554 O'' '^"^ /"^'•«'«"> Rtfitxkn, and Cilours of Light. 



trum on an horizontal table, and was there received on a chart fcven feet from tlie windo-inr., 

 1 then placed on the f;'.me table, and in the rays between the chart and tlic prifln, at three 

 inches from the chart, two lliarp knife-blades, with even edges, and fixed to a board with 

 wax, fo as to make an angle with one another ; moving them nearer aiid nearer, till I faw 

 ■ the fringes appear in the red light on the chart, and then in the orange aud other colours 

 fucceilively. I then withdrew one, and the fringes became faint andnarrow, and not all 

 within the (hadow of the remaining biife, but at its edge, and even in tlie light of the 

 fpcJtrum. LalUy, when I flowly approaclicd the other, tliey moved into the fl)4dow,'and 

 became broader and farther feparated one from another ; there being the like fringes in 

 both Ihadows. This I repeated in all ilie rays, and plainly f.iw, that at the approach of the 

 knife the fringes became broader and farther removed from one another, and from the 

 light, in the red llian in the violet, or any of the other rays. 



Ohj. IV. In repeating the foregoing Experiment, I obferved a very curious phenome- 

 non. When the angle of the knife-blades was fo held in any of the rays, as to miike the 

 hyperbolic fringes defcribcd by Newton*, and thefe being always of the colour in whicli 

 they were h.eld ; moving the angle a little, fo as to make the fringes out of the lightlhat 

 went to the top of any one divifion of the fpedlrum, and alfo out of that which went near 

 the bottom of the next, the fringes were matfcof two colours ; one part was of the higheft 

 colour, and the other of the loweft •, but both were on the ground of the highsft. Thus, if 

 held on the confine of tlie green and blue, the upper half of each fringe was blue, the under 

 green, but both parts in the blue divifion of the fpeclrum ; and trying the fame in all the 

 ravs it was evident that the red was moved farther into the orange, and the orange into the 

 yellow, than the blue was into the indigo, or the indigo into the violet. Now in Obf. III. 

 the fringes were formed by the inflexion of one knife, and were moved into its fhadow, and 

 feparated and dilated by the deflexion of the other ; and this mod in the red, and lead in 

 the violet. Likewife in Obf IV. the fringes of one colour were defledled into the region of 

 the next, and this moft in the red, and lead in the violet-, although in both obfervations 

 the violet, from the pofition of the chart, was farthefl from the angle, and conlequently, 

 had the rays been equally deflected, the violet Ihould have been farthcft moved, and moft 

 dilated bythe deflexion : but, feeing that, at equal angles of incidence in the third, and at 

 lefs in the fourth Obfervation, the red was moft, and the violet lefs, dtfledled, it is evident 

 that the moft inflexible rays are alfo moft deflexible. 



Having thus found that the parts of light difFcr Ifi flexibility, I wifhed next to learn two 

 things ; in what proportion the angle of inflexion is to that of deflexion at equal incidences ; 

 and fecondly, what proportion the different flexibilities of the diflferent rays bear to one 

 another. But the nature of the coloured fringes muft firft be undcrftood ; fo that I defer 

 this enquiry till after I have made ufe of the principles now laid 'down for the explanation 

 of natural phenomena, and proceed in the meali time to ' • ; ' 



Part U.—Of Rejlfxioii. 

 THAT bodies rcfleft light by a repulfivc power extending to fome dirtance from their 

 furfaccs, has never been denied fincc the time of Sir Ifaac Newton f. Now this power 



» Optics, Book iii. Obf. 8- ,+ Ibid; Bopkiii. f art III,, Prop. 8. 



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