Primer: Newton’s Prism Experiments and Theory of Color

Today’s Hump-Day History post is written by frequent visitor Thony Christie, a dedicated amateur historian who “once had a semi-professional background”.  He has approved a few editorial truncations and rephrasings.

Update: Not long after this blog post, Thony started his own blog, The Renaissance Mathematicus.

In 1672 the still relatively young and unknown Lucasian Professor of Mathematics at Cambridge University, Isaac Newton, published his first piece of experimental philosophy: “A Serie’s of Quere’s Propounded by Mr. Isaac Newton, to be Determin’d by Experiments, Positively and Directly Concluding His New Theory of Light and Colours; and Here Recommended to the Industry of the Lovers of Experimental Philosophy, as they Were Generously Imparted to the Publisher in a Letter of the Said Mr. Newtons of July 8.1672”  in the Philosophical Transactions of the Royal Society.  The work became a touchstone in the establishment of the short report of experimental results in a serialized publication as a major means of scientific communication.  The Philosophical Transactions had existed for seven years prior to Newton’s contribution, but had been dedicated primarily to reporting the Royal Society’s regular piecemeal correspondence rather than the systematic presentation of experiments and observations, which was at that time accomplished mainly in the book format.

A sketch by Newton of one of his prism experiments.

As to the content of Newton’s first publication, it reported a series of simple but elegant experiments with a beam of sunlight and a couple of glass prisms, in which Newton demonstrated that light is not homogeneous and white, but heterogeneous, and made up of different colours each of which has a slightly different index of refraction thereby resulting in the spectrum when refracted by a prism or the rainbow when refracted by raindrops. Previous to Newton, it had been believed that colour was an attribute or property of the objects that were coloured and that the perception of colour was somehow transmitted from the objects to the eye during the process of vision, an idea that went back to Aristotle. It was also believed that the spectrum was produced by white light being somehow dirtied or darkened on passing through the refracting medium. Newton’s paper changed all of this radically.

But not without difficulty. The two leading experts for things optical, at this time, were Robert Hooke in London and Christian Huygens in Paris, both of whom reacted very negatively to Newton’s paper. When asked for his opinion by Oldenburg, the secretary of the Royal Society, Huygens was at first lukewarm and did not appear to see anything new in Newton’s work, implying that he had not really read the paper, but, when pressed, rejected Newton’s theories out of hand. Newton was enraged and in his reply addressed Huygens, a leading figure in European natural philosophy, as if he were addressing a particularly ignorant schoolboy. Huygens said that if the discussion was to be conducted at that level, he would not contribute.

One of Newton’s major problems was that he had used his discoveries to support his own view that light was corpuscular in nature; he argued that the refracting medium imparted spin to a light particle (in the same way as a tennis player imparts spin to the ball), and the different indexes of refraction are a result of the different degrees of spin imparted to the particles of each colour. Both Huygens and Hooke had developed wave theories of light, and it was Hooke who took up the attack.  He interpreted Newton as saying that his theory of colour was dependent on a corpuscular theory of light.  Yet, as he, Hooke, had already philosophically demonstrated that light was propagated in waves, then Newton’s theory must be wrong. This was just the main one of many criticisms that Hooke brought that led to a very tempestuous exchange of letters through Oldenburg over a period of several years.

At first Newton was content to answer, and he even showed that his theory worked equally well for a wave theory of light at the same time producing the best mathematical model for such a theory in the 17th century. During this period Newton worked on a long exhaustive essay on optics covering all of his research work up until this time, which he intended to publish in the Philosophical Transactions as a glorious rebuttal of all of his critics.  However, Hooke did not let up, and Newton was further beset by criticisms from Ignace Gaston Pardies, a highly respected Jesuit scientist living in Paris who was also something of an expert for optics, and a second Jesuit, the Englishman Francis Hall, known as Linus, who lived in Liège. The dispute with Pardies passed off relative quietly, but the one with Linus dragged on for six years, and was continued by his student John Gasgoines after Linus’ death.

Although Linus was not a well-known philosopher, his objections are interesting and significant from a methodological point of view: he complained that he had been unable to repeat Newton’s experiments! This was not an isolated incident as the same thing occurred to Italian Newtonians at the beginning of the 18th century. In the case of the Italians, it turned out that the problem lay in the quality of the glass prisms that they were using and when they replaced them with better quality glass they were able to achieve the same results as Newton.  One can assume that something similar happened in the case of Linus, but we will never know.

The results of this mass of criticism were fairly monumental.  Newton’s patience, never very good at the best of times, gave out. He withdrew the extended optics essay that he had been writing and refused to have any more direct dealing with the Royal Society until 1704.  He never established a relationship with Huygens. The feud with Hooke was patched up, only to break out again in the 1680s when Hooke accused Newton of having stolen the inverse square law of gravitation from him (but that, as they say, is another story).

In 1703 Hooke died and Newton extracted his revenge.  He became active in the Royal Society and was immediately elected its president in 1704, from which position he prevented the publication of Hooke’s posthumous papers (and even is rumoured to have had his portrait burned). In the same year he finally published that long essay, to which Hooke’s criticism had driven him, as the first section of his book Opticks (which became a great classic of the scientific literature).

In the Bible the rainbow is a symbol of peace following a deluge; for Newton, unweaving the rainbow proved to be the prelude to a storm.

Literature: For all things Newton the best place to start is Richard Westfall’s masterpiece Never at Rest: A Biography of Isaac Newton [1980], a book that deserves a place in Will’s list of canonical literature. On the intricacies of 17th century optical theories and the disputes between their proponents the best source is A.I. Sabra’s Theories of Light from Descartes to Newton [1967].

NB from Will: On the difficulties of replicating Newton’s prism experiments and the controversies surrounding Newton’s claims, also see Simon Schaffer’s classic “Glass Works: Newton’s Prisms and the Uses of Experiment” in the 1989 volume The Uses of Experiment.