from the Equatorial Surface of a Magnet. 167 



second Nicol is turned through an extremely small angle from 

 the position of extinction, first righthandedly, then left- 

 handedly; and in each case the effects of the two magnetizing 

 currents are observed in the polariscope. 



About incidence 85°, the light restored by a right-handed 

 rotation of the second Nicol is strengthened by a right-handed 

 current, and so forward, the effects being undistinguishable in 

 any way from those obtained in the first experiment at the 

 same incidence, except that (under equally favourable con- 

 ditions) they are certainly and considerably fainter. About 

 incidence 80° the effects are still of the same kind, but a good 

 deal fainter — so faint, indeed, that they cannot be brought out 

 very distinctly except under the most favourable conditions 

 (the battery fresh, the initial extinction very pure, and the dis- 

 placement of the second Mcol extremely small). About inci- 

 dence 75° the regular effects disappear. About incidence 70° 

 they reappear very faintly, as faintly as at 80°, but quite dis- 

 tinctly contrary to those obtained at 80° and 85°: the light 

 restored by a right-handed rotation of the second Mcol is now 

 weakened by a right-handed current, and strengthened by a 

 left-handed current, and so forward. At incidences 65°, 60°, 

 45°, 30°, the effects are of the same kind as at 70°, still con- 

 trary, therefore, to those obtained at 85° ; about incidence 60° 

 they are comparatively clear and strong, though sensibly fainter 

 than those obtained in the first experiment at the same inci- 

 dence ; about 30° they are faint but still distinct, and clearly 

 stronger than the contrary effects obtained at 85°. It appears 

 thus that, in the second experiment, the optical effects of 

 magnetization fall under two distinct cases : — 



(1) Between grazing and principal incidences, the law is 

 the same as in the first experiment : the right-handed current 

 conspires with a right-handed rotation of the second Mcol, and 

 so forward. 



(2) Between principal and normal incidences, this law is 

 simply reversed : the left-handed current conspires with a 

 right-handed rotation of the second Mcol, and so forward. 



9. Any details that could be given as to the methods of ob- 

 servation in the second experiment would be a virtual repetition 

 of article (7) ; but some remarks are due to the case of prin- 

 cipal incidence. In my earlier observations at and about 

 incidence 75° in this experiment, I was much perplexed with 

 the results. The effects of magnetization were sometimes im- 

 perceptible, while in other cases they were quite sensible 

 though faint. Sometimes they were similar to the effects at 

 85°, and sometimes similar to the contrary effects at 60°; but 

 they were more frequently of another kind : either the right- 



