ON LIGHT AND COLOUES. 127 



candle through the interval of the opposite edges, you per- 

 ceive that the fringes are images of its flame, with the wick, 

 and that they move as the flame moves to and fro. \Yhen you 

 observe the half-moon in like. manner, you perceive that the 

 side of the fringes answering to the rectilinear side of the 

 moon, are rectilinear, and the other side circular ; and when 

 the full moon is thus viewed, the fringes on both sides are 

 circular. The circular disc of the moon is, indeed, drawn or 

 elongated as well as coloured. It is, that is to say, the fringe 

 or image which is exactly a spectrum by flexion. Like the 

 prismatic spectrum, it is oblong, not circular, and it is 

 coloured ; only that its colours are much less vivid than those 

 of the prismatic spectrum. 



PROPOSITION II. 



The rays of light, when inflected by bodies near which 

 they pass, are thrown into a condition or state which disposes 

 them to be on one of their sides more easily deflected than 

 they were before the first flexion ; and disposes them on the 

 other side to be less easily deflected : and when deflected by 

 bodies, they are thrown into a condition or state which 

 disposes them on one side to be more easily inflected, and on 

 the other side to be less easily inflected than they were before 

 the first flexion. 



Let E A (fig. 4) be a ray of light whose opposite sides are 

 R A, E' A', and let A be a bend- 

 ing edge near which the ray 

 passes, the side E' A' acquires 

 by A's inflexion, a disposition 

 to be more easily deflected by 



another body placed between A and the chart C, and the side 

 EA acquires a disposition to be less easily deflected than 

 before its first flexion ; and in like manner E' A' acquires 

 a disposition to be more easily inflected, and E A a dispo- 

 sition to be less easily inflected by a body placed between A 

 andC. 



Exp. 1. Place A' (fig. 5) in any position between A and v ', 



