as exhibited in its propagation along plates of glass . 51 
set, is comprehended between CD the edge of the plate and 
the black fringe MN, and the 2d exterior or lateral set 
between the opposite edge of the plate FE and another 
black fringe OP. 
The first interior or central set lies between MN and a b, a 
line equidistant from the two black fringes, and the 2d interior 
or central set between OP and the same line a b. The first 
exterior set contains a greater number of fringes than the se- 
cond exterior set, but in the latter they have a greater breadth ; 
and in both these sets the fringes diminish in breadth, as they 
recede from the black spaces MN, OP. The terminal fringes 
appear at the extremities MO, NP of the plate.* They are 
separated from the central fringes by a faint black space, 
which becomes lighter as the tints increase ; and from the 
lateral fringes by a diagonal black space bisecting the angles 
E, C, D, F. As the tints increase in number, the terminal 
fringes suffer particular changes, which will be described in 
the second part of this paper. 
When the glass plate extends far beyond the heated iron, 
the terminal fringes are not produced. 
Proposition VI. 
To explain the successive developement and subsequent extinction, 
of the fringes during the propagation of the heat along the 
glass plate . 
When the plate of glass CDEF, Fig. 2. (PI. II.) is set upon 
the hot iron, a fringe or wave of a pale white colour instantly 
appears along the line CD, and gradually advances upon the 
* The terminal fringes are not shown in this figure ; but they are represented in 
Hgs. 3 , 4, 8, (PI. II.) 20 and 21. (PI. III.) 
H* 
