by Reflection from the Pole of a Magnet. 327 



sensibly equal to that of the light restored formerly by mag- 

 netization in the first experiment. This being done, I watch 

 the faint light in the polariscope, and work the commutator as 

 formerly. But I must now specify the magnetic states of the 

 mirror. 



When the mirror becomes a north pole, the light flashes up 

 at once to a sensibly higher intensity, which is sustained with- 

 out change as long as the current passes. When the circuit is 

 broken and the mirror demagnetized, the light falls at once 

 from the higher intensity to the primitive faint intensity, and 

 so continues as long as the circuit is open. When the mirror 

 becomes a south pole, the light falls from the primitive faint 

 intensity, down either to perfect extinction or extremely near 

 it. In favourable cases of this kind (that is, in cases properly 

 managed and in a well-darkened room) it is very striking to 

 look at the chink through the analyzer, searching in vain for 

 the faintest trace of the streak of light, and remembering the 

 displacement of the first Nicol. When the circuit is finally 

 broken, the light reappears at once as at first. 



(2) Leaving the circuit open and everything else untouched, 

 I watch the faint light in the polariscope, and turn the first 

 Nicol backwards to the left, into the position of extinction and 

 a little beyond it, regulating the amount of rotation by the in- 

 tensity of the restored light as in the first case. I now watch 

 the light through the analyzer and work the commutator. It 

 would be superfluous to describe the magnetic changes of the 

 iron mirror, and the corresponding changes in the polariscope ; 

 the description would be word for word as before, with one 

 essential alteration. It is the south pole that now strengthens 

 the light, and the north pole that extinguishes or weakens it. 



This experiment is much more easily managed than the first. 

 Let a good sensible extinction of the streak across the chink 

 be obtained by optical trial in the manner already described 

 (6), the plane of polarization of the incident light being either 

 parallel or perpendicular to the plane of incidence ; and let the 

 first Nicol be turned to the right, so far only as to render the 

 extinction sensibly impure. When the three states of the 

 mirror (north, neutral, south) are now made to succeed one 

 another rapidly, the contrast of bright, faint, dark in the po- 

 lariscope comes out in almost every case very distinctly. 



Very often I have seen the second experiment give clear 

 effects as now described, in cases where, through partial ex- 

 haustion of the battery, the first experiment gave no sure effect 

 whatever. 



11. I have given these two experiments as a simple and 

 exhaustive summary of a large number of observations which 



