**Willebrord Snell** was a Dutch mathematician who is best known for the law of refraction, a basis of modern geometric optics; but this only become known after his death when Huygens published it.

- It is also commonly given as Willebrordus Snellius, the Latin version of Willebrord Snell, which he used for all his publications.
- Let us make a comment on Willebrord's date of birth.
- Some biographies of Snell give 1591 as the year of his birth but this is simply an error copied from an old biography.
- Rudolph Snell (1546-1613), although appointed as an extraordinary professor of mathematics at the University of Leiden in 1581, was a broad scholar who did not have a great deal of mathematical skill.
- It is in this house, filled with students, in which Willebrord grew up.
- By 1600 Snell was studying law and teaching mathematics at the university on days when the professor was not teaching.
- After spending a while in Würzburg, the two mathematicians went to Prague where Snell was introduced to Tycho Brahe by van Roomen.
- Snell spent some time with Brahe assisting him in making observations and clearly learnt much during this visit.
- Snell, still in the company of van Roomen, continued visits to mathematicians in various German towns such as Joannes Praetorius in Altdorf, Michael Mästlin in Tübingen, and Wilhelm Hatzfeld and Christophorus Vulteius in Hersfeld.
- Slowly Snell's position at Leiden became more official.
- Further work on Apollonius which Snell produced at this time was never published and has been lost.
- Snell received the degree for Master of Arts from Leiden on 12 July 1608 after defending theses on artes liberales: grammatica, rhetorica, logica, arithmetica, geometria, analysis/algebra, physica, optica, astronomia, geographia, gnonomica, statica and ethica.
- Since Rudolph died a month later, Snell was required to continue teaching but he struggled to get proper recognition from the University of Leiden.
- In 1626, at the age of 46, Snell died from colic which caused a fever and paralysis of his arms and legs.
- Snell had dinner with his wife.
- In this work Snell attempted to measure the circumference of the earth and so required a considerable number of measurements.
- Throughout his career Snell was interested in astronomy and published several works on that topic some, but not all, of which contained data from his own observations.
- In this latter work Snell strongly criticised Aristotle and stressed how harmful to the development of science it was to continue to treat his views with such reverence.
- Despite his attack on Aristotle, Snell did not accept Copernicus's heliocentric system but firmly believed in an Earth centred system.
- Although he discovered the law of refraction now known as "Snell's law" in 1621, a basis of modern geometric optics, he did not publish it and only in 1703 did it become known when Christiaan Huygens published Snell's result in Dioptrica.
- There is a manuscript by Snell's which is an outline of a treatise he intended to write on optics.
- Other remarks in the manuscript reveal that Snell had studied the existing literature on optics, particularly on refraction; various passages in the manuscript are analogous in formulation and sequence to other treatises as shown by his marginal notes and occasional references to other texts interspersed throughout the manuscript.
- Snell showed special interest in Ibn al-Haytham's experimentum elegans ....
- Snell studied the loxodrome, the path on the sphere that makes constant angle with the meridians.
- In Snellius's work we see modern physics proceed from ancient physics with continuity.

Born 13 June 1580, Leiden, Netherlands. Died 30 October 1626, Leiden, Netherlands.

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Origin Netherlands, Physics

**O’Connor, John J; Robertson, Edmund F**: MacTutor History of Mathematics Archive