L\V_> 



Mr. R. T. Grlazebrook on NicoVs Prism. 



with x'Oy 1 . But previously the plane of polarization of the 

 emergent wave was inclined at 5° 2> r to xOy. 



Thus by this rotation the Nicol and the plane My have been 

 turned through 90°, while the plane of polarization of the 

 emergent light has been rotated through 90° ±5° 3'. Thus, 

 if the incident light were inclined in this manner to the axis 

 of the Nicol at 5°, we might introduce an error of 5° 3' into 

 the measurement of a rotation of about 90°. The sign of the 

 error will depend on the direction in which the Nicol is turned. 



I have assumed that the axis of rotation of the Nicol is 

 parallel to one edge of the rhomb of which it is composed. 

 If the two are inclined at a small angle, the numbers will be 

 modified slightly, but remain much of the same magnitude. 



It seemed hopeless, from the number and complication of 

 the formulae, to try and solve the problem generally ; I was 

 therefore obliged to give numerical values to the quantities 

 involved. I have supposed, too, that the Nicol has been cut 

 directly from a rhomb of spar ; so that the face of incidence is 

 a cleavage-plane. 



Let us now consider briefly the case in which the ends of 

 the Nicol are perpendicular to its length, and therefore to the 

 direction of the incident light. As before, take the normal to 

 the face of incidence as axis of x ; let a plane perpendicular to 

 the optic axis cut the face of incidence in the axis of z, so that 

 it is still a possible direction of vibration. The angle between 

 this plane and the face of incidence will be greater by 20° than 

 it was in the previous case; so that it is now 65°. Then y 



is another possible direction of vibration. 



Clearly, by supposing the incident-wave normal to lie in a 

 plane midway between x y and x z } the intersection of the 

 wave-front and the face of incidence may be inclined at 45° 



