116 MR JAMES CLERK MAXWELL ON THE 
If two cylindric surfaces, whose axes are perpendicular to the plane of an 
indefinite elastic plate, be equally twisted in the same direction, the resultant 
compression in any direction may be found by adding the compression due to each 
resolved in that direction. 
The result of this operation may be thus stated geometrically. Let A, and 
A, (fig. 1.) be the centres of the twisted cylinders. Join A, A,, and bisect A, A, 
in O. Draw OBC at right angles, and cut off OB, and OB, each equal to OA,. 
Fig. 1. 


Then the difference of the retardation of oppositely polarized rays of light 
passing perpendicularly through any point of the plane varies directly as the pro- 
duct of its distances from B, and B,, and inversely as the square of the product 
of its distances from A, and A,. 
The isochromatic lines are represented in the figure. 
The retardaticn is infinite at the points A, and A2; it vanishes at B, and B,; 

ae <-"> 
