356 ON THE DISTRIBUTION OF THE TOLARISING FORCE 



ihey 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. a b, a r b', d e, d! V, as shown 

 in Plate VII. Fig. 1. ; the outermost, a b, a'b\ terminates at c, c, 

 and the other, d e, 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 o, 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 q 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 Isochromatic 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 f ; 

 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 



* See Phil. Trans. 1816, Plate IV. fig. 29. f Id. Plate II. fig. 3, 4. 



I Id. Plate II. fig. 8. || Id. Plate IX. fig.' 9. § Id. Plate IX. fig. 10. 



