HKRSCUKL ON COLOURED RINGS, 



187 



and of the secondary black, the defects in both sets Avere 

 still exactly alike, and as before; except that they had also 

 undergone the like transformation of colour, each having as- 

 sumed its opposite. It remains now only to show, that this 

 experiment is decisive; for by the established course of the 

 rays we saw the secondary set of rings when it had a white 

 centre by the transmitted rays marked 1, i>, 4, 5, in figure 

 13 ; and when it had a black one, by the reflected rays 6, 7, 

 2, 4, 5, of the same figure; but in neither of these two 

 cases did the rays come through the defective part of the 

 lens in their return to the eye. 



This experiment proves more than we might at first be This proves it 

 aware of; for it does not only establish, that the second t0 ^ concern- 

 surface, when properly combined with a third surface, has a nation of the 

 modifying power, whereby it can interrupt the regularity of rul S*- 

 the rings, but also one whereby it contributes to their for- 

 mation ; for, if it can give an irregular figure to them by 

 transmitting its irregularly modified rays, it follows, that 

 when these rays are regularly modified it will be the cause of 

 the regular figure of the rings. Nay, it proves more ; for if 

 it modifies the figure of the rings by transmission, it modi- 

 fies them no less by reflection ; which may be seen by follow- 

 ing the course of the rays 6, 7> ~> 4, o; for as they do not 

 pass through the defective place of the lens, they can only 

 receive their modification from it by reflection. This opens Hence we may 

 a field of view to us, that leads to the cause of all these in- beled *?* 



, . cause of the 



tricate phenomena, of which in a second part of this paper phenomena. 



I shall avail myself. 



XXVI. Of the Action of the third Surface. 



When a double convex lens is laid upon a plain metalline Action of the 

 mirror, that happens to have an emery scratch in its surface, 3tl surface « 

 we see it as a black line under the rings that are formed over 

 them. This shows, that, when a defect from want of polish 

 has not a power to reflect light in an irregular manner, it 

 cannot distort the rings that are formed upon it. 



When I laid a good 21-feet object glass upon a plain slip Defects in this 



that had some defects in its surface, the rings* 'in every part ca P ab,e ? f di *- 



* t tortin ,r the 

 of the object glass that was brought over them, were always rings," 



disfigured ; which proves, that a reflection from a defective 



third 



\ 



