338 Royal Irish Academy. 



his work on cones and sphero-conics in association with the late 

 Bishop of Limerick, his paper " On the Motion of a Particle, and the 

 Equilibrium of Flexible Strings on a Spherical Surface," published 

 in the Transactions of this Academy, showed his special aptitude for 

 mathematical study and investigation. In 1891 he was elected to a 

 Fellowship of the Royal University and to the chair of Natural 

 Philosophy in University College. 



In 1890 he published his work, "The Theory of Light," which 

 he followed up in 1894 with "The Theory of Heat." These books 

 supplied a distinct want in English scientific literature ; they are the 

 only comprehensive works on light and heat that have yet appeared 

 in our language. They are, however, more than mere works of 

 reference, more than critical resumes of the development of two great 

 branches of physics; and the maturer work, "The Theory of Heat," 

 will long remain a model on account of the philosophic spirit of its 

 method and the singular beauty of its style. 



In 1894 he was offered the Science and Art Inspectorship for 

 Ireland under the South Kensington Department, and after mature 

 deliberation the ofier was accepted. In 1897 he became a Member of 

 this Academy. He was elected Fellow of the Royal Society in 1898, 

 and in the same year the Senate of the Royal University conferred 

 upon liim the degree of D.Sc. honoris causa. For his distinguished 

 researches on "Radiation Phenomena in a Strong Magnetic Field," 

 the Council of the Royal Dublin Society awarded him the Boyle 

 Medal only last month. 



In March, 1897, Dr. Zeeman had announced his discovery that, 

 when a source of light is placed in a strong magnetic field, the spectral 

 lines become sensibly modified in appearance. From theoretical con- 

 siderations Lorenz at once predicted and Zeeman verified by the use 

 of a Mcol's prism that each modified line consisted of three lines 

 polarized in a definite manner. Preston perceived that in this dis- 

 covery lay the key to a new line of inquiry into the constitution of 

 matter. By increasing the strength of the magnetic field and by 

 using a powerful spectroscope, he succeeded in separating the lines 

 without the use of a nicol, and he was the first to illustrate the effects 

 by photography. He found there were deviations from the standard 

 triplet type ; some lines became quartets, some sextets, and some 



