as exhibited in its propagation along plates of glass. 71 
In a piece of glass shaped as in Fig. 13. (PI. II.J when 
CD was heated, part of the upper black space had the form 
a be, and the other part e f terminated at/. 
Proposition XVIII. 
To explain the effects produced upon the fringes by an interruption 
in the continuity of the glass. 
If the second exterior and the two interior sets of fringes 
were caused by the actual communication of heat to the parts 
of the glass which produce them, there was reason to believe, 
that they would not be affected by any breach of continuity in 
the glass, which did not obstruct the progress of the heat. In 
order to determine this, I broke a plate of glass ABCD, 
Fig. 14. (PL III.) through the middle mn, and having ob- 
tained a very clean fracture, I placed the upper fragment CD, 
upon the lower one. This compound plate was set upon the 
hot iron RS; but no effect was produced on the upper plate, 
the fringes developing themselves in AB, just as if CB had 
been removed. When the heat was almost uniformly dif- 
fused over AB, CD began to exhibit faint traces of the white 
fringes, AB now serving as a new source of heat. The very 
same result was obtained when the two plates were joined by 
the interposition of zuater , Canada balsam , or rosin. 
I now took a piece of glass ABCD, Fig. 15. (PI. III.) in- 
terrupted by a fissure, or crack m n extending a short way 
into the plate. When the heat was communicated to its 
lower edge, the fringes were seen above mn, as if the crack 
had not existed, and the depolarised white light appeared con- 
densed at n, like a fluid rushing round the point. The crack, 
however, suddenly extended to 0; the upper piece of glass 
