232 



NATURE 



{July 4, 1889 



OPTICAL TORQUEy 



I. 



C^EVENTY-EIGHT years have elapsed since the first dis- 

 *~-^ covery, by Arago, of the remarkable chromatic effects 

 produced by slices of quartz crystals upon light, previously 

 polarized, 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 polarization ; the 

 amount of the rotation being different for lights of different colours. 

 Ever since then, the rotation of the plane of polarization of light 

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

 devotee of research to an endless variety of experiments and 

 suggestive speculations : it has lured on the mathematician to 

 problems which tax his utmost skill : it has afforded to the lec- 

 turer 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 Spottis- 

 woode, 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 polar- 

 ization 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 polarization, but that many non- 

 crystalline bodies — -liquids, such as turpentine, 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 

 polarization of light could be effected by the application of mag- 

 netic 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 magne'o- 

 optic rotation in these substances. H. Becquerel has extended 

 them to gases, and has shown how the magnetism of the earth 

 rotates the plane of polarization of the light which, previously 

 polarized by reflection from the aerial particles which give the 

 sky its "blue," passes earthward through the oxygen of the air. 



Other experimenters have dealt with the rotatory effects 

 (whether crystalline, molecular, or magnetic) in relation to 

 lights of different colours, and have studied the dispersion 

 which arises from the greater actual angle of optical torsion 

 which is produced upon waves of short wave-length (violet and 

 blue) than that which is produced under the influence of equal 

 rotatory forces upon the waves of longer wave-length (red and 

 orange). It has also been demonstrated that the plane of polar- 

 ization of waves of invisible light, whether those of the infra-red, 

 or those of the ultra-violet species, if they have been previously 

 polarized, can b; rotated just as can that of waves of visible light. 



In 1877, Dr. Kerr, of Glasgow, discovered a point which 

 Faraday had sought for, but fruitlessly — namely, that in the act 

 of reflection at the pole or surface of a magnet, there is a rota- 

 tion of the plane of polarization of light. This discovery was 

 completed in 1884 by Kundt, of Strasburg, by the further 

 demonstration, also dimly foreseen by Faraday, that a mag- 

 neto-optic rotation of the plane of polarization is caused by 

 the passage of previously polarized ,light through a normally 

 magnetized film of iron so thin as to be transparent. 



Lastly, in this brief enumeration, we were shown a month 

 3-gOj by Oliver Lodge, how the magnetic impulses generated by 

 the rapid oscillatory discharges of the Leyden jar can pro- 

 duce corresponding rapid oscillatory rotation in the plane of 

 polarization of the waves of previously polarized light. 



You will not have failed to notice the cumbrous phrase which, 

 whether in speaking of the purely optical effects (of quartz, or 

 sugar, or turpentine), or in speaking of the magneto-optic effects 

 of more recent discovery, I have employed to connote a very 

 simple fact. You may have wondered that any lover of simple 

 English speech should indulge in such sesquipedalian words. 



Of course, at this period of the nineteenth century it is 

 no longer open to debate that light consists of waves. The 



' A Discourse delivered at the Royal Institution, May 17, 1889, by Prof. 

 Silvanus P. Thompson. 



plane of polarization of the waves of light is the plane of polar- 

 ization of the light itself. The rotation of the plane of 

 polarization is the rotation of the polarized waves, and therefore 

 of the polarized light itself. Yet I must draw attention to the 

 fact that in all the array of discoveries which I have enumer- 

 ated, that which had been observed was the rotation— whether 

 by crystalline, molecular, or magnetic means — not of natural 

 light, but of light which had by some means been previously 

 polarized. It was not known to Arago or to Biot, to Fresnel, 

 to Faraday, nor even to Spottiswoode or to Maxwell, that 

 natural unpolarized light could be rotated. They may have in- 

 ferred so, but it was not in their time even demonstrable that a 

 beam of circularly-polarized light could be rotated upon itself in 

 the same sense as that in which a beam of plane-polarized light 

 could be rotated. 



That light of any and every kind, however polarized or devoid 

 of that which is called polarization, can be, and in fact is, 

 rotated when it passes across a slice of quartz or along a mag- 

 netic field, is a wider generalization 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 discourse. 



In order the better to enable this audience to comprehend the 

 ultimate significance of this discovery, I must claim the indul- 

 gence of those amongst them who are already familiar with the 

 subject of the polarization of light, whilst I go back to the most 

 simple elementary matters. Having illustrated the fundamental 

 facts about the plane of polarization of light and its twisting, I 

 shall then go on to methods of precisely measuring the amount of 

 optical torsion produced 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 polarized 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 polarized into a definite 

 plane are also capable of impressing a rotation upon natural, 

 non-polarized light. 



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

 the meaning of the term polarization as applied to wave-motion, 

 I will show a simple apparatus, 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 movements 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 polarization of their movements being at 

 right angles to one another. Their velocity of march is equal ; 

 but in this model, as a matter of fact, their phases differ 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.] 



Now, in the case of waves of natural light from all ordinary 

 sources — sun, stars, candles, gas-flames, or electric light — the 

 waves emitted are not found to be polarized. That is to say, 

 their motions are not executed in any particular plane, nor even 

 in any particular path of any kind ; they appear to be absolutely 

 heterogeneous at least so far as this, that no vibration of the 

 millions of millions emitted in a second of time is followed by 

 moi"e (on the average) than about 50,000 vibrations of a similar 

 sort, executed along a similar path — -the plane of the polarization, 

 if any, changing after the lapse of such an incredibly short time 

 that for most purposes the vibrations in different directions are 

 as inextricably mixed as if they had all been simultaneously 

 jumbled up. Since, then, natural light is non-polarized or 

 miscellaneous, the production of polarized light must be brought 

 about by the employment of polarizing apparatus or agents which 

 will so operate on or affect the mixed waves as to bring their 

 vibrations into one direction — or, what amounts to the same 

 thing, transmit the light whilst destroying or absorbing those 



^ The convenient term torque was first proposed by Prof. James Thomson, 

 of Glasgow, for the older and more cumbrous phrase "moment of couple,' 

 or " angular force." Its general acceptance by engineers justifies the ex- 

 tension 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. 



