476 Professor Silvaniis P. Tliomjpson [May 17, 



polarised light could be rotated upon itself in the same sense as that 

 in which a beam of plane-polarised light can be rotated. 



That light of any and every kind, however completely polarised or 

 devoid of that which is called polarisation, can be, and in fact is, 

 rotated when it passes across a slice of quartz or along a magnetic 

 field, is a wider generalisation of more recent date ; but one of the 

 reality of which I hope to convince you before the warning finger 

 of the clock puts a period to my discoiu'se. 



In order the better to enable this audience to comprehend the 

 ultimate significance of this discovery, I must claim the indulgence 

 of those amongst them who are already familiar with the subject of 

 the polarisation of light, whilst I go back to the most simple ele- 

 mentary matters. Having illustrated the fundamental facts about the 

 plane of polarisation of light and its twisting, I shall then go on to 

 methods of precisely measuring the amount of optical torsion pro- 

 duced by the various substances under various conditions. And after 

 dealing with the magnetic as well as the crystalline and molecular 

 methods of producing optical torsion in the case of light that has been 

 previously polarised into a given plane, I shall be in a position to 

 speak of the nature of the torque,* or twisting force, which in the 

 several cases produces the torsion ; and shall finally endeavour to 

 indicate the scope of the researches by which it is now definitely 

 ascertained that the very same optical forces which are capable of 

 impressing a rotation upon light which has been artificially polarised 

 into a definite plane, are also ca2)able of impressing a rotation ujjon 

 natural non-polarised light. 



At the outset, to elucidate to any who may not comprehend the 

 meaniug of the term polarisation as aj^plied to wave-motion, I will 

 show a simple apj)aratus, constructed from my designs by Mr, Groves. 

 In this there are two sets of movable beads, fixed upon stems which 

 pass into a box containing a piece of mechanism actuated by means 

 of a handle. These beads, when I turn the handle, oscillate to and 

 fro in definite directions, and, by their successive motions, give rise 

 to progressive waves. One set of beads, tinted red, executes move- 

 ments in a plane inclined 45^ to the right, another set, silvered, 

 simultaneously executes movements at 45"^ to the left. There are 

 therefore here two waves, the planes of polarisation of their move- 

 ments being at right angles to one another. Their velocity of march 

 is equal ; but in this model, as a matter of fact, their phases difier by 

 one-quarter — that is to say, each successive wave of the one set is 

 always a quarter of a wave-length behind the corresponding wave of 

 the other set. [Model exhibited.] 



* The convenient term Torque was first proposed by Prof. James Thomson, of 

 Glasgow, for the older and more cumbrous phrase " moment of coupk^," or 

 " angular force." Its general acceptance by engineers justifies the extension of the 

 term to optics. As a mechanical torque is that which produces or tends to 

 produce mechanical torsion, so optical torque may be defined as that which 

 produces or tends to produce optical torsion. 



