by Reflection from tlie Pole of a Magnet. 329 



the apparatus as for the first experiment and with the greatest 

 care, I find the effects of the two magnetizations unequal in 

 almost every case. Say that the north pole restores distinctly, 

 and the south pole weakly or not at all. Leaving the circuit 

 open, I turn the first Nicol to the left as little as possible, and 

 then bring the second Nicol into the position of extinction, 

 and test by working the commutator and watching the light 

 in the polariscope. Several careful operations of this kind are 

 sometimes requisite. 



Summary and Interpretation of the facts. 



12. In these experiments light is reflected from an iron 

 mirror at an incidence of 60° to 80°, passing through a first 

 Nicol before reflection and through a second Nicol after. 



Initial conditions. — The iron mirror unmagnetized, the prin- 

 cipal sections of the two Nicols perpendicular and parallel 

 respectively to the plane of incidence. 



Essential operations. — Starting thus from pure extinction in 

 the polariscope, we apply any one or two of four operations. 

 Two of these are merely mechanical, extremely small rotations 

 of the first Mcol from its initial position, a right-handed rota- 

 tion (R) and a left-handed (L). The other two are physical, 

 intense magnetizations of the mirror, as a north pole (jN) and 

 as a south pole (S). These four operations will be named here 

 and afterwards by suggestive and easily remembered letters as 

 above ; and they will be grouped in pairs invariably, R and N 

 together, thus : 



(R,N), (L,S). 



13. When any one of the operations is applied singly, the 

 light is restored from pure initial extinction in the polariscope. 



When any two of the operations are applied simultaneously, 

 and their relations determined by comparison of effects in the 

 polariscope, they are found to be conspiring operations if they 

 belong to the same pair, and contrary operations if they be- 

 long to different pairs. Considering, then, any one of the 

 operations, we see that there are two ways of strengthening 

 its effect in the polariscope, and two ways of weakening it. To 

 strengthen the effect of R, apply either operation of the pair 

 (R, N) ; turn the first Mcol a little more to the right, or mag- 

 netize the mirror as a north pole. To neutralize or weaken 

 the effect of R, apply either operation of the pair (L, S) ; turn 

 the first Mcol a little to the left, or magnetize the mirror as a 

 south pole. 



To obtain a complete interpretation of the facts, we have 



