196 Experiments for invesilgating 



upon -them, and so much the more, as it has been asccr-» 

 tained that the first surface assists only by permitting light 

 to pass into the body of the glass. Some of the experi- 

 ;^r)t?ents that have' been instituted for examining- the action of 

 .the first surface will equally serve for investigating that of 

 the second. 



Hk lens already used with a strong emery scratch being 

 ■again placed on the mirror, but with the injured side down- 

 wards, 1 found that the rings, when brought under the 

 scratch, were not distorted ; they had only a black mark cf 

 the same shape as the scratch across them. 



The 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 occa- 

 sioned great irregularity among the rings, which were in- 

 dented and broken wherever the little polished holes that 

 make up a scabrous surface came near them ; and if by 

 gently lifting the lens a strong contact was prevented, the 

 colours of the rings were likewise extremely disfigured and 

 changed. 



. As we have now seen that a polished defect upon the 

 second surface will affect the figure of the rings that are 

 under them, it will remain to be determined whether such 

 defects do really distort them by some modification they 

 give to the rays of light in their passage through them, or 

 whether they only represent 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 

 ihe appearance of the rings when they are close to them. 

 TRe following experiment will entirely clear up this point. 



Over the middle of a 22-inch double convex lens T drew 

 a strong line with a diamond, and gave it a polish after- 

 wards that it might occasion an irregular refraction. This 

 being prepared, T laid a slip<^f glass upon a plain metalline 

 mirror, and placed the lens with the polished line down- 

 wards upon the slip of gla^s. This arrangement lias been 

 fhown to give two sets of rings. When I examined the 

 .^J- ■ " . primary 



