the Cause of coloured concentric Rings . 219 
that have been instituted for examining the action of the first 
surface will equally serve for investigating that of the second. 
The lens already used with a strong emery scratch being 
again placed on the mirror, but with the injured side down- 
wards, I found that the rings, when brought under the 
scratch, were not distorted ; they had only a black mark of 
the same shape as the scratch across them. 
The lens with a scabrous side was also placed again upon 
the mirror, but with the highly polished side upwards. In 
this position the scabrousness of the lowest surface occa- 
sioned great irregularity among the rings, which were in- 
dented and broken wherever the little polished holes that 
make up a scabrous surface came near them ; and if by gently 
lifting the lens a strong contact was prevented, the colours of 
the rings were likewise extremely disfigured and changed. 
As we have now seen that a polished defect upon the 
second surface will affect the figure of the rings that are 
under them, it will remain to be determined whether such 
defects do really distort them by some modification they give 
to the rays of light in their passage through them, or whether 
they only represent the rings as deformed, because we see 
them through a distorted medium. For although the scab- 
rousness did not sensibly affect the figure of the rings when 
it was on the first surface, we may suppose the little polished 
holes to have a much stronger effect in distorting the appear- 
ance of the rings when they are close to them. The follow- 
ing experiment will entirely clear up this point. 
Over the middle of a 22-inch double convex lens I drew 
a strong line with a diamond, and gave it a polish afterwards 
that it might occasion an irregular refraction. This being 
