93 
as exhibited in its propagation along plates of glass. 
appear at a vertical incidence, but, upon inclining the plate to 
the incident ray, in the direction of its length OP, three black 
spaces mn , OP, qr, are gradually developed. One of them 
OP passes through the centre of the plate ; and between the 
black spaces are four sets of fringes 1,1; 1,1 ; 2,2; 2,2; By 
examining these fringes with a standard plate of sulphate of 
lime, and with plates of crystallized glass, I found that the 
fringes 1, 1, 1,1, had the same character as the diagonal fringes 
A, D, while the fringes 2,2, 2,2, had the same character as 
the other two diagonal fringes C, B. In; one plate, where 
the maximum tint of the interior fringe was a faint yellow 
of the first order, the fringes 1 , 1, 2, 2, consisted of a blue of the 
first order, and in another plate where the maximum tint of 
the interior fringe was a faint yellow of the second order, the 
fringes between m n and q r consisted of a green of the second 
order. 
Proposition XXXIII. 
When a plate of crystallized glass is placed on a red hot iron , the 
number of its fringes is increased. These additional fringes 
are the same that would have been produced by combining with 
the crystallized plate an uncrystallized plate of the same form 
and thickness , and subjected to the same temperature as the cry- 
stallized plate. They disappear when the glass cools, but the 
permanent fringes are not altered unless the heat be very intense , 
in which case , they suffer a small diminution. 
The results described in the Proposition were obtained by 
placing crystallized plates upon bars of iron of different tem- 
peratures. The plate was held out of the heat of the red hot 
iron, when its effect was combined with that of an uncrystal- 
