Thomas Preston (1860-1900)

The life and work of Thomas Preston is described in an article by D. Weaire and S. O’Connor [1]. Herein is a short description of Preston’s career in physics in Trinity College Dublin and his discovery of the Anomalous Zeeman Effect. Preston was the subject of Weaire’s Trinity Monday Memorial Discourse in 1990.

Thomas Preston arrived in Trinity in 1881 and graduated in 1885 in in Mathematics and Experimental Sciences. Fitzgerald was appointed the Professor of Natural and Experimental Philosophy the year that Preston arrived. Preston’s early postgraduate work was in mainly in mathematical physics, though he also carried out some experimental work in Fitzgerald’s Laboratory and they remained lifelong friends. Preston became Professor of Natural Philosophy in University College Dublin in 1891.

Preston made his first foray into writing textbooks with W. J. McClelland. They published A Treatise in Spherical Trigonometry in 1886. Preston subsequently published two further significant textbooks on The Theory of Light in 1890 and The Theory of Heat in 1894.  As commented by Weaire and O’Connor “Trinity College had been the source of many important textbooks in Physics, going back to the publication of Richard Helsham’s lectures in 1739. Preston was following in an established tradition.” He received letters of thanks and congratulations from many eminent scientists including Kelvin, Rayleigh, Fitzgerald, Hertz, Stoney, Ball and Michelson. Many subsequent editions of the books were reprinted, and they continued to be used internationally for decades after his death.  

During his time in UCD Preston turned increasingly to experimental work. In 1897 he presented photographic evidence of the Anomalous Zeeman Effect in cadmium and zinc in a presentation to the Royal Dublin Society [2]. Preston overtook the work of Michelson, Cotton, Cornu and Zeeman and gained international recognition.

Zeeman had previously reported on the effect of a magnetic field on atomic spectral lines [3,4]. He observed a broadening of the lines and polarization effects. These results provided experimental evidence for the classical electron theory of Hendrik Lorentz who predicted a splitting into three lines. Zeeman’s observation of only spectral broadening was due to the limited resolution of his spectrometer. Using a large Rowland spectrometer at the Royal University with a borrowed electro-magnet, Preston photographed “all the (visual) appearances described by Zeeman” as described in his letter in Nature in 1897 [4]. However, Preston noticed that the behaviour of the splitting was more complex that predicted by classical electron theory. He conducted further investigations using a stronger magnet and

obtained photographic evidence of examples which were did not follow the “normal” triplet line splitting predicted by Lorentz [2]. Preston reported “It is clear that the magnetic effect depends not so much on the wavelength of the spectral line as on some hidden quality which we may refer to as the character of the line; for lines of nearly the same wavelength, even of the same substance, show effects which differ remarkably in magnitude and character. Such laws, therefore, as that the broadening of the spectral lines is proportional to the wavelength or to the square of the wavelength are shown to be utterly untenable, unless perhaps it might be possible to group the spectral lines of each substance into sets, so that some law of wavelength might apply to the lines of each set”. Over the next couple of years Preston and others published many papers characterising the anomalous Zeeman effect. However, it was not until the 1920’s that a theoretical explanation was obtained based on spin of the electron and quantum mechanics.

Preston was elected a fellow of the Royal Society in 1898 and was awarded the RDS Boyle medal in 1899. Unfortunately, he died at a young age on January 31st, 1900. His work was cut short and consequently his legacy has been somewhat overlooked. Lorentz and Zeeman were awarded the Nobel Prize in physics in 1902.


  1. Weaire, D, and O’Connor, S. “Unfulfilled Renown: Thomas Preston (1860-1900) and the Anomalous Zeeman Effect.” Annals of Science44, no. 6 (1987): 617-44.
  2. Preston, “Radiation Phenomena in a Strong Magnetic Field”, Transactions Royal Dublin Society, 6 (1898), 385-91.
  3. Zeeman, “On the Influence of Magnetism on the Nature of Light Emitted by a Substance”, Philosophical Magazine, 43 (1897) 226-39.
  4. Zeeman, “Doublets and Triplets in the Spectrum produced by External Magnetic Forces”, Philosophical Magazine, 44 (1897) 55-60 and 255-59.
  5. Preston, “The Zeeman Effect Photographed”, Nature 57, (1897), 173.
  6. Weaire for the photograph from Preston’s paper to the RDS in December 1897.