattern emerges, this would lend further support.
- 36.
Here, Newton was referring to experiments described in Hooke’s Micrographia (Hooke 1966/1665: 48), which Hooke mentioned in his response to Newton’s paper.
- 37.
Newton was clearly disappointed that Hooke had failed to recognise the epistemically special relationship between his new theory and his experiments (Newton 1959–1977: Vol. 1, 171). For, instead of considering Newton’s support for his theory, Hooke had discussed whether another hypothesis could fit the evidence just as well. However, it is useful to note that Newton misinterpreted Hooke’s objection. Where Newton took Hooke to be attempting to assert his own hypothesis in place of Newton’s, Hooke was in fact careful to point out that other hypotheses could also fit the facts (Newton 1959–1977: Vol. 1, 113).
- 38.
It is worth noting that, while both Hooke and Huygens had stakes in the debate in that they both had recently published books concerning optics, they weren’t threatened to the same extent by Newton’s new theory. Huygens had developed a mathematical wave theory, which developed the notion of wave fronts, but which didn’t deal with colour. In contrast, Hooke’s wave theory offered a new modificationist account of colour. Hooke had more to lose.
- 39.
Although Huygens did admit that he hadn’t tried this yet (“car cette pensee ne m’est venue qu’a cette heure” (Newton 1959–1977: Vol. 1, 257, n.4)).
- 40.
Nevertheless, from then on, Newton was always careful to distinguish between sunlight and white light! See (e.g. Newton 1952: 26, 63, 116).
- 41.
For a discussion of the distinction between theories and hypotheses in early modern philosophy more generally, see (Ducheyne 2013).
- 42.
For an extended discussion of the respective roles of hypotheses and queries in Newton’s natural philosophy, see (Walsh 2014).
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Walsh, K. (2017). How Many Colours?. In: Silva, M. (eds) How Colours Matter to Philosophy. Synthese Library, vol 388. Springer, Cham. https://doi.org/10.1007/978-3-319-67398-1_3
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