i $6 Dr. Herschel’s Experiments for investigating 
radius of the curvature of the surfaces between which they are 
formed. Curvatures of a short radius, cceteris paribus, give 
smaller rings than those of a longer ; but Sir I. Newton 
having already treated on this part of the subject at large, it 
will not be necessary to enter farther into it. 
I should however remark, that when two curves are con- 
cerned, it is the application of them to each other that will de- 
termine the size of the rings,sothat large ones may be produced 
from curvatures of a very short radius. A double convex lens 
of 2^-inches focus, for instance, when it is laid upon a double 
concave which is but little more in focal length, gives rings 
that are larger than those from a lens of 26 inches laid upon 
a plain slip of glass. 
VIII. Of Contact. 
The size of the rings is considerably affected by pressure. 
They grow larger when the two surfaces that form them are 
pressed closer together, and diminish when the pressure is 
gradually removed. The smallest ring of a set may be in- 
creased by this means to double and treble its former diameter ; 
but as the common or natural pressure of glasses laid upon any 
flat or curved surface is occasioned by their weight, the varia- 
tions of pressure will not be very considerable when they are 
left to assume their own distance or contact. To produce that 
situation, however, which is generally called contact, it will 
always be necessary to give a little motion backwards and 
forwards to the incumbent lens or glass, accompanied with 
some moderate pressure, after which it may be left to settle 
properly by its own weight. 
