from the Equatorial Surface of a Magnet, 173 



About incidence 75°, the light restored by compression right 

 hand down is strengthened by the right-handed current, and 

 weakened by the left-handed current, the actions of the currents 

 from reversal being regular and very clear though faint. 



About incidence 60° the effects are of the same kind, but a 

 great deal stronger. Each of the currents acts clearly now 

 from its own break (art. 7), the right-handed current conspiring 

 always with compression right hand down, therefore also with 

 tension left hand down ; and the left-handed current conspiring 

 always with tension right hand down, therefore also with 

 compression left hand down. These four cases of conspiring 

 actions, as well as the four corresponding cases of contrary or 

 mutually compensating actions (right-handed current with 

 tension right hand down, &c), are all brought out regularly 

 and strongly about incidence 60°. 



At incidence 45° the effects are still of the same kind, regular 

 and very distinct though not strong, not nearly so strong as 

 at 60°. Between principal incidence and grazing, about 85°, 

 the effects are still of the same kind, the right-handed current 

 clearly conspiring here as elsewhere, though here very faintly, 

 with compression right hand down. 



20. It is worth noticing that we might have anticipated some 

 of the results brought out in the last two experiments, those 

 of them, namely, that are obtained at incidences near 75°. It 

 has been mentioned already as a result easily obtained with 

 the present apparatus, that a small right-handed rotation of the 

 first Nicol from extinction, at or near incidence 75°, is neutra- 

 lized by compression right hand down (art. 5). From this we 

 infer that compression right hand down is optically equivalent, 

 at least in an approximate manner, to a left-handed rotation 

 of the first NicoJ. We have, then, two cases, according to the 

 fourth and third experiments : — 



(1) When the plane of polarization of the light incident on 

 the mirror is perpendicular to the plane of incidence, the right- 

 handed current conspires with a left-handed rotation of the 

 first Nicol, and should conspire therefore with the optical 

 equivalent of that rotation, compression right hand down, as 

 it does actually in the eighth experiment. 



(2) When the plane of polarization is parallel to the plane of 

 incidence, and the angle of incidence is about 75°, neither 

 current conspires with any small rotation of the first Nicol ; so 

 that neither current should conspire with compression right 

 hand down, which is actually the case in the seventh ex- 

 periment. 



21. I have now given all my positive results ; but there are 



