the Cause of coloured concentric Rings. 
'97 
IX. Of measuring Rings. 
It may be supposed from what has been said concerning the 
kind of contact, which is required for glasses to produce rings, 
that an attempt to take absolute measures must be liable to 
great inaccuracy. This was fully proved to me when I wanted 
to ascertain, in the year 1792 , whether a lens laid upon a 
metalline surface would give rings of an equal diameter with 
those it gave when placed on glass. The measures differed so 
much that I was at first deceived ; but on proper consideration 
it appeared that the Hu ygenian object glass, of 122 feet focus, 
which I used for the experiment, could not so easily be brought 
to the same contact on metal as on glass ; nor can we ever 
be well assured that an equal distance between the two surfaces 
in both cases has been actually obtained. The colour of the 
central point, as will be shown hereafter, may serve as a di- 
rection; but even that cannot be easily made equal in both 
cases. By taking a sufficient number of measures of any given 
ring of a set, when a glass of a sufficient focal length is used, 
we may however determine its diameter to about the 25th or 
30th part of its dimension. 
Relative measures, for ascertaining the proportion of the 
different rings in the same set to each other, may be more ac- 
curately taken, for in that case the contact with them all will 
remain the same, if we do not disturb the glasses during the 
time of measuring. 
X. Of the Number of Rings. 
When there is a sufficient illumination, many concentric 
rings in every set will be perceived ; in the primary set we see 
MDCCCVII. D d 
