26o 



NA TURE 



{July II, 1889 



Well, my films of jelly enclosing particles of magnetic oxide of 

 iron do faintly act on polarized light ; but their action is not as 

 marked as that of films of jelly inclosing actual small scraps of 

 iron. This film, when placed across the poles of this electro- 

 jnagnet, between two Nicol prisms at 45°, shows an action when 

 the magnet is turned on, as you see by the way in which it flashes 

 into light in the dark field. When the jelly is fresh, and of the 

 proper consistency, the action is very strong, but with the rather 

 dry sample before you I fear we can only call the effect a succes 

 d^estime. 



Incidentally, in the course of these experiments on magnetic 

 films, I came across a new m.agnetic body unknown hitherto, I 

 believe, to the chemist — namely, a magnetic double oxide of cobalt 

 and iron — a ferroso-cobaltic oxide, I think — a black powder, a 

 sample of which I have here. 



It also occurred to me, as a matter of speculation, that if I 

 could strongly magnetize a crystal of ferrous sulphate or nickelous 

 sulphate, whilst viewing it by convergent polarized light I might 

 find some interesting phenomena, which should, if they existed, 

 show some sort of a relation between the direction of the optic 

 axis and that of the lines of the magnetic field. I thought that 

 a longitudinal magnetization might possibly set up a rotatory 

 phenomenon like that in quartz in so far as to disturb the 

 central field between the arms of the black cross ; however, not 

 by the most powerful magnetizing could I discover any such 

 effect. Again, I thought that by magnetizing transversely to the 

 optic axis 1 might possibly succeed in turning the uniaxial crystal 

 into a biaxial, or producing by magnetism an effect resembling 

 the action of heat on crystals of selenite. Owing probably to 

 the small depth of any crystals that can be obtained, I have failed 

 so far to obtain any such effect, though I am convinced that it 

 must exist. 



An effect precisely analogous to the magnetic effect which I 

 vainly sought has, however, been lately discovered by Prof. 



Fjg. 18. 



Fig. 19. 



Rontgen. I sought a distortion of the optic axis by transversely 

 magnetizing, and I sought it in crystals of .sulphate of nickel : he 

 has found a distortion of the optic axis by transversely electrifying, 

 and he has found it in crystals of quartz. 



Suppose a piece of a quartz crystal is cut as a square prism, 

 its long faces being principal planes of section respectively 

 parallel to and at right angles to two of the natural faces of the 

 hexagonal prism. Fig. 18 shows the form of the portion cut. 

 The + and - signs in this figure refer to the pyro-electric poles 

 of the crystal. Such a piece viewed by convergent light shows 

 the usual rings and black cross with a coloured centre (Fig. 19). 

 If now two opposite faces be covered with tinfoil, and the 

 crystal be electrified transversely, the rings are distorted into 

 lemniscates, the direction of the distortion changing with the 

 sign of the electrification. It is necessary to use a red glass, or 

 still better sodium light, to observe the changes in form on 

 reversing the sign of the charges. Figs. 20 and 21, 22 and 23, 

 show the changes of form, but these sketches grossly ex'^ggerate 

 the effects. As you see upon the screen, when the charges im- 

 parted by this fine Wimshurst machine are rapidly reversed, 

 there is a decided distortion of the rings, but it is small in 

 amount. 



Returning to the phenomena of the rotation impressed by 

 magnetism, on polarized light, I may point out that the torque 

 which a magnetic field exerts on the light-waves appears to be 

 really an action upon the matter through which the light-waves 

 are passing. It is as though the magnetic field were really a 

 portion of space rotating rapidly on itself, or perhaps as though 

 the ether were there rotating, and that this rotation in some wa)t 

 dragged the particles of matter along with it. It has long been 

 supposed necessary, in order to account for the refractive and 

 dispersive properties of transparent bodies, to consider that their 

 particles are in some way concerned in and partake of the vibra- 



tions going on in the ether within them or between their molecules. 

 It is impossible to explain the phenomena of magneto-optic 

 rotation by the supposition that any skew structure is imparted to 

 the medium ; for these phenomena, unlike those of quartz, do 

 not exhibit skew symmetry. There seems to be no other way of 

 explaining the magneto-optic torsion of light than by supposing 

 that the molecules of matter in the magnetic field are actually 

 subjected to rotatory actions ; as indeed was suggested long ago 

 by Sir William Thomson. 



However, there is room here not only for speculation but for 

 experiment. Some day, when facts enough have been collected, 

 we shall be ready to build thereon the wider generalization 

 which at present seems to escape us. 



So far we have been applying an optical torque to previously 

 polarized light, and producing a torsion of it. It remains for 

 me yet to describe the means by which, in the hands of Prof. 

 Abbe and Prof Sohncke, it has been demonstrated that natural, 

 non-polarized light is actually rotated when subjected to an 

 optical torque. 



The way of doing this is to make use of the principle of in- 

 terference. Here is a slit from which a narrow beam of light- 

 waves issues. At a point a little distance away is a Fresnel's 

 biprism which splits up the light (without polarizing it) into two 

 beams, just as if we had two slits or sources of light. These two 

 beams pass along, and meet upon this distant screen, and give 

 us — what ? A set of interference fringes, having a bright line 

 down the middle, because this part of the screen is exactly 

 equidistant from the two sources of light. 



Fig. 20. 



Fig. 21. 



Fig. 23. 



But these dark interference fringes that lie right and left can only 

 exist because, in the first place, the vibrations have travelled un- 

 equal paths differing by an odd number of half wave-lengths ; and 

 secondly, because (owing to the method adopted of using two 

 images of one slit) the phases of the emitted waves from the 

 two sources are identical. 



This being so, let us now introduce across the two interfering 

 baams of light a special biquartz, made of right- and left-handed 

 quartz of only 188 mm. thick. This will rotate — if it rotates 

 natural light at all — the yellow light in one beam 45° to the 

 right and that of the other beam 45° to the left. The angles 

 will be a little more for green and blue, a little less for red and 

 orange. Consequently we shall not get quite a perfect result 

 for all kinds of colours. But for the main body of the light the 

 result is this : that because the two beams have had their re- 

 spective vibrations turned so that, whatever their primitive 

 positions, they are now at right angles to one another, they 

 cannot interfere. In other words, if it be true that the quartz 

 rotates natural light, the interference bands will die out. [Experi- 

 ment shown.] 



Here I have the light passing through the biprism only, and 

 giving us this narrow series of interference bands. You must 

 notice carefully- — ^with opera-glasses if you have them — the 

 narrow bright and dark stripes. Now I shift this little diaphragm 

 so that the light passes through the biquartz as well. Instead 

 of sharp interference bands we have merely a dull line of 

 nebulous light. The disappearance of the fringes proves that 

 quartz does twist the not-previously-polarized waves of light. 



That the magnetic field can also exert a magnetic torque on 

 non-polarized light is readily proved, at least when one already 



