# Todhunter, Isaac (DNB00)

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TODHUNTER, ISAAC (1820-1884), mathematician, was second son of George Todhunter, independent minister of Rye, Sussex, and Mary, his wife, whose maiden name was Hume. Isaac was born on 23 Nov. 1820. His father’s death in 1826 left the family in narrow circumstances and the mother opened a school at Hastings. Isaac, who as a child was ‘unusually backward,’ was sent to a school in the same town kept by Robert Carr, and subsequently to one newly opened by Mr. J. B. Austin from London; by the influence of this latter teacher his career was largely determined. He next became assistant master at a school at Peckham, and while thus occupied managed to attend the evening classes at University College, London, where he had for his instructors Key, Malden, George, Long, and Augustus De Morgan, to all of who he always held himself greatly indebted, but especially to the last. In 1842 he graduated B.A. and obtained a mathematical scholarship in the university of London, and, on proceeding M.A., obtained the gold medal awarded for that examination. Concurrently with these studies he filled the post of mathematical master in a large school at Wimbledon conducted by Messrs. Stoton and Mayer.

In 1844, acting on De Morgan’s advice, he entered at St. John’s College, Cambridge. In 1848 he gained the senior wranglership and the first Smith’s prize, as well as the Burney prize. In the following year he was elected fellow of his college. From this time he was mainly occupied as college lecturer and private tutor, and in the compilation of the numerous mathematical treatises, chiefly educational, by which he became widely known. Of these, his Euclid (1st ed. 1862), a judicious mean between the symbolism of Blakelock and the verbiage of Potts, attained an enormous circulation; while his algebra (1858), trigonometry, plane and spherical (1859), mechanics (1867), and mensuration (1869), all took the place which they for the most part still retain as standard text-books. No mathematical treatises on elementary subjects probably ever attained so wide a circulation; and, being adopted by the Indian government, they were translated into Urdu and other Oriental languages. He was elected F.R.S. in 1862, and became a member of the Mathematical Society of London in 1865, the first year of its existence. In 1864 he resigned his fellowship on his marriage (13 Aug.) to Louisa Anna Maria, eldest daughter of Captain (afterwards Admiral) George Davies, R.N. (at that time head of the county constabulary force). In 1871 he gained the Adams prize, and in the same year was elected a member of the council of the Royal Society. In 1874 he was elected an honorary fellow of his college. In 1880 an affection of the eyes proved a forerunner of an attack of paralysis which eventually prostrated him. He died on 1 March 1884, at his residence, 6 Brookside, Cambridge. A mural tablet and medallion portrait have since been placed in the ante-chapel of his college by his widow, who, with four sons and one daughter, survived him.

Todhunter’s life was mainly that of the studious recluse. His sustained industry and methodical distribution of his time enabled him to acquire a wide acquaintance with general and foreign literature; and besides being a sound Latin and Greek scholar, he was familiar with French, German, Spanish, Italian, and also Russian, Hebrew, and Sanscrit. He was well versed in the history of philosophy, and on three occasions acted as examiner for the moral sciences tripos. His habits and tastes were singularly simple; and to a gentle kindly disposition he united a high sense of honour, a warm sympathy with all that was calculated to advance the cause of genuinely scientific study in the university, and considerable humour.

Besides the text-books above enumerated, he published: 1. ‘A Treatise on the Differential Calculus and the Elements of the Integral Calculus,’ 1852. 2. ‘Analytical Statics,’ 1853. 3. ‘A Treatise on Plane Co-ordinate Geometry,’ 1855. 4. ‘Examples of Analytical Geometry of three Dimensions,’ 1858. 5. ‘The Theory of Equations,’ 1861. 6. ‘History of the Progress of the Calculus of Variations during the Nineteenth Century,’ 1861. 7. ‘History of the Mathematical Theory of Probability from the Time of Pascal to that of Laplace,’ 1865. 8. ‘History of the Mathematical Theories of Attraction from Newton to Laplace,’ 1873. 9. ‘The Conflict of Studies and other Essays on Subjects connected with Education,’ 1873. 10. ‘Elementary Treatise on Laplace’s Functions,’ 1875. 11. ‘History of the Theory of Elasticity,’ a posthumous publication edited by Dr. Karl Pearson (1886).

Todhunter’s publications were the outcome of great research and industry, and he made in them many valuable contributions to the history of mathematical study. His most original work is his ‘Researches on the Calculus of Variations’ (the Adams prize for 1871), dealing with the abstruse question of discontinuity in solution.

[In memoriam: Isaac Todhunter, by Professor J. E. B. Mayor; Dr. Routh in Proceedings of the Royal Society, vol. xxxvii; The Eagle, a magazine supported by the members of St. John’s College, xiii. 94 sq.]