IN PLATES, TUBES, AND CYLINDERS OF GLASS. 361 



The angle <p, which the straight lines of equal tint form with 

 the edges of the plate, will be found by the formula, 



T „ T - B 



lang. <p — y^r, — -g>. 



When x : y — B : B', and when similar sides of the plates 

 cross each other, we shall have r = 0, that is, the line of no- 

 polarisation will be the diagonal of the parallelogram formed 

 by the sides of the two plates *. 



When B - B', and T = T, then 

 Br , 



y = 2T " """ *' 



the straight lines of equal tint will be inclined 45° to the edges 



T 

 of the plates, for -rp = 1, which is the tangent of 45°. 



When a plate of glass with two axes is combined with a 

 plate of bent glass f, we have 



<2Ty : _ T x 2 



c = -^r- + 1 — .3l2-B 2 » and 



when the concave side of the bent plate crosses a plate with 

 two axes, in which the principal axis is negative ; or, 



h' .312 B 2 



when the convex side of the bent plate crosses a plate with 

 two axes, in which the principal axis is positive, and vice 

 versa. Hence, it follows, that the lines of equal tint are here 

 Parabolas. 



Vol, VIII. P. II. Z z When 



* See Phil. Trans. 1816, Plate IX. fig. 9. f Id. Plate IX. Fig. 10. 



