the Cause of coloured concentric Rings. 
263 
39. Coloured Appearances cannot he produced between the plain 
Surfaces of two parallel Pieces of Glass applied to one another. 
As the production and modification of the figure of the 
coloured appearances, that have hitherto been considered, has 
in the last article been ascribed to curved surfaces, it will be 
necessary to examine whether such phenomena may not also 
be seen between the plain surfaces of two parallel pieces of 
glass applied to each other directly in contact, or inclined to- 
wards each other in some certain extremely small angle. 
The latter of these cases has already been considered in 
the 31st article of the first part of this paper, where I have 
shown that two plain surfaces, let the angle of the wedge of 
air between them be as small as you please, will not give 
coloured streaks. I have indeed seen two thin plain pieces of 
glass, with a slip of platina of an extraordinary thinness be- 
tween them at one end tied together, which showed some 
streaks near the place where the glasses were in contact, but 
when I removed the thread that bound them together, the 
streaks vanished, which proves that the glasses had been con- 
strained, and thus had probably assumed some curvature at 
the point of contact. 
I have also tried two flat surfaces of glass, which were so 
perfect that no colour could be perceived unless they were by 
unequal pressure somewhat disfigured, and when that was 
the case large flashy coloured appearances became visible, 
and their configuration followed very evidently the stress 
which I laid upon the different parts of the glasses. 
It is however unnecessary to dwell on proofs, that streaks 
cannot be seen when two plain parallel pieces of glass are 
M m a 
