Newton’s prisms and the uses of experiment

Newton’s prisms and the uses of experiment, D. GOODING, T. PINCH, S. SCHAFFER, The Uses of Experiment, CUP, 1989.


‘Instruments are in truth reified theorems.’

Gaston Bachelard, 1933

‘Perhaps said the Marchioness, Nature has reserved the Merit of demonstrating the Truth to the English prisms; that is , to those by whose means she at firts discovered herself.’

Francesco Algarotti, 1737

Experimental controversy involves contest about authority. The acceptance of a matter of fact on the basis of an experimental report involves conceding authority to the reporter and to the instruments used in the experiment. In the seventeenth century, experimental philosophers used a wide range of means to make authority for their work. Conviction was thought to result from a long series of trials or from a single decisive experiment. It might result from being present as a witness at such a trial, achieving a replication of such a trial, or by reading a report given in so much circumstancial details that such direct witnessing was obviated. Authority might be held to lie in the credit of a single experimenter or in the communal assent of the experimental community. In their controversies, experimental philosophers often strenuously debated these differing ways of making conviction. Such lights show some of the uses of experiment in reaching agreement among disputants. Furthermore, they show how experimental instruments play a central role in these usages, and are resources which experimenters deploy in their struggles to achieve authority.

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The controversy discussed in this paper centred on Newton’s work on light and colours between the 1660s and the 1720s. His trials with prisms, notably the celebrated experimentum crucis, were emblematic of experimental philosophy. They became so in at least two ways. After 1704, Newton claimed his trials had been replicated by competent experimenters, making facts which natural philosophers must acknowledge and use in their own work. Second, Newton also claimed that amongst these trials it was possible to pick out those which were ‘crucial’, and which would decisively settle dispute. The authority of these emblems was retrospectively located in the events surrounding the first punlic announcements Newton made in the early 1670s. But these emblematic uses were developed over several decades - they were not swiftly achieved, nor were they ever uncontested. Newton’s programme was a site at which natural philosophers debated the boundary between experiments, ‘concluding directly and without any suspicion of doubt’, and hypotheses, ‘conjured by barely inferring ‘tis thus because not otherwise or because it satisfies all phaenomena’. To some of his critics, Newton seemed to violate the rules of the experimental life. He was attacked as dogmatical, overestimating the authority due to his reports, providing too few trials to license his conclusions, and reporting experiments which could not be replicated. Thus, in contrast to Robert Boyle’s celebrated emphasis on ‘histories’ of many trials, Newton interrupted his first published account of optical trials to state that ‘the historical narration of these experiments would make a discourse too tedious & confused, & therefore I shall rather lay down the Doctrine first and then, for its examination, give you an instance or two of the Experiments, as a specimen of the rest’. Comments such as these highlighted differences over proper conduct in experimental work and reportage.

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Both the problem of the ‘cruciality’ of experiments and that of their ‘replicability’ are typical of the experimental sciences. It is misleading to treat the authority of such experiments as self-evident, for this obscures the detailed character of experimental controversy. The ground of such auhtority was often the matter in dispute. The resolution of such disputes masks the process by which agreement is accomplished. Agreement includes consensus about the conduct and meaning of a particular trial. Where experiments are interpreted as conveying unarguable lessons about the contents of Nature, this indicates that a controversy has already reached a stage of provisional closure. Only then will experiments be defined through an exemplary method, standardised tools and an agreed matter of fact. This paper examines the career one of Newton’s experiments, a trial with two prisms which he first recorded in a note book in 1666 as the ‘forty-fourth’ of a long list of experiments on light and colour. In significantly changed format, this trial was made into an ‘experimentum crucis’. Newton’s experimentum crucis was the object of considerable debate during the 1670s and has remained a central topic of philosophical and historical attention. The term was not used in his notebooks, drafts or lectures before 1672, nor did it appear in the Opticks in 1704. Nevertheless, the label remained current among Newton’s readers and disciples. However, the reference of the term changed markedly. There was no consensus among the experimenters on the lesson which its author intended should be taught by this trial, nor on the proper method for conducting it. Some tried their version of this experiment, obtained results different from those which they held Newton had reported and Newton’s account of light and colour. These experimenters treated the trial as crucial but used that cruciality to undermine Newton’s theory. Others, notably Robert Hooke, replicated Newton’s trial, but then argued that the trial was not crucial, and denied that this replication licensed Newton’s account. ‘Cruciality’ was an accomplishment which varied with out-comes of attempted replication.

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The character of this accomplishment was intimately connected with issues of instrumentation, specifically, with the evaluations experimenters gave of the quality and arrangement of their prisms. There was no uncontroversial way making these evaluations authoritative. For one community of experimenters during one period of time, Newton’s experimentum crucis could be associated with an obvious procedure, involving complex arrangements of specially crafted prisms and lenses and a self-efficient matter of fact, involving the chromatic homogeneity and the fixed refrangibility of primitive colour-making light rays. In the crucial experiment, a prism was used to make ‘primitive’ rays, and then one of these rays subjected to a second refraction in a second prism. Newton sometimes claimed that if white light were transmitted through a prism it could be separated into a set of ‘primitive’ ray could not then be further colours. But for Newton and his allies, a ‘primitive’ ray could be defined as a ray which could not be split by a second refraction. Then experimenters who managed to split such a ray could be criticised by Newton for their failure to produce ‘primitive’ rays. This argument established a troubling circle, akin to what H.M. Collins calls the “experiementer’s regress”. The criterion of a good experiment was that it produced the matter of fact which Newton sought to establish. Experimenters had to be convinced of theis matter of fact before they could share this criterion. Once conviction had been achieved, then this criterion seemed unchallengeable. After closure, the procedures for making ‘primitive’ rays became self-evident. This paper documents the process by which this self-evidence was accomplished.

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The unarguable meaning which individuates an experiment is not achieved without struggle. How, then, do experiments acquire their identity? Scientific instruments play a decisive role in this process. Newton’s arguments suggested that good prisms were those which made ‘primitive’ rays. His critics were told they were using bad prisms. Instruments help make experiments compelling because the self-evidence which is attached to instrumental procedures after closure links complex experiments to agreed matters of fact. This closure makes instruments into what are seen as uncontestable transmitters of messages from nature, that is, it makes them “tranparent” [....]

Prisms have become so uncontentious that it is now hard to recapture the sense of their contingent and controversial use. Yet it is that contingent and controversial use which must be recovered in order to understand how ‘transparency ‘ is accomplished.

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