336 Dr. J. Kerr on Rotation of the Plane of Polarization 



The four operations (R, N), (L, S) were found in the second 

 experiment to be related to one another, two and two, as con- 

 spiring or contrary ; they are now seen to be related to one 

 another more generally, and in the same combinations, as like 

 or unlike. With reference to effects in the polariscope, the 

 operations R and S are as clearly unlike as are the operations 

 R and L, or the operations N and S ; and, on the other hand, 

 and with reference always to eftects in the polariscope, R and 

 N are as clearly like as are any two operations R, or any two 

 operations N. It was assumed, in explanation of the facts 

 brought out in the second experiment, that the optical eftects 

 of the four operations (R, N), (L, S) are the same in kind 

 for all, and similarly directed for those of either pair, but op- 

 positely directed for those of different pairs. All the new 

 facts agree with this hypothesis and confirm it- 

 It has been observed already that the eftects of the opera- 

 tions R and L are fully impressed upon the light before inci- 

 dence, while the eftects of N and S are impressed in the pro- 

 cess of reflection ; but, as far as we can judge from the present 

 experiments (17, 18, 19), and as far as changes of phase of 

 the principal components are concerned, this difference be- 

 tween the mechanical operations and the magnetic has little 

 influence upon the final effect in the polariscope. We may 

 therefore assume provisionally that, as far as changes of phase 

 by metallic reflection are concerned, the rotation due to mag- 

 netic force is impressed effectively before incidence. We come 

 now to the second method proposed in 14. 



21. Sixth experiment. — Angle of incidence about 75°, initial 

 arrangements as in the first experiment, plane of polarization 

 of the incident light sometimes parallel and sometimes perpen- 

 dicular to the plane of incidence, initial extinction as pure as 

 possible. 



(1) Leaving the first Nicol untouched, I turn the second 

 Nicol righthandedly through a very small angle ; and watching 

 the faint light thus restored, I work the commutator as for- 

 merly. The operation N strengthens the light ; and this effect 

 is distinct and regular. The operation S has sometimes no 

 effect, and sometimes weakens the light, always less distinctly 

 than N strengthens it, and generally less and less distinctly as 

 the rotation of the second Nicol is diminished. 



(2) The second Nicol is turned to the left from its initial 

 position through a very small angle. N and S now inter- 

 change eftects ; but otherwise the phenomena are as in the first 

 case. 



The effects obtained in repeated and careful trials were, with 

 few exceptions, as I have now described them ; but they were 



