ihe Cause of coloured concentric Rings, 1 1 9 



reflection, the centre of the same set, with the same pres- 

 sure of the ji^lasses, seen by transmission, will be white *. 



I have only mentioned black and white; but any other 

 alternate colours, which the rings or centres of the two sets; 

 may assume, are included in the same predicament. 



XVI I. Why two <^n?iected Sets of Rings are of alternate 



Colours, 



It has already been shown, when two sets of rings arc 

 seen, that their colours are alternate, and that the approach 

 of the shadow of a penknife will cause a sudden change of 

 them to take place. I shall now prove that this is a very 

 obvious consequence of the course of rays that has been 

 proposed. Let figure 7 and 8 represent the arrangement 

 given in a preceding article, where a 1 6-inch lens was laid 

 upon a looking-glass, and gav€ two sets of rings with cen- 

 tres of different colours : but let figure 7 give them by one 

 set of rays, and figure 8 by another. Then, if the incident 

 rays come in the direction which is represented in figure 7, 

 it is evident that we see the primary set with \U centre at 2 

 by reflection, and the secondary one at 4 by transmission. 

 Henc€ it follows, in consequence of the admitted principle, 

 that if the contact is such as to give us the primary set with 

 a black centre, the secondary set must have a white one ; 

 and thus the reason of the alternation is explained. 



But if the rays come as represented in figure 8, we see 

 the primary set by transmission, and the secondary one by 

 reflection ; therefore, with an equal pressure of the glasses, 

 the primary centre must now be white, and the secondary 

 one black. 



Without being well acquainted with this double course of 

 rays, we shall be liable to frequent mistakes in our estima- 

 tion of the colour of the centres of two sets of rings ; for by 

 a certain position of the light, or of the eye, we may see one 

 set by one light and the other set by the other. 



XVIir. Of the Cause of the sudden Change of the Colours, 



Having thus accounted for the alternation of the central 



• See Article XI. of this Paper. 



H 4 colours. 



