1889.] on Optical Torque. 493 



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 points of Maxwell's 

 celebrated electro-magnetic theory of light, that the better a bodv 

 conducts electric currents, the greater is its tendency to absorb light 

 and become opaque. Xow, suppose it were possible to obtain a 

 substance such as to possess greater electric conductivity in one 

 direction than in another, such a substance ought to absorb those 

 vibrations of light which are executed in the direction 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 

 polarise 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 of force, when it is 

 powerfully magnetised. More recently it has been discovered — I 

 myself observed it in tinfoil, and announced the discovery to the 

 Physical Society a few days before the announcement of the same fact 

 by Eighi — 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 magnetised 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 oblain a similar result by making 

 a transparent maguet out of a film of magnetic oxide of iron, precipi- 

 tated chemically. In this too 1 have not succeeded. I have tried to 

 precipitate a transparent film cf 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 fi.lni. In this way I hoped to get, 

 by placing the preparation in a strong magnetic field, a sort of mag- 

 netic 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 polarisers ; and second, that 

 a mere film of paint, such as indigo, can, if a proper mechanical drag 

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



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. 



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

 do faintly act on polarised light : but their action is not as marked 

 as that of films of jelly enclosing actual small scraps of iron. 

 This film, when placed across the poles of this electromagnet, betsveen 

 two crossed Xicols at -ib', sLows an action when the magnet is tiirntd 

 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 &ucch d'estime. 



Incident-all V. in the course of tliese experiments on magnetic films, 



■2 L 2 



