as exhibited in its propagation along plates of glass. i o i 
Proposition XXXVIII. 
To explain the origin and form of the different sets of fringes 
described in the preceding Propositions. 
j. On the fringes produced by rectangular plates. 
It is i>6t easy to ascertain in what manner the various sets of 
opposite fringes are produced during the heating and cooling 
of glass, (See Prop. XXXIX.) but it is obvious from the pre- 
ceding experiments, that when a plate of glass is either transi- 
ently or permanently crystallized, all the elementary crystals 
of which it is composed, turn one of their neutral axes in the 
direction of the current of heat. The principal axes of the 
crystals which form the exterior fringes, are parallel to the one 
edge, and perpendicular to the other. Thus in Fig. 49, (PI. V. ) 
the axes of the exterior fringes are perpendicular to AD and 
BC, and the axes of the terminal fringes are perpendicular to 
AB and DC, while the axes of the interior fringes are parallel 
to AD and BC. 
Let us now consider, what change should take place in the 
position of the crystals situated at the angles A, B, C, D. An 
elementary crystal at E will have its neutral axes perpendi- 
cular to AD, as it is out of the reach of the forces which act 
upon the crystals at the edges AB, DC ; but, a crystal G in 
the diagonal AH, BH being similarly situated with respect to 
the edges AB, AD, will have a tendency to turn its axis both 
in the direction AB, and in the direction AD, and being unable 
to obey both these solicitations, it will turn it in the direction 
of the diagonal AH, forming angles of 45 0 , with the axes of 
all the other crystals of which the glass is composed. Any 
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