40 



POLARISATION OF LIGHT. 



Fig. 42. 



. 43. 



the axes of the solid, while a third, 

 equally inclined to those two. and form- 

 ing; an angle of 45 with the common 

 section, passes through the centre of the 

 icosatetrahedron. The other two planes 

 of the pentahedron are halves of two 

 of the adjoining trapezia which form the 

 surface of the general solid. 



If we suppose the crystal to have a 

 cubical form, and expose it to polarised 

 light, incident perpendicular to any of its 

 faces, we shall find that all the planes 

 passing through the diagonals are planes 

 Of no double refraction and polarisation, 

 as shown in fig. 43. The black lines at 

 ris:ht angles to one another show the 

 planes where there are no polarised 

 tints, and the intermediate shades repre- 

 sent the different orders of colours which 



of course depend on the thickness of the 

 crystal. This effect is produced when 

 any of the two axes of the cube, or those 

 lying in a plane perpendicular to the po- 

 larised ray, are inclined 45 to the plane 

 of primitive polarisation. When any of 

 these two axes, however, are in the plane 

 of primitive polarisation the tints disap- 

 pear, and continue invisible while the 

 crystal is made to revolve round that 

 axis ; but when the axis is inclned to 

 that plane, the tints re-appear, and reach 

 their maximum intensity when the in- 

 clination becomes 45. 



In order to convey an idea of the 

 structure of the complete crystal we have 

 represented the icosatetrahedron in fig. 

 44, with its planes of no double refraction 

 and polarisation, and the tints of the in- 



Fis. 44. 



termediate solids. The dark shaded lines 

 represent the planes in which there is no 

 double refraction and polarisation, and 

 the fainter shadings represent the tints. 

 The appearances, however, shown in 



this figure, and in fg. 43, can never be 

 seen by the observer at once. 



The tints polarised by analchre are 

 those of Newton's scale, and they are 

 negative in relation to each of the four 



