by Reflection from the Pole of a Magnet. 341 



S that strengthens the light, and N that weakens or extin- 

 guishes it. 



The phenomena now mentioned are very faint, a good deal 

 fainter than those obtained in the second experiment ; but 

 they are certain, distinct, and perfectly regular. I need hardly 

 say that this experiment is decisive, and that the effects are 

 certainly due to rotations virtual and actual of the plane of 

 polarization of the light which is presented to the analyzer, 

 the virtual rotations being produced by displacements of the 

 second Nicol, and the actual by the operations N and S. 

 N conspires with a righthanded rotation of the second Nicol ; 

 and therefore N turns the plane of polarization to the left : 

 S conspires with a lefthanded rotation of the second Nicol ; 

 and therefore S turns the plane of polarization to the right. 



29. Ninth experiment. — No change in the arrangements, 

 the initial extinction perfect. 



(1) The first Nicol is turned righthandedly (from C as 

 point of view) through a small angle, giving a faint but di- 

 stinct restoration. S strengthens the light thus restored, and 

 N weakens and sometimes extinguishes it. 



(2) The first Nicol is turned lefthandedly, through pure 

 extinction, to faint restoration. N strengthens the restored 

 light, and S weakens or extinguishes it. 



The phenomena are precisely as in the eighth experiment, 

 and equally distinct and regular, but with reversed relations 

 of S and N to movements of the Nicol : and this was to be 

 certainly expected, because the first Nicol simply carries the 

 plane of polarization with it, while the second Mcol simply 

 leaves that plane behind it. As an illustration of this state- 

 ment, and of the consistency of the results obtained in this 

 article and the preceding, I offer the following optical experi- 

 ment, though it will be to many of my readers unnecessary. 



30. Tenth experiment. — The arrangements unchanged, the 

 initial extinction perfect, the circuit kept open. 



(1) First Mcol to the right, giving a faint restoration. 

 The restored light is weakened to extinction by rotation of 

 the second Mcol to the right ; strengthened clearly, ab initio^ 

 by rotation of the second Mcol to the left. 



(2) First Mcol to the left, giving a faint restoration. 

 The light is weakened to extinction by rotation of the second 

 Mcol to the left ; strengthened clearly, ab initio , by rotation 

 of the second Nicol to the right. 



These effects are certain and regular ; but sensibly perfect 

 extinctions are obtained only in careful work, and with very 

 small displacements of the first Mcol. 



31. Eleventh experiment. — Starting with the same arrange- 



