474: Professor Silvanas P. Thompson [May 17, 



WEEKLY EVENING MEETING, 



Friday, May 17, 1889. 



John Rae, M.D. LL.D. F.R.S. Vice-President, in the Chair. 



Professor Silyaxus P. Thompson, B.A. D.Sc. 3LB.I. 



Optical Torque.* 



Seventy-eight years have elapsed since the first discovery, by Arago, 

 of the remarkable chromatic effects produced by slices of quartz 

 crystals upon light, previously polarised, which was caused to traverse 

 them. These effects were shown, one year later, by Biot, to be caused 

 by a peculiar action of the quartz in rotating the plane of polarisation ; 

 the amount of the rotation being different for lights of different 

 colours. Ever since then, the rotation of the j^lane of polarisation of 

 light has been a topic familiar to physicists. It has stimulated the 

 devotee of research to an endless variety of experiments and sug- 

 gestive speculations : it has lured on the mathematician to problems 

 which tax his utmost skill : it has afforded to the lecturer an array 

 of beautiful and striking illustrations. Here, in this place, made 

 classical by the researches and expositions of Thomas Young, of 

 Michael Faraday, and of William Spottiswoode, and last, but not 

 least, by the labours of those eminent men whom we rejoice still to 

 number amongst the living — here, I say, on this classic ground, the 

 rotation of the plane of polarisation of light is almost a household 

 word, and its phenomena are amongst the most familiar. We know 

 now that not only certain actual crystals, such as quartz, bromate of 

 soda, and cinnabar, rotate the plane of polarisation, but that many 

 non-crystalline bodies — liquids, such as turj^entine, oil of lemons, 

 solutions of sugar and of various alkaloids, and even certain vapours, 

 such as that of camphor — possess the same property. 



In 1845, at the very culminating point of his unique career of 

 research, Faraday opened a new field of inquiry, linking together for 

 the first time the science of optics with that of magnetism, by his 

 discovery that the rotation of the plane of polarisation of light could 

 be effected by the application of magnetic forces. This effect he 

 observed first in his peculiar " heavy-glass," when it lay in a powerful 

 magnetic field. Subsequently he found other bodies to possess 

 similar properties : some of these being magnetic liquids, such as 

 solutions of iron, others being diamagnetic. Time will only permit 

 me in passing to refer to the researches of Verdet, and those of Lord 

 Rayleigh and of Mr. Gordon upon the numerical values of the 



* The blocks of the woodcuts illustrating this discourse have been kindly lent 

 by the publishers of Nature. 



