Why the Phenomenon [Book II L 

 drawn from the fun to the drop, which will be parallel 

 to a line S M drawn from the eye of the fpedtator 

 to the fun. The courfe of part of the decompounded 

 ray S B may be firft by refra6r.ion from B to A, then 

 by reflection from A to W, laftly by refraction from 

 W to M. Now all drops, which are in fuch a fitua- 

 ticn that the incident and emergent rays S B, M W, 

 produced through them make the fame angle, S N M 

 will be the means of exciting in the fpectators the 

 fame idea of colour *. Let M W turn upon H O as 

 an axis till W meets the horizon on both fides and 

 the point W will defcribe the arc of a circle, and all 

 the drops placed in its circumference will have the 

 property we have mentioned, of tranfmitting to the eye 



a par- 



* Half the angle between the incident and emergent rays is 

 pqual to the difference between m times the angle of refradion 

 and the angle of incidence j m being equal to the number of 

 reflations added to unity. B C L=C B N-f C N B, and alio 

 B C L=C A B + C B A~z. C B A. 



.-. C B N-f C N E 2. C B O. 



.-. C N B= 2. C B A C B N- 



C N B is half the angle between the incident and emergent 

 rays, and 2 = //j, there being in this cafe only one reflection ; and 

 by purfuing the enquiry in the fame manner when the number 

 pf reflections is increafed, it will appear that C N B always equals 

 m. CBA CBN. 



This angle C N B, if the angle of incidence increafes from no- 

 thing, firft increafes and then decreafe?, therefore m C B A 

 G B N will in lome places be a maximum ; that is, where m times 

 the fluxion of C B A, is equal to the fluxion of C B N. 

 Let C B A=A and C B N==B. 



.-. A : B : : i : m 

 and radius being equal to unity 



S A S B S A SB 



cof. A ' co). 'B ' ' cof. A ' col. B 

 : tang. B : : . 

 Jang. A ; tang. B : : j 



J>ut tang. A : tang. B : : S ' A - S ' B 

 cof. A ' col. jj. 



Pence 



