from the Equatorial Surface of a Magnet. 163 



the magnet and the movable stand of the lamp, each of the 

 stands presenting suitably a long straight edge in contact with 

 the table. 



It will be observed that, according to these arrangements, 

 the lines of magnetic force at the point of incidence remain 

 sensibly parallel to the trace of the plane of incidence on the 

 reflecting surface, through all changes of incidence from near 

 grazing to near normal. 



3. Specification of Rotations of the JSficols. — I shall have 

 occasion repeatedly to speak of right-handed and left-handed 

 rotations of the two Nicols. In the employment of these 

 terms, I shall always view the second Nicol from the point E, 

 and the first Mcol from the point C. By a right-handed ro- 

 tation of the second Mcol, I mean, therefore, a rotation of the 

 analyzer which is with watch-hands when viewed from E ; 

 and by a left-handed rotation of the first Mcol, I mean a rota- 

 tion of the polarizer which is against watch-hands when viewed 

 from the point of incidence C. 



4. Specification of Magnetizations of the Reflector. — In the 

 statement of results, I shall always specify the two magnetiza- 

 tions of the reflector by giving the direction of the magnetizing 

 current, the current being supposed to circulate spirally round 

 the bar A B, out of one coil into the other. These directions 

 of magnetizing currents may of course be conceived and re- 

 membered more simply, as the directions of Amperean currents 

 in the magnetized bar. By a right-handed current I shall 

 always understand here, a magnetizing current whose effective 

 direction, round the reflecting bar A B, is with watch-hands 

 when viewed from the point F. 



The three points of reference now fixed will be carefully 

 adhered to: — E for movements of the second Mcol or analyzer ; 

 C for movements of the first Mcol or polarizer ; and F for 

 directions of the magnetizing current, or for directions of Am- 

 perean currents in the magnetized reflector. 



5. Optical Observations loith the Mirror and the two Nicols. — 

 These belong properly to Optics, and present nothing new ; 

 some of them, also, were described in my former paper ; but 

 they are given here for their bearing on subsequent methods 

 and results, and for the useful practice which they afford in 

 the management of the apparatus. Suppose the pieces placed 

 as in the diagram (2). 



Whatever be the angle of incidence, there are only two ar- 

 rangements of the Nicols which give pure extinction of the 

 reflected image. In one of these arrangements the principal 

 section of the first Mcol is parallel to the plane of incidence; 

 and in the other it is perpendicular. 



M2 



