the Cause of coloured concentric Rifigs . 223 
the introduction of a fourth ; this, at all events, must prove 
that such a surface is not essential to the formation of rings, 
but as not only in direct transmission, but also when two sets 
of rings are to be seen, one of which may be formed by trans- 
mission this fourth surface must be introduced ; I have ascer- 
tained by the following experiments how far the same has any 
share in the formation of rings. 
In direct transmission, where the light comes from below, 
the fourth surface will take the part which is acted by the first, 
when rings are seen reflected from a metalline mirror. Its 
office therefore will be merely to afford an entrance to the 
rays of light into the substance of the subjacent glass ; but 
when that light is admitted through the first, second, and 
third surfaces, the fourth takes the office of a reflector, and 
sends it back towards the point of contact. It will not be 
required to examine this reflection, since the light thus turned 
back again is, with respect to the point of contact, in the same 
situation in which it was after its entrance through the first 
surface when it proceeded to the same point ; but when two 
sets of rings are to be formed by rays, either coming through 
this point directly towards the fourth surface, or by reflection 
from the same point towards the place where the secondary 
rings are to be seen, it will then be necessary to examine 
whether this surface has any share in their formation, or 
W'hether these rings, being already completely formed, are 
only reflected by it to the eye. With a view to this, I selected 
a certain polished defect in the surface of a piece of coach- 
glass, and when a 26-inch lens was laid upon it, the rings of 
the set it produced were much distorted. The lens w^as then 
put upon a perfect slip of glass, and both together were laid 
G g 2 
