VOL. LXXXVI.] PHILOSOPHICAL TKANSACTIOfM, 72g 



angle with each other ; moving them nearer and nearer, till I saw the fringes 

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

 successively. I then withdrew one, and the fringes became faint and narrow, 

 and not all within the shadow of the remaining knife, but at its edge, and even 

 in the light of the spectrum. Lastly, when I slowly approached the other, they 

 moved into the shadow, and became broader, and farther separated one from 

 another, there being the like fringes in both shadows ; this I repeated in all the 

 rays, and plainly saw that at the approach of the knife, the fringes became 

 broader, and farther removed from each other, and from the light, in the red 

 than in the violet, or any of the other rays. 



Obs. 4. In repeating the foregoing experiment, I observed a very curious phe- 

 nomenon. When the angle of the knife-blades was so held in any of the rays 

 as to make the hyperbolic fringes described by Newton, (Optics, book 3, obs. 

 8), and these being always of the colour in which they were held, moving the 

 ano^le a little, so as to make the fringes out of the light that went to the top of 

 any one division of the spectrum, and also out of that which went near the bot- 

 tom of the next, the fringes were made of 2 colours; one part was of the 

 highest colour, and the other of the lowest, but both were on the ground of 

 the highest. Thus, if held on the confine of the green and blue, the upper 

 half of each fringe was blue, the under green, but both parts in the blue divi- 

 sion of the spectrum ; and trying the same in all the rays, 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 Obs. 3, 

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

 shadow, and separated and dilated by the deflection of the other ; and this most 

 in the red and least in the violet : likewise in Obs. 4, the fringes of one colour 

 were deflected into the region of the next, and this most in the red, and least in 

 the violet ; though in both observations the violet, from the position of the 

 chart, was farthest from the angle, and consequently, had the rays been equally 

 deflected, the violet should have been farthest moved, and most dilated by the 

 deflection ; but seeing that at equal angles of incidence in the 3d, and at less in 

 the 4th observation, the red was most and the violet less deflected, it is evident 

 that the most inflexible rays are also most deflexible. 



Having thus found that the parts of light differ in flexibility, I wished next to 

 learn 2 things : in what proportion the angle of inflection is to that of deflection 

 at equal incidences ; and 2dly, what proportion the different flexibilities of the 

 different rays bear to each other. But the nature of the coloured fringes must 

 first be understood, so that I defer this inquiry till after I have made use of the 

 principles now laid down, for the explanation of natural phenomena, and proceed 

 in the mean time to 



VOL. XVII. 5 A 



