330 POLARISATION. 



The surface of elasticity is consequently always an ellipsoid of 

 revolution in uniaxial media; moreover, the axis of revolution 

 coincides in optically negative media with the least diameter, 

 and in optically positive media with the greatest. The sectional 

 surfaces of such an ellipsoid are generally ellipses likewise ; they 

 become circles only when at right angles to the axis of revolution. 

 The changes of position and excentricity, which the optically effective 

 ellipse undergoes during the rotation of the ellipsoid, may be so 

 easily traced since the problem is considerably simplified owing 

 to the equality of two axes that a special exposition of them 

 appears to us necessary. 



3. DETERMINATION OF THE AXES OF ELASTICITY. 



The estimation of the above-developed dependence of the optical 

 action on the position and form of the ellipsoid of elasticity is 

 based upon the principle of reciprocity. If it is possible to pre- 

 determine from the directions and relative magnitudes of the three 

 axes the process of the phenomena which we observe on rotation 

 round any given perpendicular or horizontal axis, it must, con- 

 versely, be also possible to deduce the directions and relative magni- 

 tudes of the axes of elasticity from the known optical phenomena, 

 and thus to construct the ellipsoid of elasticity, as it were, in the 

 refracting medium. This is precisely the task which presents itself 

 in researches in polarised light ; with its solution the object, which 

 is usually the question in point, is fully attained. 1 



We have therefore to discuss how the determination of the axes 

 of elasticity is to be practically effected. The observer must take 

 into consideration two unknown factors, which, however, we 

 will regard separately, viz., the directions of the axes and their 

 relative magnitudes. As regards the first point, the axial directions, 

 it is hardly necessary for us to make any introductory remarks 

 for those readers who are acquainted with the phenomena of 

 polarisation as explained in the text-books of Physics, since it 



1 Further questions for instance, whether an irregular distension alters the 

 relation of the axes of elasticity, and how this takes place, whether the cause 

 of double refraction lies in the individual micella) (molecular groups) of 

 organised bodies, or in their arrangement, &c. are, at any rate, of secondary 

 consideration, and take for granted the solution of this problem. 



