HERSCIIEL ON COLOURED RINGS. 141 



or reflections are .to be regarded only as convenient ways to 

 get a good sight of them. 



With this definition in view, and with the .assistance of a 

 principle which has already been proved by experiments, we 

 may explain some very intricate phenomena; and the satis- 

 factory manner of accounting for them will establish the 

 truth of the theory relating to the course of rays that has 

 been described. 



The principal to which I refer is, that, when the pressure 

 is such as to give a black centre to a set of rings seen by re- 

 flection, the centre of the same set, with the same pressure 

 of the glasses, seen by transmission will be white*. 



I have only mentioned black and, white, but any other al- 

 ternate colours, which the ring3 or centres of the two set* 

 may assume, are included in the same predicament. 



XVII. Why two connected Sets of Rings are of alternate 

 Colours., 



It has already been shown, when two sets of rings are Whytwocou- 



seen, that their colours are alternate, and that the approach nected sets ar<* 

 3 ' ' r of alternate co» 



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



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

 vious consequence of the course of rays that has been propo- 

 sed. 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 gave two sets of rings with centres of dif- 

 ferent 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 its centre at 2 by reflection, 

 and the secondary one at 4 by transmission. Hence it fol- 

 lows, in consequence of the admitted principle, that if the 

 | contact is such as to give us the primary set with a black cen- 

 tre, 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 reflec- 

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



* See Article XI, of this Paper, p. 1S5. 



primary 



