as exhibited in its propagation along plates of glass. 67 
struggle between the advancing and retiring fringes, had a 
curious appearance. Before the usual series of fringes vanished, 
the external fringes became broader, while the middle one gra- 
dually diminished. The two black spaces met in the middle 
of the plate, forming a broad undefined dark space, and the 
new or unusual series were seen advancing from the edges of 
the glass. At this instant there were two white spaces in the 
middle of the plate, and two external ones, but one of the 
middle white spaces quickly died away, and the unusual series 
was speedily developed. 
If plates of glass that exhibit the usual fringes are taken 
from the hot iron and allowed to cool in the open air, the 
fringes will gradually pass away as described in Prop. VI. 
but, as soon as they disappear, or a little before their disap- 
pearance, the opposite sets begin to advance upon the plate in 
the manner already described. 
Proposition XV. 
When similar fringes of the usual and unusual series are combined 
symmetrically , the polarised tint is that which is due to the dif- 
ference of the thicknesses , but when they are combined trans- 
versely . , the tint is that which is due to the sum of the thick- 
nesses of the plates. When dissimilar fringes of the tzvo series 
are combined symmetrically , the polarised tint is that which is 
due to the sum of the thicknesses , but when they are combined 
transversely , the polarised tint is that which is due to the diffe- 
rence of the thicknesses of the plates that produce them. 
The preceding truth was established by combining the 
fringes produced by 15 plates of crown glass cooled from the 
heat of boiling water, |with those produced by a plate of glass 
Ke 
