the Cause of coloured concentric Rings. 217 
bent glasses merely acts as an inlet to the rays that afterwards 
form the rings. 
To confirm the idea that the mere admission of light would 
be sufficient, I used a slip of glass polished on one side but 
roughened with emery on the other ; this being laid upon a 
21-feet object-glass, I saw a set of rings through the rough 
surface ; and though they appeared hazy, they were otherwise 
complete in figure and colour. The slip of glass when laid in 
the same manner upon the letters of a book made them ap- 
pear equally hazy ; so that the rings were probably as sharply 
formed as the letters. 
Having now already great reason to believe that no modifi- 
cation, that can be given by the first surface to the incident 
rays of light, is essential to the formation of the rings, I made 
the following decisive experiment. 
Upon a small piece of looking-glass I laid half a double 
convex lens of 16-inches focus, with the fracture exposed to the 
light, as represented in figure 11. Under the edge of the per- 
fect part of the lens was put a small lump of wax, soft enough 
to allow a gentle pressure to bring the point of contact 
towards the fractured edge, and to keep it there. In this ar- 
rangement it has already been shown that there are two dif- 
ferent ways of seeing two sets of rings : by the rays 1, 2, 3, 
we see a primary set; and by 1, 2, 4, 5, the secondary set 
belonging to it : by the rays 6, 7, 2, 3 we see a different pri- 
mary set ; and by 6, 7, 2, 4, 5, we see its secondary one. 
That this theory is well founded has already been proved; 
but if we should have a doubt remaining, the interposition of 
any small opaque object upon the looking-glass near the frac- 
ture will instantly stop the latter two sets of rings, and show 
