1913] Applications of Polarized Light 727 



WEEKLY EVENING MEETING, 



Friday, April 18, 1913. 



Sir Francis LakinCx, Bart., G.C.V.O. K.C.B. M.D. LL.D., 

 Vice-President, in the Chair. 



Thomas Martix Lowry, Esq., D.Sc. F.C.S. 



Applications of Polarized Ligiit. 



Ox November 80, 1812, just over 100 years ago, the French physicist 

 Biot communicated to the Institute of France a memoir " On a new 

 kind of oscillation which the molecules of light experience in travers- 

 ing certain crystals." In this paper, which extends over 371 pages 

 of the printed memoirs, the phenomenon of " Rotatory Polarization " 

 was described for the first time. This phenomenon, as most of you 

 will be aware, depends on the property which certain substances 

 possess of taking a beam of polarized light and imparting a twist to 

 the plane of polarization : the beam of light enters with all the 

 vibrations compressed, say, into a vertical plane ; it emerges 

 apparently unchanged, but careful examination shows that the com- 

 ponent vibrations are no longer vertical, but inclined either to the 

 right or to the left. The importance of this discovery to physicists 

 and to crystallographers was immediately obvious. In our own 

 generation its fertility has been realized also by chemists, who have 

 found in the polarimeter an instrument which promises to render 

 to the science services not less notable than those which have been 

 accomphshed with the help of the spectroscope. 



A. — Sources of Polarized Light. 



If one were to ask what progress has been made in the f aciUties 

 for applying polarized light to the study of chemical and physical 

 problems, the answer would be two-fold. On the one hand it must 

 be acknowledged that the " Iceland spar," by means of which 

 Huyghens in 1678 first detected the polarization of light, is still the 

 best substance for producing this effect. But the increasing demand 

 for the spar has not been accompanied by any corresponding increase 

 in the supply, and large clear pieces of the mineral are becoming 

 increasingly difficult to procure. It may indeed be doubted whether 

 large polarizing prisms such as those which have been handed down 

 as heirlooms at the Royal Institution could now be purchased at any 

 Vol. XX. (No. 107) 3 c 



