2 26 Dr. IIerschel's Experiments for investigating 
to a cause that may be found in the modifications arising 
from the action of the surfaces which have been proved to be 
the only essential ones in the formation of rings. 
XXX. Co?ice?itric Rings cannot be formed by an alternate 
Reflection and Transmission of the Rays of Light. 
One of the most simple methods of obtaining a set of con- 
centric rings is to lay a convex lens on a plain metalline 
mirror ; but in this case we can have no transmission of rays, 
and therefore we cannot have an alternate reflection and 
transmission of them. If to get over this objection it should 
be said that, instead of transmission, we ought to substitute 
absorption ; since those rays which in glass would have been 
transmitted will be absorbed by the metal, we may admit the 
elusion ; it ought however to have been made a part of the 
hypothesis. 
XXXI. Alternate Fits of easy Refection and easy Transmis- 
sion, if they exist, do not exert themselves according to va- 
rious Thicknesses of thin Plates of Air . 
In the following experiment, I placed a plain well polished 
piece of glass 5,6 inches long, and 2,3 thick, upon a plain me- 
talline mirror of the same length with the glass ; and in order 
to keep the mirror and glass at a distance from each other, I 
laid between them, at one end, a narrow strip of such paper as 
we commonly put between prints. The thickness of that 
which I used was the 640th part of an inch ; for 1 28 folds of 
them laid together would hardly make up two-tenths. Upon 
the glass I put a 39-inch double convex lens ; and having 
