358 ON THE DISTRIBUTION OF THE POLARISING FORCE 
tint will be the sum of the ee tints. Hence, in this case 
we have 
urate Ulam 
3128? 312 B? 
V+T—- Tots Be 
oo ve — 
y? = .312B TY ) “7-319 BE * 
The lines of equal tint will therefore be El/ipses, as shewn in 
Plate VIL. Fig.2. when either the maximum tints, or the breadths. 
of the combined plates are not equal. 
But, when T = T’ and B = By, we have 
—-T+T — 
, and 
ye =.312 B? (2 eet —a", 
Consequently, the lines are in this ease Circles, as shewn in 
Phil. Trans. 1816, Plate IT. 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 = :Toa:t; and 
Pp 
a :T’=y:t, which give 
c= a and ’ = ay. Consequently, 
a 2Te + 2Ty 
LS +r a » and 
Be TR 
I= oP — TR | 
Hence, the lines of equal tint will be Straight lines. 
The 
