**TODHUNTER, ISAAC** (1820-1884), English mathematician,
son of George Todhunter, a Nonconformist minister, was born
at Rye on the 23rd of November 1820. He was educated at
Hastings, at which town his mother had opened a school after
the death of his father in 1826. He became an assistant master
at a school at Peckham, attending at the same time evening
classes at the University College, London. In 1842 he obtained
a mathematical scholarship and graduated as B.A. at London
University, and was awarded the gold medal on the M.A.
examination. About this time he became mathematical master
at a school at Wimbledon. In 1844 he entered St John's College,
Cambridge, where he was senior Wrangler in 1848, and
gained the first Smith's prize and the Burney prize; and in 1849
he was elected to a fellowship, and began his life of college lecturer
and private tutor. In 1862 he was made a fellow of the Royal
Society, and in 1865 a member of the Mathematical Society of
London. In 1871 he gained the Adams prize and was elected
to the council of the Royal Society. He was elected honorary
fellow of St John's in 1874, having resigned his fellowship on
his marriage in 1864. In 1880 his eyesight began to fail, and
shortly afterwards he was attacked with paralysis. He died
at Cambridge on the 1st of March 1884.

Works.—*Treatise on the Differential Calculus and the Elements of*
*the Integral Calculus* (1852, 6th ed., 1873), *Treatise on Analytical*
*Statics* (1853, 4th ed., 1874); *Treatise on the Integral Calculus* (1857,
4th ed., 1874); *Treatise on Algebra* (1858, 6th ed., 1871); *Treatise*
*on Plane Coordinate Geometry* (1858, 3rd ed., 1861); *Plane Trigonometry*
(1859, 4th ed., 1869); *Spherical Trigonometry* (1859); *History*
*of the Calculus of Variations* (1861); *Theory of Equations* (186I, 2nd
ed. 1875); *Examples of Analytical Geometry of Three Dimensions*
(1858, 3rd ed., 1873); *Mechanics* (1867), *History of the Mathematical*
*Theory of Probability from the Time of Pascal to that of Lagrange*
(1865); *Researches in the Calculus of Variations* (1871); *History of*
*the Mathematical Theories of Attraction and Figure of the Earth from*
*Newton to Laplace* (1873); *Elementary Treatise on Laplace's, Lamé's*
*and Bessel's Functions* (1875); *Natural Philosophy for Beginners*
(1877). An unfinished work, *The History of the Theory of Elasticity*,
was edited and published posthumously in 1886 by Karl Pearson.
Todhunter also published keys to the problems in his textbooks on
algebra and trigonometry; and a biographical work, William
Whewell, account of his writings and correspondence (1876), in addition
to many original papers in scientific journals.

See obituary notices in the *Proc. Lond. Math. Soc.* (1884), and
*Proc. Roy. Soc.* (1884).