the Cause of coloured concentric Rings. q i 5 
Upon a plain metalline mirror I laid a double convex lens, 
having a strong emery scratch on its uper surface. When I 
saw the rings through the scratch, they appeared to have a 
black mark across them. By tilting the lens, I brought the 
center of the rings upon the projection of the scratch, so that 
the incident light was obliged to come through the scratch to 
the rings, and the black mark was again visible upon them, 
but much stronger than before. In neither of the situations 
were the rings disfigured. The stronger mark was owing to 
the interception of the incident light, but when the rings had 
received their full illumination the mark was weaker, because 
in the latter case the rings themselves were probably complete, 
but in the former deficient. 
I placed a lens that had a very scabrous polish on one 
side, but was highly polished on the other, upon a metalline 
mirror. The defective side being uppermost, I did not find 
that its scabrousness had any distorting effect upon the rings. 
I splintered off the edge of a plain slip of glass ; it broke as 
it usually does with a waving striated, curved slope coming 
to an edge. The splintered part was placed upon a convex 
metalline mirror of 2-inch focus, as in figure 10. The irre- 
gularity of the striated surface through which the incident ray 
1,2, was made to pass had very little effect upon the form of 
the rings ; the striae appearing only like fine dark lines, with 
hardly any visible distortion ; but when, by tilting the return- 
ing ray, 2, 3, was also brought over the striated surface, the 
rings were much disfigured. This experiment therefore 
seems to prove that a very regular refraction of light by the 
first surface is not necessary; for though the rings were 
much disfigured when the returning light came through the 
F f 2 
