494 



JAMES CLERK MAXWELL. 



the points where the action is greatest ; and the intensity of 

 the action at any other point is determined by its position 

 with respect to the isochromatic curves. 



" The direction of the principal axes of pressure at any point 

 is found by transmitting plane polarised light, and analysing 

 it in the plane perpendicular to that of polarisation. The light 

 is then restored in every part of the triangle, except in those 

 points at which one of the principal axes is parallel to the plane 

 of polarisation. A dark band formed of all these points is seen, 

 which shifts its position as the triangle is turned round in its 



Fig. 5. 



own plane. Fig. 5 represents these curves for every fifteenth 

 degree of inclination. They correspond to the lines of equal varia- 

 tion of the needle in a magnetic chart. 



" From these curves others may be formed which shall indicate, 

 by their own direction, the direction of the principal axes at any 

 point. These curves of direction of compression and dilatation are 

 represented in Fig. 6 ; the curves whose direction corresponds 

 to that of compression, are concave toward the centre of the 

 triangle, and intersect at right angles the curves of dilatation." 



