326 



POLARISATION. 



axes a a and b b of the ellipsoid. It assumes, therefore, nearly the 

 form represented in Fig. 185 A, since b I is of course greater than 

 c c and less than a a. If we now turn the ellipsoid round the axis 

 b b, and let it revolve like a rolling sphere from left to right, then 

 another ellipse of elasticity evidently represents each position. 

 In all ellipses which here become successively effective, however, 

 the one axis has the constant length b b, since it is the axis of 

 revolution, while the other gradually assumes all values between 

 a a and c c, and after half a revolution again becomes equal to a a. 

 In the plane of the circular section (k k and k'k' in Fig. 181) the axes 



are of course equal in length (Fig. 185 B) ; then the transverse axis 

 becomes less than b b, and decreases continuously till a rotation of 

 90 has been made, when it coincides with the axis c c of the 

 ellipsoid (Fig. 185 (7). On continued rotation all possible forms 

 of the sectional surface repeat themselves in opposite sequence, 

 till a a for the second time lies in the plane of the field of view. 

 All possible positions of the ellipsoid are thus exhausted ; for since 

 the two vertical angles are equal, the return to the horizontal 

 position is equivalent to the return to the starting point. 



The positions of the ellipsoid, in which the circular sections 

 lie horizontal, may be further considered. The elasticity of the 

 ether is in these positions equally great in all directions through- 

 out the field of view, precisely as in an isotropic substance. 



