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



pression are reversed, and therefore interchanged. It is now 

 compression that extinguishes the light ; tension strengthens 

 it from first to last. 



When the angle of rotation of the first Nicol is too large, 

 which it may be while still very small, the neutralization by 

 tension or compression is incomplete, the light fading to a 

 very sensible minimum and then increasing ; but the extinc- 

 tion is still perfect when the initial intensities have reached 

 much greater values than those obtained by magnetization in 

 the first experiment. 



I found the present experiment a very interesting one, from 

 the simplicity of the means, the purity of effects, and the 

 beautiful distinctness of the contrasts. However, I do not 

 give the experiment here for its own sake. The only use of it 

 is to characterize the effects of R and L in the polariscope ; and 

 this work it does perfectly. 



18. Fourth experiment. — The angle of incidence about 75°, 

 and all the arrangements and procedure as in the first experi- 

 ment, with addition of the compensator. As the intensity in 

 the polariscope is very faint at the best, all proper means are 

 adopted for increasing it — the room well darkened, the battery 

 in good order, the surface of the mirror fresh, the chink be- 

 tween wedge and core merely wide enough to give a good 

 object, and the initial extinction sensibly perfect. 



When the light is restored from pure extinction by the ope- 

 ration N, and the compensator is placed and strained as in the 

 third experiment, the light is weakened by tension and 

 strengthened by compression, and the weakening by tension 

 proceeds to pure extinction. The effect of the operation S is, 

 on the contrary, weakened to extinction by compression, and 

 strengthened from first to last by tension. 



This is the general result; but some precautions had to be 

 taken in the actual experiment. Sometimes heat from the 

 hand, possibly also from the breath, gave rise in the compen- 

 sating slip to strains which had large effects in the polariscope, 

 effects larger indeed than that to be compensated. In such a 

 case the slip was laid aside and a fresh one employed. It was 

 found necessary also to keep the plate faces of the slip accu- 

 rately perpendicular to the reflected beam, as a very small 

 displacement from this direction gave a noxious effect. Ob- 

 serving these and other precautions, and working with proper 

 care, 1 found after some practice that the phenomena were 

 perfectly under control. Sitting down in front of the polari- 

 scope, and getting an assistant to hold the submagnet and work 

 the commutator, I bring the compensator suddenly into the 

 standard position, and find the extinction still pure. The cir- 



