364 



POLARISATION. 



FIG. 207. 



On this supposition the sphere acts in any given position, at 

 least in its marginal parts, as a middle lamella B B (Fig. 207) 



which we may imagine to be cut from 

 it or as a transverse section of the 

 cylinder. For it may readily be shown 

 that the spherical calotte turned to the 

 observer and the one turned from him 

 always act in the same way as the 

 imaginary middle piece, and that they 

 consequently increase its effect, but do 

 not change it. This is at once apparent 

 with uniaxial elements whose optic axis 

 is situated radially. We arrive at the 

 same result with biaxial elements if 

 we consider that the tangential axes of 

 elasticity are situated in every position, 



and therefore act upon the transmitted ray of light with a 

 value which is mid- way between the minor and the major 

 axes. The effect of the superposed elements is clearly the 

 same as if, with similar position of the tangential axes, they 

 were turned rapidly round their radial axis, and therefore 

 also the same as with uniaxial elements with radially situated 

 optic axis. 



Hence the image which a spherical object gives in polarised 

 light decides for us the question, whether the radial axis 

 of the ellipsoid of elasticity is greater or less than the mean 

 value of the two tangential ones; but it leaves undecided 

 whether the elements are uniaxial or biaxial. Assuming the 

 latter case, it further remains doubtful whether the radial 

 axis is the mean one, or, in accordance with the character 

 of the colour, the major or the minor i.e., it is doubtful 

 whether the optic axes lie in a tangential plane or in a dia- 

 metral one. 



The neutral cross, which separates the acceleration and 

 retardation quadrants from each other, acts exactly as in 

 transverse sections of cylinders. It appears rectangular if the 

 polarising planes of the Nicols carried through the centre of 

 the layers intersect the layers themselves at right angles; and 

 in the opposite case it appears oblique-angled. In circular layers 

 the former is, of course, always the case ; but in elliptical ones 



