irheii reduced into thin Pellicles. 333 



the origin oi' these colours, to establish their comparison 

 with those owing to absorption, and finally I shall endeavour 

 to ascribe to their true cause some phaenomcna, which to 

 this hour have been differently explained. 



I shall point out, in the first place, the principal effects 

 upon which it is important for my purpose that the attention 

 of my readers should be fixed. 



When the light falls upon the very thin and slender bo- 

 dies which yield these colours, 



1st. At the spots where the colours arise upon the thin 

 body, each complex bundle of rays, or, if you please, the 

 wliite light, is divided into two portions, in a variable manner, 

 and one of them is reflected while the other cannot come 

 out of the substance except by transmission. 



2d. This division varies according to a law depending 

 upon the thickness of the body, its density, and the incli- 

 nation of the luminous rays. 



3d. Each ray acts in particular as if endowed with the sin- 

 gular property of having fits of easy reflection at periodical 

 intervals, and fits of easy transmission at other intervals, al- 

 ternate with the first : these various results are equally be- 

 yond all dispute. 



But whence comes this disposition of the rays ? — Newton 

 considered it as inherent in the rays themselves, not only in 

 that part of their transmission comprised between the two 

 extreme surfaces of the body which they traverse, but also 

 over the whole of the progress of these rays, from the mo- 

 ment of their first emission from a luminous body *. Here 

 there is a kind of hidden cause, of which it seems very dif- 

 ficult to form a precise idea : some distinguished philoso- 

 phers have therefore testified much repugnance in admitting 

 it. 



But Newton himself, at the end of his work, puts us in 

 the right path whevi he asks, if it is not in virtue of a simi- 

 lar principle that the rays are reflected or refracted by bodies, 

 and bent before they actually arrive at the bodies f- 



We have much cause to regret that this great man has not 



* Oj-,tic8, book ij. part 3. prop. 12. t l'"-"^- •^"''^ "'• ^""2'- ■^• 



treated 



