from the Equatorial Surface of a Magnet, 165 



by compression left hand down. By a contrary strain with 

 the same hand down in each of these cases, or by a similar 

 strain with the other hand down, the effect of rotation of the 

 first Nicol in the polariscope is very clearly strengthened ab 

 initio. These observations with the slip of strained glass were 

 described and explained in my former paper. It may be re- 

 membered that a right-handed rotation of the first Mcol was 

 there compensated by tension right hand down, and not, as 

 here, by compression right hand down. The reason of this 

 difference is obvious, the plane of reflection being there vertical, 

 but here horizontal. I proceed now to the observations with 

 the magnetized mirror and the two Nicols, placed always as 

 in the diagram (2). 



6. First Experiment. — The plane of polarization of the 

 light incident upon the mirror is constantly parallel to the 

 plane of incidence ; and the initial extinction is made as pure as 

 possible. 



(1) The second Nicol is turned righthandedly through an 

 extremely small angle from the position of extinction ; and the 

 light thus restored faintly in the polariscope is watched for 

 changes of intensity when the reflecting bar is magnetized suc- 

 cessively by contrary currents. 



(2) The second Nicol is turned lefthandedly through an 

 extremely small angle past the position of extinction ; and the 

 optical effects of contrary magnetizing currents are observed 

 as in the former case. The following is an accurate statement 

 of the results : — 



(1) The light restored from extinction by a very small 

 right-handed displacement of the second Nicol is always 

 strengthened by a right-handed magnetizing current, and 

 always weakened by a left-handed current. 



(2) The light restored from extinction by a very small left- 

 handed displacement of the second Nicol is always weakened 

 by a right-handed current, and always strengthened by a left- 

 handed current. 



The intensity of these optical effects of magnetization varies 

 very noticeably with the angle of incidence. About incidence 

 85° the effects are very faint, but perfectly regular and much 

 better than merely sensible ; about incidence 75° they are 

 more distinct, and very sensibly stronger ; about incidences 

 65° and 60° they are comparatively clear and strong, a good 

 deal stronger than at 75° ; about incidence 45° they are still 

 pretty strong, but very sensibly fainter than at 60° ; about 

 incidence 30° they are again very faint, much the same as at 85°. 



Between 30° and the normal I have made very few obser- 

 vations, and these not satisfactory ; between 85° and 90° I 



