the Cause of coloured concentric Rings. 185 
the curvature of its upper surface, as well as of the mirror, 
may be such as to give them any focal length, provided the 
radius of their convexities is less than that of the concavity of 
an incumbent lens ; in all which cases complete sets of con- 
centric rings will be obtained. 
Fifth Method. Into the concavity of a double concave glass 
of 8 inches focal length I placed a 7-inch double convex lens, 
and saw a very beautiful set of rings. 
Sixth Method. Upon a 7 feet concave metalline mirror I 
placed the double convex 26-inch lens, and had a very fine 
set of rings. 
Generalization. With these two last methods, whatever 
may be the radius of the concavity of the subjacent surface, 
provided it be greater than that of the convexity of the incum- 
bent glass ; and whatever may be the figure of the upper 
surface of the lenses that are placed upon the former, there 
will be produced concentric rings. The figure of the lowest 
surface of the subjacent glass may also be varied at pleasure, 
and still concentric rings will be obtained. 
II. Of seeing Rings by Transmission. 
The great variety of the different combinations of these 
differently figured glasses and mirrors will still admit of fur- 
ther addition, by using a different way of viewing the rings. 
Hitherto, the arrangement of the apparatus has been such as 
to make them visible only by reflection, which is evident, 
because all the experiments that have been pointed out may 
be made by the light of a candle placed so that the angle of 
incidence and of reflection towards the eye of the observer, 
may be equal. But Sir I. Newton has given us also an 
