328 Dr. J. Kerr on Rotation of the Plane of Polarization 



were at first very perplexing, so irregular and apparently in- 

 consistent were the phenomena. The chief cause of my per- 

 plexity I found afterwards to be a very interesting thing ; and 

 that was what I may truly call the exquisite delicacy of the 

 magnetic mirror as a test for fixing the position of the plane 

 of polarization of the incident light. One or two simple notes 

 of actual observations will illustrate this point more distinctly 

 than any general statement could do. 



Things often happened thus. Working as in the first expe- 

 riment and with ordinary caution, I started from good extinc- 

 tion, and found the north pole restoring the light, and the 

 south pole much the same as open circuit. Trying to obtain 

 better initial conditions if possible, I threw the two Nicols well 

 out of position, and worked them carefully back to good extinc- 

 tion ; and now, without any other observable change in the 

 conditions, I found things reversed, the south pole clearly re- 

 storing the light, and the north pole much the same as open 

 circuit. Here the magnetic mirror simply detected the impu- 

 rity of the initial extinction, and characterized it, by strong 

 contrasts of intensity in the polariscope, as due to a slight mis- 

 placement of the first Nicol (otherwise barely or not at all 

 detectable), to the right in the first case, and to the left in the 

 second. 



Working sometimes with one of the mirrors (that which had 

 been polished by chamois leather, and which was not so well 

 planed as the other), at a particular part of its surface, and at 

 large angles of incidence, I found the upper end of the streak 

 clearly restored by the north pole and the lower end not, 

 while the lower end of the streak was clearly restored by the 

 south pole and the upper end not. There can be no doubt 

 that in this case the magnetic mirror detected a slight differ- 

 ence of slope at those parts of the mirror which reflected the 

 upper and lower ends of the streak. Say that the one part 

 sloped a little downwards to the left, and the other a little 

 downwards to the right ; then the planes of incidence at the 

 two places would be out of coincidence with the plane of po- 

 larization of the incident light, to the left in the first case, and 

 to the right in the second. 



Similarly, I have sometimes seen the right side of the streak 

 restored by the north pole and the left side not, while the left 

 side was restored by the south pole and the right side not. 

 Irregularities and inconsistencies of this kind were explained 

 perfectly by the second experiment as soon as it was discovered. 



Finally, I observe here that the arrangements for the first 

 experiment are best obtained through those for the second ; 

 and this is a point of some practical importance. Arranging 



