356 ON THE DISTRIBUTION OF THE POLARISING FORCE 
they cut perpendicularly every similar ellipse drawn within the: 
plate. Hence, when the ellipse is nearly a circle, the distance 
of the fringes will be almost nothing, and each of the fringes 
will form two straight lines at right angles to one another, as 
in elliptical plates, when the transverse or the conjugate axis is 
in the plane of primitive polarisation. 
The lines of equal tint at the angles of rectangular plates of 
glass, are highly deserving of attention. They are produced. 
by the external fringes, and the maximum tint at the angles 
is always less than at the edges, and generally higher than the 
maximum tint in the middle of the plate. When the external 
fringes are two in number, viz. ab, ab’, de, d’e’, as shown 
in Plate VII. Fig. 1. ; the outermost, ab, a’b’, terminates at c¢, e, 
and the other, de, d’e’, branches off at e, e’, so as to form | 
the ellipsis efe’e. The tint increases from this elliptical line 
to its centre 0, but decreases from o to p, and from o to q. 
When the glass plate is a perfect square, the curve efe’e is a 
circle *. In a plate whose maximum tint, in the middle, was a 
Blue of the 2d order ; the tint at o was a Pink of the 2d order ; 
and that at p and g a White of the 1st order. 
3. On the Lines of Equal Tint formed by the Transverse 
_ Combination of Plates of Glass. 
In the Philosophical Transactions for 1816, I have repre- 
sented the Jsochromatic lines, or lines of equal tint, form- 
ed by the transverse combination of plates of glass, when 
the principal axes are both negative, or both positive + ; 
when the principal axis in one is negative, and in the 
other positive |; when the two plates receive their structure 
by bending || ; and when a bent plate is combined with a plate 
formed by heat §. In these various cases, the lines of equal 
tint 
ee SS ane. = San La ee 
* See Phil. Trans. 1916, Plate IV. fig. 29. + Id..Plate II. fig. 3, 4. 
+ Id. Plate If. fig. 8. || Td. Plate IX. fig. 9. § Id. Plate IX. fig. 10. 
