Thomas Harriot: The Discovery of Refraction
How an English mathematician beat Snell and Descartes to one of optics' most fundamental laws — and was forgotten for centuries
A. Thomas Harriot was one of the most gifted natural philosophers of the late sixteenth and early seventeenth centuries — a mathematician, astronomer, navigator, and experimental scientist whose range of original contributions rivalled those of any of his contemporaries. He produced the first telescopic observations of the Moon before Galileo made his own more celebrated drawings; he made the first systematic observations of sunspots; he formulated the sine law of refraction — the relationship between the angle of an incoming light ray and the angle of refraction at the boundary between two media — a full decade before its independent discovery by Willebrord Snell. Yet Harriot published almost nothing in his lifetime and is largely unknown today, his contributions overshadowed by the achievements of contemporaries who were less reluctant to claim public recognition for their work.
B. Harriot was born in Oxford around 1560 and studied at Merton College. His early career took him to North America as the scientific member of Sir Walter Raleigh's first expedition to Virginia in 1585, where he spent a year documenting the natural history, ethnography, and geography of the Algonquian-speaking peoples of the Outer Banks. His account of this expedition, A Briefe and True Report of the New Found Land of Virginia, published in 1588, was widely read and translated into several languages, providing one of the earliest and most scientifically careful accounts of indigenous American life. It was the only work Harriot published in his lifetime.
C. Harriot's work on light and optics began in the 1590s and appears to have reached its most significant results around 1601 to 1602. Working with glass prisms and careful measurement of the angles of incident and refracted light rays, he established the mathematical relationship between these angles that is now known as Snell's Law: the ratio of the sines of the angles of incidence and refraction is constant for a given pair of media, and this constant is characteristic of the two media involved. Harriot recorded his findings in unpublished manuscripts that were not discovered and properly analysed until the twentieth century, when historian of science John Shirley identified the key calculations and established Harriot's priority.
D. Why Harriot did not publish his optical findings remains a matter of scholarly discussion. The precarious political situation of Harriot's primary patrons — both Walter Raleigh and Henry Percy, Earl of Northumberland, his two main supporters, faced accusations of atheism and treason and both spent time in the Tower of London — may have made publication of any kind seem politically risky. Harriot himself shared lodgings with Raleigh's household and was associated with circles viewed with suspicion by the authorities. Some scholars have also suggested that Harriot was a perfectionist who was reluctant to publish anything until he was satisfied that his analysis was complete, a standard he may never have felt was fully achieved.
E. The circumstances of Harriot's later life were marked by illness and the continued failure to bring his substantial unpublished work to the public. He was diagnosed with a cancer of the nose in 1615 — one of the earliest cancer diagnoses recorded in English medical history — and died in 1621, leaving over 8,000 pages of manuscript material covering mathematics, astronomy, optics, navigation, and natural philosophy. His executor made efforts to organise and publish the manuscripts but died before completing the task. The materials eventually entered the British Library, where they remained incompletely analysed for centuries.
F. The rehabilitation of Harriot's historical reputation began in earnest in the twentieth century, as historians of science began the systematic analysis of his manuscripts. The establishment of his priority in formulating the sine law of refraction was a significant finding, but the full scope of his contributions — including his work in algebra, which anticipated several results later attributed to Descartes and others — is still being assessed. Harriot's story has become a touchstone for discussions about the relationship between scientific discovery and publication, about the multiple dimensions of credit and priority in the history of science, and about how many significant discoveries may have been made and then lost because their makers lacked either the opportunity or the inclination to claim public recognition.