John Casey

John Casey (1820 to 1891) was an Irish geometer, born at Kilbehenny in County Limerick and active in Dublin. He is best known for Casey's theorem, which describes a circle tangent to four other circles and extends Ptolemy's theorem, and for new proofs and perspectives in Euclidean geometry. He began as a teacher under the Board of National Education and later headed the Central Model Schools in Kilkenny. He entered Trinity College Dublin in 1858 and took his degree in 1862, then taught mathematics at Kingston School before holding a chair of higher mathematics and mathematical physics at the Catholic University of Ireland and a lectureship at University College Dublin. He was elected a Fellow of the Royal Society in 1875, and the Royal Irish Academy gave him its Cunningham Gold Medal in 1878. His textbooks on Euclid, analytic geometry and trigonometry were widely used. He and Emile Lemoine are regarded as co-founders of the modern geometry of the circle and the triangle.