22 5 
the Cause of coloured concentric Rmgs. 
pass through the point of contact, or through one of the 
surfaces about the same point to the other to be reflected 
from it. 
IV. And that in all these cases a set of rings will be formed, 
having their common center in the place where the two 
surfaces touch each other. 
XXIX. Considerations that relate to the Cause of the formation of 
concentric Rings. 
It is perfectly evident that the phagnomena of concentric 
rings must have an adequate cause, either in the very nature or 
motion of the rays of light, or in the modifications that are 
given to them by the two essential surfaces that act upon them 
at the time of the formation of the rings. 
This seems to reduce the cause we are looking for to an 
alternative that may be determined ; for if it can be shown 
that a disposition of the rays of light to be alternately re- 
flected and transmitted cannot account for the phasnomena 
which this hypothesis is to explain, a proposition of accounting 
for them by modifications that may be proved, even on the 
very principles of Sir I. Newton to have an existence, will 
find a ready admittance. I propose, therefore, now to give 
some arguments, which will remove an obstacle to the 
investigation of the real cause of the formation of the con- 
centric rings ; for after the very plausible supposition of the 
alternate fits, which agrees so wonderfully well with a number 
of facts that have been related, it will hardly be attempted’ 
if these should be set aside, to ascribe some other inherent pro- 
perty to the rays of light, whereby we might account for 
them ; and thus we shall be at liberty to turn our thoughts 
