86 Experiments for investigating 



IX. Of meastiring Rings. 



It may be supposed from vvhai 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 1 792, 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 Huygenian ob-» 

 ject glass, of 122 feet focus, which I used for the experi" 

 ment, 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 direction ; 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. nnjni^' 



Relative measures for ascertaining the proportion of the 

 different rings in the same set to each other, may be more 

 accurately 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 Numler of Rings, 



When there is a sufficient illumination, many concentric 

 rings in every set will be perceived ; in the primary set we 

 see generally 8. 9, or JO, very conveniently. By holding.the 

 eye in the most favourable situation I have often counted 

 near 2Q, and the pumber pf them is generally lost when they 

 grow too narrow and minute to be perceived, so that we can 

 never be said fairly to have counted them to their full extent. 

 In the second set I have seer^,,as,jmany as in the first, and 

 they are full as bright. The third set, when it is seen by a 

 metalline mirror under two slips, will be brighter than the 



second. 



