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



at the conical end. The surface of the boring is well dimmed 

 with a coating of lampblack. To ensure perfect stability of 

 position when the piece rests upon its conical end, the original 

 rectangular -volume of the block was restored, the part added 

 being a hard stone of plaster of Paris, which was easily 

 moulded to the block of iron in the usual way. This is the 

 first submagnet that gave me good and constant effects in the 

 case of normal incidence ; and it appears to be much the best 

 that I have yet tried. Without a submagnet of some kind, I 

 have never obtained a suspicion of an effect. 



27. Placing of the pieces. — The old magnet (3) is placed 

 on a solid table near the edge, with its polar surfaces horizon- 

 tal ; and the submagnet just described is laid upon one of the 

 polar surfaces, its conical end downwards, the axis of core 

 and boring coincident, and the block and core separated by a 

 wide ring of writing-paper, or very thin card. Above the 

 block, as in Norremberg's polariscope, stands a mirror of un- 

 silvered glass, which receives a horizontal beam from the first 

 Nicol, and reflects it downwards through the boring, perpen- 

 dicularly to the surface 

 of the magnetic mirror. 

 In the diagram, H F G 

 is the polar surface, L 

 the source of light, which 

 is the same paraffin-flame 

 as formerly, E the ob- 

 server's eye, A and B the 

 first and second Nicols, 

 C the transparent mir- 

 ror. The course of the 

 light from L to E is 

 L A C F C B E. All the 

 pieces are placed very stably, and the room is well darkened. 



28. Eighth experiment. — All the pieces are placed as in 

 the diagram, and so that the observer sees at F, through B, a 

 bright and steady image of part of the flame L : the first 

 Nicol is so laid that the plane of polarization of the light in- 

 cident at C coincides with the plane of incidence ; and the 

 second Nicol is turned into the position of pure extinction. 



(1) The second Nicol is turned righthandedly through a 

 small angle, giving a distinct but faint restoration. The 

 operation N strengthens the light thus restored ; and the 

 operation S weakens and sometimes extinguishes it. 



(2) The second Nicol is turned lefthandedly through a 

 small angle beyond pure extinction. The results are as in 

 the first case, with reversal of actions of N and S. It is now 



^B- 



-*o 



