358 ON THE DISTRIBUTION OF THE POLARISING FORCE 



tint will be the sum of the separate tints. Hence, in this case 

 we have 



.312 B 2 .312 B /! 



— T 4- T' — —— --1— nnr? 



f « .312 B- 2 ( T+ T T - - 7 ) ^ ^ 



T .312_FV 

 312 B 2 * 



The lines of equal tint will therefore be Ellipses, as shewn in 

 Plate VII. Fig.2. when either the maximum tints, or the breadths 

 of the combined plates are not equal. 



But, when TzT' and B zz B/, we have 



tf = .312 B' 2 Q^ — ) — x\ 



Consequently, the lines are in this ease Circles, as shewn in 

 Phil Trans, 1816, Plate II. Fig. 8. 



When the two combined plates receive their structure by 

 bending, the tints do not vary, as the squares of their distances 

 from the centre, but simply as their distances. Hence, T being 

 the maximum tint at the edge of the plate, and B B' the 



breadths of the plate as formerly, we have -— ■ : T == a? : t j and 



TV 



— :T' zz y : r , which give 



2 T x 2T« 



t = ^ , and If zz ^ - . Consequently, 



* = —g- — B7 ^, and 



V - 2T T'B* 



Hence, the lines of equal tint will be Straight line** 



The 



