5<;2 Omt/u L'J!t.\ion, Rijlixioti, ami Cdjiirs of L'lglt. 



9', Tcfpe£livc1y. Now the natural fines of 4', 30"; s\ 7 'anil 9', areas the numbers 1309, 

 1454, 2035 1, and 2617, which aie as the fines of inculeiicc, deflexion, and inflexion, of the 

 viok-t,- green, and red. Thus the angles of flexion of the extreme and mean rays being 

 given, thofc of the other rays are found by dividing tlie ditVerence between 1454 and 2617 

 in the harmonical ratio ; for then the red will be equal to 145 ', the orange 87^^, the yel- 

 low 155-t\) 'l^c green 193,, the blue 193J, the indigo 129J, and the violet 258^; and by 

 adding to the number 1454 the violet, and to their funi the indigo, and fo on, we get the 

 fltxibility of the red from 2617 to 24714. of the orange from 2471^ to 2384I, ofthe 

 vcllow from 238.4!- to 2229,-, of the green from 2229}- to 2035^, of the blue from 2035^ 

 to 1841', of the indigo from 1841I to 1712^, and of the violet from 1712.J to 1454, the 

 common fine of incidence being 13C9. It is therefore evident that the flexibility of the 

 ■red is not to that of the violet as the refrangibility of the violet to that-of the red ; and a 

 little attention will convince us that we had no reafon to expe£l the analogy fhould be 

 kept up in this refpe£l; for the refrangibility of the rays depends on the fpecies of the re- 

 frafting medium, and follows no general rule ; whereas our calculation has been made 

 concerning the aclion of the bending power at a cert.iin diftance, greater than that whereat 

 the particles of media acl on the rays in refracling them. It was obferved in the mathe- 

 matical propofitions prefixed to this paper, that the angle of flexion is lefs than tliat of 

 incidence, when in the cafe of inflexion the angle made by the ray and the body is acute, 

 and when in the cafe of deflexion that angle is obtufe ; and when the ray is perpendicular, 

 ,or parajlel, the angle of incidence vanifhes in both cafes. It is evident therefore, that in 

 both thefe fiiuations of things the ratio of 1309 to 2036 being that of a kfs to a greater, 

 will not enable us to find the angle of flexion, although it ferves very well when the ray 

 before inflexion makes an obtufe, and before the deflexion an acute angle. I have there- 

 fore mentioned the angle made by the bent ray with the incident, which gives a general 

 formula ; for let the angle of incidence be I, and that which the bent ray makes with the 

 incident B, then F being the angle of flexion, we have F = B ± I ; fo that if I = O, F = B, 

 or if the incident makes an obtufe angle with the body in the cafe of deflexion, and aa 

 acute in that of inflexion, then F = I — B, and in the remaining cafe F = I + B. 



Thefe obfervations enable us to give a very fhort fummary of optical fcience. When 

 particles of light pafs at a certain dillance from any body, a repulfive power drives tliem 

 ofl^; at a diftance a little lefs this power becomes attraftive : at a flill lefs diftance it 

 again becomes repulfive •, and at the lead diftance it becomes attraftive, as before, always 

 adling in the fame direftion. Thefe things hold, whatever be the diredion of the particles; 

 but if, when produced, it pafles through the body, then the neareft repulfive force drive* 

 the particles back, and the neareft attradlive force either tranfmits them or turns them out 

 of tlieir courfc during tranfmiflion. Farther, the particles difl^er in their difpofitions to be 

 acted upon by this power in all thefe varieties of exertion ; and thofe which are moft ftrongly 

 aflFe£tcd by its exertion in one cafe, are alfo moft ftrongly afleded by that exertion when 

 varied, except in the cafes of refraftion, of which we before fpokc ; and thefe difpofitions of 

 the parts are in all the cafes in the fame harmonical ratio. Laftly, the caufe of theCe dif- 

 ferent difpofitions is the magnitude of the particles being various. 



All that remains now to be done on this part of the fubjed, is to explain one or two 

 phenomena relating to reflexibility. 



2 I. It 



