562 On the InJUxion, Refiexian, and Coleiin if Light, 



the fupplement of GB A, the fum of the angles of reflexion and incidence ; wherefore HB : 

 HF : : the fine of the fum of the angles of reflexion and incidence, to the fine of tlie angle 

 of reflexion ; fo that if 1 be the angle of incidence, R that of reflexion, V the velocity of 



light, and F the reflefling force ; F = - ^ '"^ \^ By accommodating this formula 



to the dllTcrent cafes, we obtain F in all the rays ; and the ratio of F in one fct to 

 F in another being required, we have (by (Iriking out V, which is condant) F : F' : : 



£2j^-i2 . f'"- (R' +jO suppofe we would know F and F' in the red and violet re- 

 lin. R ■ fin. K' '^'^ 



.. _ / \i\i « / .1. 17 c/ .fin. ioi*o' fin. loj"?' 



fpeaively; I = 50<» 48'-R = So» 21', and R'=si° >S i thenF . F •■ ^^^-^. ■ ^,„^^„,^, 



Performing the divifion in each by logarithms, and finding the natural fines correfponding 

 to the quotients; F : F':: 1275 : 1253. But the force exerted on the red is to that ex- 

 erted on the violet as the fize of the red to the fize of the violet (by hypothefis) ; therefore 

 the red particles are to the violet as 1275 to 1253. This may be extended to all the other 

 colours by fimibr calculations ; their fizes lying between 1275 ^"^ 1253, which are the 

 extreme red and extreme violet ; thus the red will be from 1275 to I272i; the orange 

 from 1272;- to 1270; the yellow from 127010 1267; the green from 1267 to 1264 ; the 

 blue from 1164 to 1260; the indigo from 126010 1258; and the violet from 1258 to 

 i?53. 



All this fellows mtithematically, on the fuppofition that the parts of light are aftcd upon 

 in proportion to their fizcs : and to fay the truth, I fee no other proportion in which we 

 Ciin reafonably fuppofe them to be influenced ; for fuch an aftion is not only conformable 

 to the univerfal laws of attraflion and repulfion, but alfo to the following arguments. If 

 the action be not in the fimple ratio, it muft either be in a lower or in a higher. Let it be 

 in a lower, as that of the fquare root, then the fize of the red would be to the fize of the 

 violet as the fquares of the forces; that is, as 1625625 to 1572009: a diflisrence evidently 

 too great ; and ii fortiori of the cube or any other loot. On the other hand, if the aflion 

 were in a higher ratio, as that of the fquare, then the particles would be as the fquare root5 

 of the forces, or nearly as 35-70 to 35.39, » difference evidently too fmall; for if the fize 

 of the red particles were only -s^cths the greater than that of the violet, and the velocity of 

 both were equal, the momentum and confequently the intenfity of the red could iiot fo 

 much exceed that of the violet as we find it does; and as feems to me to be proved by the 

 experiment of Buffon (on accidental colours], who found, that after looking at a white obje£V, 

 when he (hut his eyes, it firft became violet, then blue or a mixture of blue and the other 

 colours, and lafl of all red: fo in the impreflion of the white, compounded of the impref- 

 fions of all the other rays mixed together, the violet was firft obliterated or weakeft, and the 

 red laft or (Irongeft. To this reafoning on the intenfity of the particles as owing to their 

 fize, I fee only two objeilions that can be made. The one is, that the intenfity is increafed 

 when the rays are thrown into a focus : but we muft recolleft, that the rays in this cafe arc 

 mixed, and their particles fo blended as to be increafed in fize ; for the number of feparate 

 riys thrown into one place will not increafe their intenfity fenfibly. The other obje£lioa 

 is that paflage in Ncsvton, where he fays, " that tlie orange and yellow are the moft lumi- 



4 nous 



