Experiment

Newton famously claimed that hypotheses are not admissible in natural philosophy. The proper method of inference was deduction from the phenomena:

whatever is not deduced from the phenomena is to be called an hypothesis; and hypotheses, whether metaphysical or physical, whether of occult qualities or mechanical, have no place in experimental philosophy. In this philosophy, particular propositions are inferred from the phenomena, and afterwards rendered general by induction. (p. 547)

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As several historians and philosophers of science have pointed out, however, there was a gap between Newton's methodological pronouncements and his actual scientific work. Newton himself frequently made use of hypotheses. His various deductions from the phenomena relied on various theoretical assumptions, and thus the "deduced" propositions were not, strictly speaking, deduced from the phenomena.

Newton was a prominent member of the Royal Society, and its president from 1703 until his death. His main experimental contributions concerned the mathematical science of optics. In his experimental work Newton attempted to come up with "crucial experiments" that would enable him to choose among competing hypotheses of the phenomena under investigation. Robert Hooke (1635–1703), in his Micrographia (1665), coined the term experimentum crucis.

During the 1660s Newton carried out a series of experiments on light, using a familiar instrument, the prism. Based on those experiments, he concluded that light was a composite entity, consisting of distinct rays, whose refractive properties depended on their color. Newton reported the experiments he had carried out in a paper that was published in the Philosophical Transactions of the Royal Society in 1672, where he adopted the customary presentation style of the Royal Society. That paper got him involved in a prolonged controversy with Hooke, who was then the curator of experiments in the Royal Society, a controversy that lasted until 1678. Hooke did not dispute the results of Newton's experiments, which he managed to reproduce. Rather he challenged Newton's inferences from those results and, in particular, Newton's conclusions on the composition of light.

After that controversy Newton remained silent on optics until Hooke's death. He then published his experimental and theoretical investigations on light in Opticks (1704), a book that was written in the vernacular without the use of mathematics. In that book Newton developed a corpuscular theory of light, which encountered opposition in continental Europe. Newton took advantage of his presidency of the Royal Society and his ever-growing power to face that opposition. He directed the work of the official experimentalists of the Royal Society, Francis Hauksbee (c. 1666–1713) and John Desaguliers (1683–1744), who effectively promoted the Newtonian worldview through their experimental researches, public lecturing, and textbook writing.