by Reflection from the Pole of a May net. 339 



addition made to e 2 by the weaker magnetic operation passes 

 from positive, through zero, to negative. 



(3) In the case of principal incidence, cos = 0, and the 

 additions made by N and S to the primitive intensity e 2 in the 

 polariscope are equal and always positive. 



25. Seventh experiment, a repetition of the sixth, to test 

 the preceding inferences. 



(1) Angle of incidence about 70°. All the effects recovered 

 as predicted, and as already obtained roughly in the sixth 

 experiment. Recovered also perfectly at various incidences 

 from 60° to 75°. 



(2) Angle of incidence very large, about 85°. ~No sensible 

 effect obtained in any case by application of the operations N 

 and S, with the arrangements of the sixth experiment, or with 

 those of the second. The reason very probably is that, as the 

 angle of incidence approximates to 90°, the ratio of the ampli- 

 tudes p' and e becomes excessively small, by diminution of 

 the rotation p towards zero. 



Angle of incidence about 80°. The effects very faint, but 

 clearly contrary to what was predicted : 1ST strengthens the 

 light as in the first case ; S either weakens it or has no 

 effect. 



(3) Equal positive effects of N and S in the polariscope 

 were never observed at 75° or any other incidence. The 

 hypothesis advanced in 20 is therefore inexact : the rotation 

 due to magnetic force is not impressed effectively before inci- 

 dence. Neither is it impressed effectively after reflection 

 (10 . . . 19). The difference of phases of the two reflected 

 vibrations, p r in O Y, and 1 in X, has therefore some value 

 \cf) intermediate between <£> and ; and the intensity in the 

 sixth experiment is equal to 



e 2 + p /2 — 2ep' cos \$. 

 Judging from the earlier experiments, second to fifth, I 

 think we are bound to assume that X is very nearly equal to 1 : 

 but certainty upon the subject can be reached only through 

 exact measurements. I come now to the third method men- 

 tioned in art. 14. 



Case of Perpendicular Incidence. 



26. Submagnet. — The old wedge C of art. 7 is now in- 

 adequate. The piece which I substitute for it is a block of 

 soft iron, 2 inches square and 3 inches long, rounded at one 

 end into a frustum of a very obtuse cone, of which the small 

 base is hardly -J inch in diameter. A small boring is drilled 

 through the block, and along the axis of the cone, narrowing 

 regularly from £ inch at the flat end of the block to -^ hich 



Z2 



