Preston, Thomas

Thomas Preston (1860-1900) was born at Ballyhagan, Kilmore, Co. Armagh, and was one of the most famous ex-pupils of The Royal School in Armagh. He studied for the examinations of the Royal University in Ireland before enrolling in Trinity College Dublin where he graduated with both honours in mathematics and experimental science in 1885. He there came under the influence of George Francis Fitzgerald. Preston became an integral part of the exciting Maxwellian research programme led by Fitzgerald and George Johnstone Stoney in which Dublin was at the very forefront of international research into electromagnet and spectroscopic sciences. Preston found an important niche and as Fitzgerald had too many commitments to write textbooks himself he encouraged Preston’s gift here.

Preston proved to be a magnificent textbook writer and was the author of two renowned textbooks in physics. The Theory of Light (1890) and the Theory of Heat (1894), both of which reached several editions, and remained in use long after his death. These books were used all over the English-speaking world in both physics and engineering degree courses and unquestionably the best Maxwellian textbooks available in these two key engineering research areas being developed in Dublin at that time. He continued his mathematical studies while teaching at what is now University College Dublin, and was appointed Professor of Natural Philosophy there in 1891. When leading spectroscopists in Europe and America engaged, during 1897, in exploring the recently discovered Zeeman Effect, they were perhaps surprised to be taken over by a relatively obscure researcher working in Dublin. Preston was, however, coming to these researches from training in Trinity College which made his achievement in discovering the Anomalous Zeeman Effect less surprising. His enormous contribution was immediately recognised as Preston was awarded the second Boyle Medal by the Royal Dublin Society in 1899, the same year as the first medal had been awarded to G.J. Stoney. He was a mere 39 years of age but unfortunately too ill to attend his award and his untimely death a year after has deprived posterity until now a full account of his life and qualities.

Preston is today widely celebrated in the history of science for his work in splitting the spectral lines in the presence of a strong magnetic field. Dutch physicist Pieter Zeeman (1865-1943), had found in 1896 that the spectral lines of certain elements were split into doublets and triplets. This splitting could be explained by classical theory. However, Preston reported, in an important paper published in The Scientific Transactions of The Royal Dublin Society, read on 22nd December 1897, and published the following April, that he reported results more complicated than Zeeman had reported. Following this up further, he reported in a second paper in the RDS Scientific Transactions, read on 18th January 1899, and published the following June, that he had found results that were ’very startling’ and appeared ‘quite contrary to all theoretical explanations’. He had discovered what is called the Anomalous Zeeman Effect, which challenged classical theories. The full explanation had to wait for the theory of relativity and the introduction of quantum mechanics, which were to shake the rigid framework of Newtonian conceptions of absolute time and space. Preston’s results were an important step in this development.

Preston went on to establish empirical rules for spectral lines, though he could not give a theoretical explanation, and these rules are still associated with his name. He was elected to Fellowship of the Royal Society in 1898 which was the most appropriate and fitting recognition of his pioneering work. In modern times this has led directly to the MRI scanner that has become perhaps the most important tool in medical diagnostics. 


Weaire D and O’Connor, S, (1987) Unfilled renown: Thomas Preston (1860-1900) and the anomalous Zeeman Effect, Annals of Science, Vol. 44, 6, 617-644.

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