356 



POLARISATION. 



The latter are impossible, because one always of the three 

 positions taken up is not capable of being combined with the other 

 two, as may easily be proved. For instance, from combination 

 14, L denoting the longitudinally situated axis of the ellipse (of the 

 middle and marginal views), R the radial axis, and Tthe tangential 

 axis, we obtain the impossible inequalities : R < T, E > L, L > T. 

 The proof for the other combinations is just as easily supplied. 



The conclusions which may be drawn from the thirteen possible 

 combinations naturally arrange themselves in two series. The first 

 series contains those cases where one of the three axes of the 

 ellipsoid is parallel to the axis of the cylinder ; the second 

 embraces all the remainder having any oblique position of the 

 tangential axes. Both series are placed side by side with the 

 combination series in the following table, for more convenient com- 

 parison. The signs we have used therein are to be interpreted as 

 follows : i positive and negative i.e., undecided whether the one 

 or the other ; plane of the axes = plane of the optic axes ; tan- 

 gential = parallel to a tangential plane situated in the surface of 

 the cylinder ; radial = in the plane drawn through the radius and 

 the longitudinal axis (L) ; transversal = in a plane perpendicular 

 to the axis L ; L = longitudinal axis i.e., axis of the ellipsoid of 

 elasticity, which in the first series is parallel to the axis of the 

 cylinder, and in the second series is inclined less than 45 

 to it ; T = tangential axis i.e., the axis of the ellipsoid, which is 

 perpendicular to the plane drawn through L and the radius of 

 the cylinder ; R = radial axis. 



