20 5 
the Cause of coloured concentric Rings. 
to 13 ; and lastly, from 13 they are reflected to 14 ; so, that 
again four sets of rings will be seen at 8, 10, 12, and 14. 
This being a theoretical way of conceiving how the rays of 
light may produce the effects, it will be required to show by ex- 
periments that this is the actual progress of the rays, and that 
all the sets of rings we perceive are really reflected or trans- 
mitted in the manner that has been pointed out ; but as we 
have so many reflections and transmissions before us, it will 
be necessary to confine these expressions to one particular 
signification when they are applied to a set of rings. 
When the center of the rings is seen at the point of contact, 
it is a primary set ; and I call it reflected, when the rays which 
come to that point and form the rings undergo an immediate 
reflection. But I call it transmitted, when the rays after hav- 
ing formed the rings about the point of contact are immediately 
transmitted. 
Thus in figure 3 and 4 the rays a b c, d ef, give reflected 
sets of rings ; and the rays g h i, k l m, in figure 5 and 6, give 
transmitted sets. 
In this denomination, no account is taken of the course of 
the rays before they come to a, d, g, k; nor of what becomes 
of them after their arrival at c,f , i, m : they may either come 
to those places or go from them by one or more transmissions 
or reflections, as the case may require ; but our denomination 
will relate only to their course immediately after the formation 
of the rings between the glasses. 
The secondary and other dependent sets will also be called 
reflected or transmitted by the same definition : and as a set 
of these rings formed originally by reflection may come to 
the eye by one or more subsequent transmissions ; or being 
mdcccvii. E e 
