the Cause of coloured concentric Rings. 1 89 
will furnish us with an opportunity of examining rings in situa- 
tions where they have never been seen before, which will be 
of eminent service for investigating the cause of their origin, 
and with the assistance of the shadows to be formed, as has 
been explained, we shall not find it difficult to see them in 
these situations. 
First Method. Upon a well polished piece of good looking- 
glass lay down a double convex lens of about 20 inches focus. 
When the eye glass has been adjusted as usual for seeing one 
set of rings, make the shadow of the penknife in the order 
which has been described, pass over the lens ; then, as it some- 
times happens in this arrangement that no rings are easily to 
be seen, the shadow will, in its passage over the surface, show 
where they are situated. When a set of them is perceived, 
which is generally the primary one, bring the third shadow of 
the penknife over it, in which situation it will be seen to the 
greatest advantage. 
Then, if at the same time a secondary set of rings has not 
yet been discovered, it will certainly be perceived when the 
second shadow of the penknife is brought upon the primary 
set. As soon as it has been found out, the compound shadow, 
consisting of all the three shadows united, may then be thrown 
upon this secondary set, in order to view' it at leisure and in 
perfection. But this compound shadow should be taken no 
farther from the point than is necessary to cover it ; nor 
should the third shadow touch the primary set. The two sets 
are so near together, that many of the rings of one set inter- 
sect some of the other. 
When a sight of the secondary set has been once obtained, 
it will be very easy to view it alternately with the primary one 
mdcccvii. C c 
