166 Dr. J. Kerr on Reflection of Polarized Light 



have obtained no sure effect, nothing that could be recovered 

 with certainty ; but within the preceding range of incidence, 

 85° to 30°, I have always recovered the effects easily, and 

 always in the forms now stated. 



The right-handed current conspires with (or strengthens the 

 effect of) a right-handed rotation of the second Nicol, and so 

 forward consistently, the optical effect of the current being 

 reversed by reversal of the current, and also by reversal of ro- 

 tation of the second Nicol. 



7. Returning for a moment to the method of observation, 

 suppose the second Nicol turned righthandedly through a 

 small angle from extinction. 



(1) Only right-handed currents are applied, the circuit 

 being closed and broken at intervals. When the circuit is 

 closed, the light is strengthened in the polariscope ; when the 

 circuit is broken, the light is weakened. 



(2) Only left-handed currents are applied. When the 

 circuit is closed, the light is weakened ; when the circuit is 

 broken, the light is strengthened. 



(3) Contrary currents are transmitted in succession, the 

 reversal being made by a rapid half-turn of the commutator. 

 In this case, the passage from left-handed current to right- 

 handed strengthens the light in the polariscope, and the con- 

 trary passage from right-handed current to left-handed weakens 

 the light. 



The magnetic changes of the bar in (3) are more than twice 

 as great as those in (1) or (2), because of the imperfect de- 

 magnetization of the bar at the instant of break. It is observed 

 accordingly in the experiment, that the optical changes in (3) 

 are far superior to those in (1) and (2), generally much more 

 than twice as strong. Sometimes, indeed, about the extreme 

 incidence 85° or 30°, and when circumstances are unfavour- 

 able, I find the effects in (3) still quite distinct, while those in 

 (1) and (2) are almost or altogether imperceptible. Another 

 fact of the same kind which I have noticed in (1) and (2) is, 

 that the effect of make, whether it be an increase or a diminu- 

 tion of intensity in the polariscope, is generally more distinct, 

 strikes the eye more sharply, than the effect of break. I may 

 state here finally that, when the effects are very weak, the 

 observer may often bring them out better and better by making 

 the rotation of the second Nicol smaller and smaller till the 

 restoration in the polariscope is merely an imperfect extinction. 



8. Second Experiment. — The plane of polarization of the 

 light incident on the mirror is constantly perpendicular to the 

 plane of incidence. All the other arrangements, and the 

 observations, are precisely as in the first experiment. The 



