HERSCHF.r. ON COLOURED RINGS. 183 



received thai full illumination, the mark was weaker, because 

 in the latter case the rings themselves were probably com- 

 plete, but in the former deficient. 



I placed a lens that had a very scabrous polish on one side, scabrousness, 

 but was highly polished on the other, upon a metalline mir- 

 ror. The defective side being uppermost, I did not find that 

 its scabrousness had a-uy distorting effect upon the rings. 



I splintered off the edge of a plain slip of glass; it broke as or irregularity, 

 it usualty does with a waving, striated, curved slope coming 

 to an edge. The splintered part was placed upon a convex 

 metalline mirror of 2 inches focus, as in PI. V, fig, 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 at rise appearing only like fine dark lines, with 

 hardly any visible distortion; but when, by tilting, the return- 

 ing ray, 2, 3, was also brought over the striated surface, the 

 rings were much disfigured. This experiment therefore seems 

 to prove, that a very regular refraction of light by the first 

 surface is not necessary; for though the rings were much dis- 

 figured, when the returning light came through the splintered 

 defect, this is no more than what must happen to the appear- 

 ance of every object, which is seen through a distorting 

 medium. 



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

 •focus with a diameter of 1*5 upon a plain mirror, and when I denc^ ha^no 

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

 contact to the very edge of the lens, both towards the light 

 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 difference in their size 

 or appearance could be perceived. This seems to prove, 

 that no modification of the first surface in which the angle of 

 incidence is concerned, such as refraction and dispersion, has 

 any share in the production of the rings, and that it acts 

 merely by the intromission of light ; and though even this is 

 not without being influenced by a change of the angle, it 

 can only produce a small difference in the brightness of the 

 rings. 



A more forcible argument, that leads to the same conclu- Further nro#f. 



sion. 



