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



stand, and so placed that, when I look fairly through it, I see 

 the image of the flame in the iron mirror. 



6. Principal azimuths of first Nicol. — As the polariscope is 

 worked here in the usual way, by restoration from the best 

 possible extinction, there are only two positions of the first 

 Nicol which are suitable to start from. The plane of polari- 

 zation of the light incident upon the iron mirror must be either 

 parallel to the plane of incidence or perpendicular to it, 

 because in every other case the reflected light is elliptically 

 polarized and therefore inextinguishable by the analyzer. I 

 generally make the plane of polarization coincide with the 

 plane of incidence ; and I manage this in the first place very 

 approximately by trial. I lay the first Nicol with its principal 

 section sensibly horizontal. Looking through the second 

 Nicol, and watching the image of the flame in the polar mirror, 

 I turn the second Nicol quickly through the position of mini- 

 mum intensity backwards and forwards, while the first Nicol 

 is turned slowly, also backwards and forwards, until I obtain a 

 minimum-intensity zero. 



It is a matter of capital importance in the experiments to 

 have the Nicols placed in this position of pure extinction ; and 

 the arrangement is not so easily made as might be supposed. 

 Perhaps it is from imperfection of polish, and perhaps from 

 the very nature and structure of the reflecting metal ; but 

 whatever be the reason, the mirror is never perfectly black in 

 the polariscope ; and though the intensity of the illumination 

 is very faint when the Nicols are in exact position, it is still 

 sufficient to embarrass the observer's judgment when he has 

 to decide between pure extinction and impure. The difficulty 

 can be overcome by a simple and regular process, as will be 

 seen immediately. In the mean time I assume that we can 

 obtain a pure initial extinction in the polariscope. 



7. Submagnet. — I have now mentioned every thing that is 

 of any importance in the arrangements, except one condition, 

 without which I have never obtained any optical effect ; and 

 that is, an intense concentration of magnetic force upon the 

 iron mirror. For this purpose I employ a block of soft iron, 

 one of several polar pieces belonging to the magnet, 2 inches 

 square and 3 inches long, which has been planed off at one 

 end into a blunt wedge with well-rounded edge. Two splin- 

 ters of hard wood, which have been thinned and toughened by 

 hammering, are laid upon the sloping polar surface about an 

 inch apart, and parallel to the plane of incidence. Holding 

 the wedge in my left hand, I plant it edge downwards upon 

 the splinters, with its rounded edge perpendicular to the plane 

 of incidence, and right above the centre of the mirror. The 



