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the Cause of coloured concentric Rings. 
by a measure of the whole space thus taken up, I found that 
the breadth of a ring of a mean size was about the 308th part 
of an inch. 
Now, according to Sir I. Newton’s calculation of the action 
of the fits of easy reflection and easy transmission in thick 
glass plates, an alternation from a reflecting to a transmitting 
fit requires a difference of part of an inch in thickness ;* 
and by calculation this difference took place in the glass plate 
that was used at every 80th part of an inch of its whole 
length; the 12 rings, as well as the central colour of the 
secondary set, should consequently have been broken by the 
exertion of the fits at every 80th part of an inch ; and from 
the space over which these rings extended, which was about 
,13 inch, we find that there must have been more than ten such 
interruptions or breaks in a set of which the 308th part was 
plainly to be distinguished. But when I drew the glass plate 
gently over the small mirror, keeping the secondary set of 
rings in view, I found their shape and colour always completely 
well formed. 
This experiment was also repeated with a small plain glass 
instead of the metalline mirror put under the large plate. In 
this manner it still gave the same result, with no other differ- 
ence but that only six rings could be distinctly seen in the 
secondary set, on account of the inferior reflection of the sub- 
jacent glass. 
* Newton’s Optics, p. 277. 
MDCCCVIL 
tth 
