196 Experiments for invesiigating 



having a strong emery scratch on its upper surface. Wheii 

 I saw the rings through the scratch, they appeared to have; 

 a black mark across them. By tilting the lens, I brought 

 the centre of the rings upon the projection of the scratch, 

 so that the incident hght was obliged to come through the 

 scratch to the ritigs, and the black mark was again visible 

 upon them, but much stronger than before. In neither of 

 the situations were the rings disfigured. The stronger mark 

 was owing to the interception of the incident light ; but 

 when the rings had received their full illumination the mark 

 was weaker, because in the latter case the rings themselves 

 were probably complete, but in the former deficient. 



I placed a lens that had a very scabrous polish on one 

 side, but was highly polished on the other, upon a metalline 

 mirror. The defective side being uppermost, I did not find 

 that its scabrousness had any distorting effect upon the rings. 



I splintered off the edge of a plain slip of glass 5 it broke 

 as it usually does with a waving striated, curved slope com- 

 ing to an edge. The splintered part was placed upon H, 

 convex metalline mirror of 2-inch focus, as in figure 10. 

 The irregularity of the striated surface through which the 

 incident ray 1,2, was made to pass had very little effect 

 upon the form of the rings ; the striae appearing only like 

 fine dark lines, with hardly any visible distortion ; but when, 

 by tilting, the returning ray, 2, 3, was also brought over the 

 striated surface, the rings were much disfigured. This ex- 

 "pcrimcnt therefore seems to prove that a very regular refrac- 

 tion of light by the first surface is not necessary ; for though 

 the rings were much disfigured when the returning li^ht 

 came through the splintered defect, this is no more than 

 what must happen to the appearance of every object which 

 is seen through a distorting medium. 



I laid the convex side of a plano-convex lens 2*8-incIi 

 focus with a diameter of 1*5 upon a plain niirror, and when 

 I saw a set of rings I tilted the lens so as to bring the point 

 of contact to the very ti\gQ of the lens, both towards the 

 Fight and from the 'light, which, on account of the large 

 diameter of the lens, gave a great variety in the angle of 

 incidence to the rays which formed the rings ; but no diffe- 

 rence 



