326 Dr. J. Kerr on Rotation of the Plane of Polarization 



good conditions, at the instant when the circuit is closed, the 

 light shows itself faintly in proper form, size, and position 

 across the formerly uniform chink, and so continues without 

 sensible change as long as the current passes. Break, and the 

 light immediately disappears. Reverse, and the light again 

 appears and continues till the instant of break, when it disap- 

 pears at once. 



The beam reflected by the mirror of magnetized iron is cer- 

 tainly not plane-polarized, as is the incident beam (and the 

 reflected beam also before magnetization) ; for when the light 

 is restored by magnetic force from pure extinction as above, 

 it cannot be extinguished by any rotation of the second Nicol 

 in either direction ; nor (as far as I can judge of these faint 

 effects and with the present means) is the light sensibly weak- 

 ened by any such rotation. The analyzer's position of extinc- 

 tion before magnetization is also (exactly or nearly) the posi- 

 tion of minimum intensity after magnetization. 



In many repetitions of this experiment, the angle of inci- 

 dence varied from 60° to 80°, and was generally about 75°. 



9. At this point I must ask the reader's attention to several 

 terms,' and to the sense in which I shall use them. By a rota- 

 tion of the first Nicol to the right, I mean a rotation which is 

 right-handed (like the motions of the hands of a -watch which 

 faces the observer) when viewed from the point of incidence 

 on the iron mirror. By the north pole of a magnet, I mean 

 "that which points, on the whole, from the north, and, in 

 northern latitudes, upwards " *. 



10. Second experiment. — Taking this experiment as a con- 

 tinuation of the first, and providing for the best effects, I sup- 

 pose all the arrangements as before : I suppose also that make, 

 break, and reverse of the commutator give bright, black, 

 bright in the polariscope distinctly, however faintly. 



(1) Leaving the circuit open and every thing else untouched, 

 I simply turn the first Nicol ever so little to the right. The 

 amount of the rotation is important. I have said it was ever 

 so little ; and this generally gives effects distinct enough. But 

 when working for the best results, I determine the displace- 

 ment of the first Nicol by this condition, that the intensity 

 of the light restored in the polariscope by the displacement be 



* Sir W. Thomson's papers on Electrostatics and Magnetism, § 445. 

 It will be seen from the quotation that this is], no innovation of mine. 

 Having had this nomenclature brought to my attention recently by Sir 

 William Thomson and very strongly recommended by him, I made it a 

 matter of careful consideration and have determined to adopt it. Like 

 poles of the great Earth-magnet and of our artificial magnets ought to be 

 similarly named ; and the northern pole of the Earth-magnet cannot with 

 any propriety, be called a south pole. 



