Attempts have been made, principally founded on some remarks of Huygens, to show that Descartes had learned the principles of refraction from the manuscript of a treatise by Willebrord **Snell**, but the facts are uncertain; and, so far as Descartes founds his optics on any one, it is probably on the researches of Kepler.

He then returned to Balliol as a **Snell** exhibitioner; became vicar of High Ercall, Shropshire, in 1750; canon of Windsor, 1762; bishop of Carlisle, 1787 (and also dean of Windsor, 1788); bishop of Salisbury, 1791.

WILLEBRORD **SNELL** (1591-1626), commonly known as

In 1613 he succeeded his father Rudolph **Snell** (1546-1613) as professor of mathematics in the university of Leiden.

(A later recalculation gave 57,033 toises =121,569 yds., after the application of some corrections to the measures indicated by himself.) **Snell** also distinguished himself as a mathematician, and discovered the law of refraction, in 1621 (see Light).

A new departure, however, was made by Willebrord **Snell** of Leiden in his Cyclometria, published in 1621.

For the purposes of the calculator a solution erring in excess was also required, and this **Snell** gave by slightly varying the former construction.

C. Huygens, in his De Circuli Magnitudine Inventa, 1654, proved the propositions of **Snell**, giving at the same time a number of other interesting theorems, for example, two inequalities which may be written as follows 8 - chd B }- 4 chd Bsin a (chd 0-sin >chd 8+3 (chd 0-sin 0).

As in the case of the process of 6 It is thus manifest that by his first construction **Snell** gave an approximate solution of two great problems of antiquity.

His third and only surviving son, George Robert Gleig (1796-1888), was educated at Glasgow University, whence he passed with a **Snell** exhibition to Balliol College, Oxford.

His parents were Presbyterians, but he early turned towards the Scottish Episcopal Church, and was confirmed in his first year at Oxford, having entered Balliol College in October 1830 as a **Snell** exhibitioner from the University of Glasgow.

In 1771, in the hope of gaining a **Snell** exhibition and proceeding to Oxford to study for the English Church, he went to Glasgow, where he attended the classes of Thomas Reid.

**Snell**, articles in the United Service Magazine (1906-1907).