186 HERSCIIEL ON COLOURED RINGS. 



were not distorter]; they had only a black mark of the same 

 shape as the scratch across them. 



Scabrous. 'J'he lens with a scabrous side was also placed again upon 



the mirror, but with the highly polished side upwards, in this 

 position the scabrousness of the lowest surface occasioned 

 great irregularity among the rings, which were indented and 

 broken wherever the little polished holes that make up a sca- 

 brous surface came near them; and if by gently lilting the 

 lens a strong contact was prevented, the colours of the rings 

 were likewise extremely disfigured and changed. 



is the distor- As we have now seen that a polished defect upon the second 



tion occasioned sur f acc will affect the figure of the rings that are under it, it 



by the rings be- . ° 



ing Jeen Wtjl remain to be determined, whether such defects do really 



throngh an ir- distort them bv some modification they give to the rays of 

 regular me- . .■* , " " * 



.dium? light in their passage through them, or whether they only re- 



present the rings as deformed, because we. see them through 

 a distorted medium, For although the scabrousness did not 

 sensibly affect the figure of the rings when it was on the first 

 surface, we may suppose the little polished holes to have a 

 much stronger effect in distorting the appearance of the rings 

 when they are close to them* The following experiment will 

 entirely clear up this point. 

 Effect of a v>o- Over the middle of a 22-inch double convex lens I drew a 

 nshrtltine. strong line with a diamond, and gave it a polish afterward) 

 that it might occasion an irregular refraction. This being 

 prepared, I laid a slip of glass upon a plain metalline mirror, 

 and planed th'e ltens with the polished line downwards upon 

 the slip of glass. This arrangement has been shown to give 

 The primary two sets of rings. When I examined the primary set, a 

 f 1 rtd^and stron » disfiguring of the rings was visible; they had the 

 likewise the appearance of having been forced asunder, or swelled out, so 

 secondary. as to ^ e m uch broader one way than another. The rings of 

 the secondary set had exactly the same defects, which, being 

 strongly marked, could not be mistaken. The centres of the 

 two sets, as usual, were «jf opposite colours, the first being 

 Mack, the second white; and all those defects, that were of 

 one colour in the first set, were of the opposite colour in the 

 second. When, by the usual method, I changed the colours 

 of the centres of the rings, making that of the primary white 



and 



