134 HERSCHEL ON COLOURED RINGS. 



IX. Of measuring Rings. 



The rin^s dif- ^ ma Y ^ e supposed from what has been said concerning 

 fault to mea- the kind of contact, which is required for glasses to produce 

 lute! * . 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 Huygenian ob- 

 ject 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 ac- 

 tually 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. 

 But their pro- Relative measures, for ascertaining the proportion of the 

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

 it!?i« S m«T° ie accurately taken, for in that case the contact with them all 



CdSIlj IT J Gel* •* 



sured. will remain the same, if we do not disturb the glasses during 



the time of measuring. 



X. Of the Number of Rings, 

 Number of When there is a sufficient illumination, many concentric 



nn S s « rings in every set will be perceived ; in the primary set we 



see generally 8, 9, or 10, very conveniently. By holding 

 the eye in the most favourable situation I have often counted 

 near 20, and the number of 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 seen as many 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, and almost as bright as the iirst: I have 

 easily counted 7, 8, aud 9 rings. • 



XI. 



