168 Dr. J. Kerr on Reflection of Polarized Light 



handed current or the left-handed, sometimes one and some- 

 times the other, conspired feebly with each rotation of the 

 second Mcol, the right-handed and the left-handed in succession. 

 The latter effects were certainly due to slight misplacements of 

 the first Mcol, which impaired the purity of the initial extinc- 

 tion. All these irregularities disappeared in the most carefully 

 conducted of my later observations, where due attention was 

 given to two principal conditions — the assignment of a proper 

 value (75° exactly or nearly) to the angle of incidence, and 

 the purification of the initial extinction by exact adjustment 

 of the first Mcol. Upon the whole, the result of my observa- 

 tions is that, in the second experiment, at and about incidence 

 75°, neither magnetization has any regular effect in the pola- 

 riscope. I have little doubt that, with higher powers, magne- 

 tization would have a distinct optical effect, and that effect an 

 increase of intensity in every case ; but I merely describe 

 things at present as I have seen them. 



10. Third Experiment. — The two Mcols are placed initially 

 at pure extinction, the plane of polarization of the light inci- 

 dent on the mirror being parallel to the plane of incidence. 

 The second Mcol remains fixed. 



The first Mcol is turned through an extremely small angle 

 from the position of extinction, first righthandedly and then 

 lefthandedly ; and in each case the effects of the two magne- 

 tizing currents are observed in the polariscope. 



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

 rotation of the first Mcol is weakened by a right-handed cur- 

 rent, and strengthened by a left-handed current ; while the 

 light restored by a left-handed rotation of the first Mcol is 

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

 left-handed current. The effects are fainter than those obtained 

 at the same incidence in the first experiment, but they are 

 distinct and perfectly regular. About incidence 80° the 

 effects are still regular and of the same kind, but very faint, 

 and requiring extremely small displacements of the first Mcol 

 to bring them out clearly. About incidence 75° the regular 

 effects disappear ; and some irregular effects, which make their 

 appearance here as in the second experiment, are eliminated 

 by the assignment of a proper value to the angle of incidence, 

 and by exact adjustment of the second Mcol. 



At incidences 65°, 60°, 45°, 30°, the effects are contrary to 

 those obtained at 85° : the light restored by a right-handed 

 rotation of the first Mcol from extinction is strengthened by 

 a right-handed current, and so forward. At incidence 60°, 

 ^nd even at 45°, the effects are very distinct and comparatively 

 strong, but always fainter than those obtained at the same in- 



