the Cause of coloured concentric Rings. 199 
description, which certainly would fall short of an actual view 
of these phenomena, will not be necessary. 
When the rings are produced by curves of a very short 
radius, and the incumbent lens is in full contact with the slip of 
glass, they will be alternately black and white ; but by lessen- 
ing the contact, I have seen, even with a double convex lens 
of no more than two-tenths of an inch focus, the center of the 
rings white, red, green, yellow, and black, at pleasure. In this 
case I used an eye-glass of one inch focus ; but as it requires 
much practice to manage such small glasses, the experiment 
may be more conveniently made by placing a double convex 
lens of 2-inch focus on a plain slip of glass, and viewing the 
rings by an eye glass of inches ; then having first brought 
the lens into full contact, the rings will be only black and 
white, but by gently lifting up or tilting the lens, the center of 
the rings will assume various colours at pleasure. 
XII. Of diluting arid concentrating the Colours . 
Lifting up or tilting a lens being subject to great uncer- 
tainty, a surer way of acting upon the colours of the rings is 
by dilution and concentration. After having seen that very 
small lenses give only black and white when in full contact, 
we may gradually take others of a longer focus. With a 
double conv ex lens of four inches the outward rings will begin 
to assume a faint red colour. With 5, 6, and 7, this appear- 
ance will increase ; and proceeding with lenses of a larger 
focus, when we come to about 16, 18, or 20 inches, green rings 
will gradually make their appearance. 
This and other colours come on much sooner if the center 
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