by Reflection from the Pole of a Magnet. 335 



cuit is now closed, and the light reappears through the com- 

 pensator. I now compress the slip along its length with a 

 force increasing slowly from zero, and find that the light in- 

 creases continuously and very distinctly as the compression 

 increases. I therefore pronounce the mirror a north pole, 

 which the assistant finds right. To verify by the polariscope, 

 I stretch the slip with a force increasing slowly from zero, and 

 find that the light fades to pure extinction and then brightens. 

 The effects are very faint, but quite unmistakable. In the 

 last-mentioned case, for instance, I put the light out of sight 

 by careful increase of the tension up to a certain small value, 

 and keep it out of sight as long as I please by sustaining the 

 force, straining my eye all the time to catch the faintest 

 glimpse, till the instant when the slip is relieved of strain 

 without change of position ; and then the light reappears as at 

 first. Working similarly in another case, I find these optical 

 effects of tension and compression interchanged, compression 

 extinguishing the light and tension strengthening it ; and the 

 mirror is found accordingly to be a south pole. 



19. Fifth experiment. — This is a repetition of the second 

 experiment with addition of the compensator ; it is more easily 

 managed than the fourth ; and the results are equally convin- 

 cing. In the first half of the second experiment, as already 

 described (10), the three sets of operations applied successively 



were (B,N), R, (R, S); 



and the intensities in the polariscope in the three cases respec- 

 tively were bright, faint, dark. 



When the effects in the first and second cases are tested by 

 the compensator, exactly as in the third and fourth experi- 

 ments, they are both compensated to pure extinction by ten- 

 sion, and both strengthened from first to last by compression. 

 And similarly in the second half of the second experiment, 

 the single effect of L and the joint effect of L and S are both 

 strengthened by tension, and both weakened down to sensible 

 extinction by compression. 



20. Summary of the results obtained in the last three expe- 

 riments. 



The effects of the operations R and L in the polariscope are 

 compensated respectively by tension and by compression of 

 glass in the standard direction : the effect of N is compensated 

 precisely as that of R, and the effect of S precisely as that of 

 L ; the joint effect of R and N is compensated precisely as the 

 separate effects of R and N, and the joint effect of L and S 

 precisely as the separate effects of L and S : and in all these 

 cases the compensation proceeds to sensible extinction. 



