the Cause of coloured concejiiric Rings. 197 



primary set, a strong disfiguring of the rings was visible ; 

 they had the appearan^cc of having been forced asnndcr, or 

 swelled out, so as to be much broader one way than an- 

 other. The rings of the secondary set had exactly the 

 ffame defects, which being strongly marked, could not he 

 mistaken. The centres of the two sets, as usual, were of 

 opposite colours, the first being black, the second whiti^; 

 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 vvhite and of the secoQ- 

 dary black, the defects in both sets were still exactly alike, 

 and as before ; except that they had also undergone the 4ike 

 transformation of colour, each having assumed its opposite^ 

 It remains now only to show that this experiment is deci;- 

 sive ; for by the established course of the rays we saw the 

 sepondary set of rings when it had a white centre by the 

 transnjitted r^ys marked 1, 2, 4, 5, in figure 13; and when 

 it had a black ope, by the reflected rays 6, 7, 2, 4, 5, of 

 the same figure ; but in neillher of these two pases did the 

 rays come through the defective part of the kna iq. their 

 return to the eye. 



This experiment proves more than we might at first be 

 aware of; for it does not only establish that the second 

 surface, when properly combined with a third surface, has 

 a modifying.power whereby it can interrupt the regularity 

 of the rings, but also one whereby it contributes to their 

 formation ; 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 foU 

 bwing the course of the rays 6, 7, 2? 4, 5 ; 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 a field of view to us that leads to the cause of all these 

 intricate phsenomena, of which in a second part of this 

 paper I shall avail myself. < -Sjtj^^^y. 



N3 XXVI. Of 



