162 Dr. J. Kerr on Reflection of Polarized Light 



fine. One lateral face of the bar (7 by ■§-) was planed and 

 carefully polished by a skilled workman. 



It may be worth mentioning that three such bars were 

 forged at the same time, were cemented together, planed and 

 polished in block, and then separated. The middle one was 

 kept as the best reflector ; but most of my results were ve- 

 rified upon each of the three. 



2. Arrangements. — The electromagnet stands upright upon 

 a solid table ; the reflecting bar lies flat and stably on the 

 poles of the horseshoe, in the position of an armature, its 

 length horizontal, and its polished face vertical ; the two 

 Nicols and the lamp stand upon the same table as the magnet, 

 and at the same height as the mirror. The diagram shows all 

 the pieces, in horizontal section through the lamp L and the 



observer's eye E. N is the first Nicol, C the point of incidence 

 on the reflector AB, and W the second JSTicol. The poles of 

 the horse-shoe, below the bar, are indicated by the dotted 

 circles. The piece P between the lamp and the first Mcol was 

 often found useful in the more delicate observations : it is a 

 metallic screen, containing a long horizontal slit about |- of an 

 inch wide. Sometimes the flat flame L has its edge presented 

 to C ; and then, the piece P being in position, the object seen 

 at C through W is a small segment of the flame, sensibly 

 square. But generally, except when the angle of incidence is 

 near 90°, the width of the flame is presented ; and then the 

 object seen in the mirror is a long horizontal rectangle, uni- 

 formly illuminated, strongly outlined above and below, and 

 bringing out small changes of small intensity very delicately. 



In the diagram, the axis of the bar A B is produced through 

 the end B next the observer ; the extremity F of the axis thus 

 produced is used afterwards as a point of reference. 



In all my ]ater observations, the values of the angle of in- 

 cidence | LCE were assigned beforehand as carefully as 

 possible, but by a method which cannot pretend to great 

 accuracy. A broad sheet of drawing-paper, which had been 

 cut away at the proper angle through one of its corners, was 

 laid flat on the table, and aligned against the fixed stand of 



