384 



face at the other end. Then causing the rhomb to revolve 

 about the ray, we shall find two positions of it in which the 

 emergent hght will be plane-polarized, these positions being 

 readily indicated by a Nicol's prism interposed between the 

 rhomb and the eye ; for such a prism, by being turned round 

 the ray, can make the light totally disappear when it is 

 plane-polarized, but not otherwise. These two positions of 

 the rhomb will be exactly 90° from each other ; in one of 

 them the principal plane of the rhomb (the plane of reflexion 

 within it) will be parallel to the major axis of the elliptic vi- 

 bration, and the angle which it makes with the plane of in- 

 cidence on the metal will be equal to 6 : while in the same 

 position the angle which the principal plane makes with the 

 plane of polarization of the emergent ray (as given by the 

 Nicol's prism) will be equal to /3. In the other position, the 

 principal plane will be parallel to the minor axis of the 

 eUiptic vibration, and the corresponding angles will be equal 

 to 90°— and 90°— j3 respectively. This, however, proceeds 

 on the supposition that the rhomb is exact. When it is not 

 so, which is of course the proper supposition, and a very ne- 

 cessary one in the experiments with which we are concerned, 

 there will still be, generally speaking, two positions of it in 

 which the emergent ray will be plane-polarized, or in which 

 a disappearance of the hght may be produced by the Nicol's 

 prism ; but these positions will no longer be 90° from each 

 other, nor will the principal plane, in either of them, coincide 

 with an axis of the elhptic vibration. If we now measure the 

 angles between the different planes as before, and denote 

 them by 6', /3' in the first position, and by 90°~9", 90°-j3'' 

 in the second, we shall find that 6' and 0" are unequal, but 

 we shall have j3' equal to /3". The values of 6 and /3 will 

 then be given by the formulae 



^ = -2-' '^°^^^ = ^3^W=0'T ^"^^ 



The error of the rhomb may easily be found. Supposing 



