4s ESSAYS AND OBSERVATIONS 



38. On fuppoiition that the different re-- 

 frangibility of the rays of light arifes folely 

 from their different velocities before inci- 

 dence ; the(e velocities muft be, to one an- 

 other, nearly as their fines of refraiftion. 



39. Sir ]faac found their fines of refra- 

 <5lion from glafs into air, beginning from 

 the extreme violet, to be * as 78, 77I-, yy^, 



77t' 77 h 77h 77h 77--> "^^^ ^"e of in- 

 cidence being 50 : from whence their fines 

 of refradliorr otit of air into glafs, begin- 

 ning from the extreme red, and ending with 

 the extreme violet, are found to be as -f* 



78000, 77S73, 77797^ 77^^^^ 77^9^^ 

 77330, 77220, 77000 ; the fine of incidence 

 being 120 120. Thefe numbers, therefore, 

 nearly exprefs the velocities in air, of the 

 feveral rays, before their incidence J. 



40. Hence 



* Nenut. Opt. book i. part 2. prop. 5. 



-f- The extreme fines are plainly reciprocal to the former ; 

 and thofe of intermediate colours are fourth proportionals 

 to the fine in Sir lfaac'% experiment, 77 and 78. 



X The quantities which give the accurate proportion of 

 the velocities, before incidence, mufl be in a conflant ratio: 

 the fines of refraftion, by which the a' ove calculations are 

 made, have this condition • but, it is oiherways manifeft, 

 that they give only a grofs approximation to the truth. 



f roia 



