]835.] Composition of White Light. 361 



withdraw the prism to a greater distance from them, the 

 violet fringe will intersect the red fringe, producing by the 

 mixture the same crimson colour ; and after passing through 

 the red it will reach the yellow, its place in the red being 

 supplied by the blue, and the whole of the colours will gra- 

 dually disappear as before. If we recollect that the violet 

 light is turned out of its course more than the green, and 

 the green more than the red, we shall perceive that the 

 most refrangible colours must necessarily overtake, and 

 then intersect, the less refrangible ; and thus, not only fill 

 up the interval produced by the dark ground, but, also, 

 when the distance of the prism is sufficiently increased, 

 cause the disappearance of the complementary colours upon 

 its boundaries by their intersection. 



It is thus that the crimson fringes are formed upon the 

 bars of a window when received through a prism ; and it 

 is upon this principle that these bars, and the fringes upon 

 them, disappear when the prism is withdrawn to a sufficient 

 distance.* The only difference in the two experiments is, 

 that when we look at the cards we see two distinct surfaces 

 of reflected light ; and when we look at the window we see 

 distinct surfaces of direct light : it is the intersection of the 

 spectra, formed by these distinct surfaces, which produce 

 the phenomena we have been investigating. 



It is not necessary, in order to produce colours in white 

 refracted light, that the light of any part of the surface 

 should be entirely removed, the slightest inequality being 

 sufficient for this purpose ; for, to preserve an uniformly 

 white surface in refracted light, it is required that the rays 

 of any colour removed from one part, should be exactly re- 

 placed by rays of the same colour, and of the same intensity, 

 from another part ; which, in like manner, must be com- 

 pensated from another throughout the whole, in succession ; 

 but if the compensating rays, though of the same colour, 

 be of greater or less intensity, the colour in excess, in one 

 case, and the complementary colour in the other, will ap- 

 pear with an intensity proportioned to the difference. Per- 

 haps the description will be rendered more intelligible by 



* By viewing a window (an open casement) at a distance of twenty or thirty 

 yards, the horizontal divisions are rendered invisible through an equi-angulai 

 prism. 



