July II, 1889] 



NATURE 



259 



C-lie Ahiens reflector, Fig. 7), and analyzer, A (the Ahrens triple 

 s])ar prism, Fig. 6), are crossed, so that here is the dark field. 

 < )n turning on the current, light is at once restored, being 

 twisted to the right when the current circulates right-handedly. 

 'I'o measure the rotation, I must turn the anal)z?r ; and now I 

 find that, owing to the greater rotation of blue waves than of 

 rod, complete extinction does not occur. Introducing a half- 

 shadow plate, and using coloured glasses, it is very easy to 

 verify the greater amount of rotation for blue light, and to show 

 that reversing the current reverses the rotation. You will per- 

 haps better understand it if I use (as in Fig. 16) the 24-ray star, 

 s, which I have previously employed. It is now obvious to you 

 that there is a large rotation — over 50° in fact — which is reversed 

 when I reverse the magnetizing current. We have thus repeated 



the fundamental experiment of magneto-optics. But now we 

 meet with another consideration. Reflect that the circulation of 

 current, if it be taken as right-handed when regarded from one 

 end of the coil, will be left-handed when regarded from the 

 other end of the coil. This is, therefore, no case of skew sym- 

 metry : it clearly indicates that something is going on in the 

 glass which tends to twist the light quite irrespective of which 

 way the light enters. 



The next magneto-optic phenomenon is that discovered by. 

 Dr. Kerr, of the rotation of the plane of polarization by reflec- 

 tion at the surface of a magnet. To observe this at all requires 

 good apparatus and a keen eye. So far as I am aware, it has 

 never been projected on the screen. If I can succeed in doing 

 so, it will only be because I have special means of the most 



IT 



i 



Fij. 16. — Projection of magnetic rotation of plane of polarization, c. condensing lenses; p, reflecting polarizer; m, magnetizing coll surrounding 

 of heavy-glass ; s, ml:a disk of twenty-four rays ; a, analyzer (Ahrens's triple prism). 



bar 



favourable character for so doing. We withdraw the bar of 

 heavy glass from the coil, and replace it (Fig. 17) by an iron 

 core polished at its coned end. This will be intensely magnetized 

 when the current is turned on. 



Now we must throw the beam of light obliquely down the 

 hollow of the coil, polarizing it by one of my improved Nicol 

 prisms, P, as it goes down. After reflection it is focussed by a 

 lens which sends it through the analyzing prism, A. You see 

 the dim sjjot of reflected light upon the screen. Now for the 

 current: "on," "off," "on," "off." Reversing its direction 

 ought to double the amount of torsion. 



Whilst Mr. Thomas is making the needful arrangements for the 

 next experiment, I may mention that it was found by Kerr that 

 the effect was approximately proportional to the magnetic induc- 

 tion through the iron. I have myself tried some further experi- 

 ments : for example, using a bar of lodestone instead of an iron 

 core. The lipht reflected from lodestone is also twisted. I 



Fig. 17, — Apparatus fjr projecting rotation of plane of polarization by reflec- 

 tion at pole of magnet, p, polarizer ; m, mignetlzing coll with coned iron 

 core ; a, analyzer. 



should expect the ferro aluminium alloy which Sir H. Roscoe 

 showed us a fortnight ago to do the same thing, because that 

 alloy is, as I have found, susceptible of magnetization. But I 

 should not expect manganese steel to rotate the light, because 

 of its singularly non-magnetizable nature. 



The experiment of Kundt, transmitting polarized light through 

 a thin transparent film of iron, magnetized noraaally whilst the light 

 is passing through it, is another difficult of repetition before an 

 audience. The small disks here are covered with films of iron, 

 kindly prepared forme by Mr. Crookes, by squirting them elec- 

 trically in a high vacuum. But the thin ones barely transmit 

 enough light to make the observation of the efifect possible even 

 t) the solitary observer. I have observed the effect projected on 

 the screen, using this very coil and these transparent mirrors. 

 It requires, however, an absolutely dark room, and is at best so 

 faint that it would be hopeless to attempt to show it to a large 

 audience. Prof. Kundt has not only observed similar rotations 



in other magnetic films of nickel and cobalt, but has even shown 

 that the degree of rotation of the light is proportional not to the 

 magnetizing force, but to the resulting magnetic induction. This 

 is a result of utmost importance in considering the theory of the 

 phenomenon. He has further shown that, whereas the magnetic 

 rotations m elementary bodies, whether magnetic or diamag- 

 netic, are in the same sense as that in which the current circu- 

 lates, the magnetic rotations in compound magnetic bodies, such 

 as a solution of sulphate of iron in water, are in the opposite 

 sense. 



These experiments with transparent mirrors of iron raise interest- 

 ing speculations as to the probable nature of a transparent magnet, 

 if such there could be. It is one of the cardinal p >ints of Max- 

 well's celebrated electro-magnetic theory of light, that the better 

 a body conducts electric currents, the greater is its tendency to 

 absorb light and become opaque. Now, suppose it were possible 

 to obtain a su'istance such as to possess greater electric conduc- 

 tivity in one direction than in another, such a substance ought to 

 absorb those vibrations of light which are executed in the direc- 

 tion of the greater electric conductivity more than those in the 

 direction at right angles. In other words, such a substance ought, 

 like the tourmaline, to polarize light by absorption. Now, since 

 the researches of Sir W. Thomson in 1856, we have known that 

 the electric conductivity of iron is altered in the direction of the 

 magnetic lines offeree, when it is powerfully magnetized. More 

 recently it has been discovered — I myself observed it in tinfoil, 

 and announced the discovery to the Physical Society a fjw days 

 before the announcement of the same fact by Righi — that non- 

 magnetic metals alter their resistance in the magnetic field. 

 Notably so do bismuth and tellurium, I had therefore conceived' 

 it possible that a film of iron or possibly of tellurium, if strongly 

 magnetized in its own plane, might exhibit polar absorption and 

 act like a tourmaline. Unfortunately, if the effect exists it is so 

 faint as to be yet undiscovered, though I have made many efforts 

 to find such. I have further tried to obtain a similar result by 

 making a transparent magnet out of a film of magnetic oxide of 

 iron, precipitated chemically. In this too I have not succeeded. 

 I have tried to precipitate a transparent film of magnetic oxide 

 in the midst of a transparent jelly. And I have mixed particles 

 of precipitated oxide with melted gelatine so as to get a film. 

 In this way I hoped to get, by placing the preparation in a 

 strong magnetic field, a sort of magnetic structure which would 

 operate upon waves of light. That such a task was not hopeless 

 was shown by two facts : first, that many mere vegetable and 

 animal structures can act as polarizers ; and second, that a mere 

 ^Int of paint, such as indigo, can, if a proper mechanical drug is- 

 given to it so as to produce structure, also act as a polarizer. 



The film of indigo-carmine which I have here, acts nearly as 

 strongly, though not quite as evenly, as a tourmaline slice, and 

 costs but a fraction of a penny. 



