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the Cause of coloured concentric Rings. 
an inch thick, between the slip of glass and the mirror, so as 
to keep up that end of the slip which is towards the light. This 
arrangement is represented in figure 9, where both sets of 
rays are delineated. Then if we interpose a narrow tapering 
strip of card, discoloured with japan ink, between the slip of 
glass and the mirror, so as to cover it at 7, we do not only still 
perceive the primary set, but see it better than before : which 
proves that being situated above the slip of glass the card below 
cannot cover it. If on the contrary we insert the strip of card 
far enough, that it may at the same time cover the mirror both 
at 4 and at 7, we shall lose the secondary set, which proves that 
its situation was on the face of the mirror. 
When several sets of rings are to be perceived by the samo 
eye-glass, and they are placed at different distances, a particular 
adjustment of it wall be required for each set, in order to see it 
well defined. This will be very sensible when we attempt to see 
three or four sets, each of them situated lower than the pre- 
ceding ; for without a previous adjustment to the distance of 
the set intended to be viewed we shall be seldom successful; 
and this is therefore a corroborating proof of the situation that 
has been assigned to different sets of rings. 
XX. Of the Connection between different Sets of Rings , 
It will now be easy to explain in what manner different sets 
of rings are connected, and why they have been called primary 
and dependent. When the incident rays come to the point of 
contact and form a set of rings, I call it the primary one : 
when this is formed some of the same rays are continued by 
transmission or reflection, but modified so as to convey an 
image of the primary set with opposite colours forward 
