i 
IN PLATES, TUBES, AND CYLINDERS OF GLASS. 361 
. The angle ¢, which the straight lines of equal tint form with 
the edges of the plate, will be found by the formula, 
iE B 
Tang. sS Oi = Py 
When x:y=B: By, and when similar sides of the plates 
cross each other, we shall have += 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= By’, and T =T, then 
Br 
Y= op and. 
the straight lines of equal tint will be inclined 45° to the edges 
of the plates, for - = 1, which is the tangent of 45°. 
When a plate of glass with two axes is combined with a 
plate of bent glass +, we have 
ay op 
a Bs 2 aR a) 
v= .312B(1— 7 + ape), 
when the concave side of the bent plate crosses a plate with 
two axes, in which the principal axis is negative ; or,, 
2 2 : 
2 
1 Ae Gio: |, aie Be OO 
2— 312 B a _ 2Ty 
Po we Ge ee, 
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. 
Vou. VIII. P. IT. ZZ When 
 * See Phil. T'rans. 1816, Plate IX. fig. 9. + Id. Plate IX. Fig. 10. 
