ON THE NATURE OF LIGHT AND COLOURS. A6S 



longer space than that which conies from the other, by an interval which is 

 equal to the breadth of one, two, three, or more of the supposed undulations, 

 while the intervening dark spaces correspond to a difference of half a sup- 

 posed undulation, of one and a half, of two and a half, or more. 



From a comparison of various experiments, it appears that the breadth of 

 the undulations constituting the extreme red light must be supposed to be, 

 in air, about one 36 thousandth of an inch, and those of the extreme violet 

 about one 60 thousandth; the mean of the whole spectrum, with respect to 

 the intensity of light, being about one 45 thousandth. From these dimen- 

 sions it follows, calculating upon the known velocity of light, that almost 500 

 millions of millions of the slowest of such undulations must enter the eye in 

 a single second. The combination of two portions of white or mixed light, 

 when viewed at a great distance, exhibits a few white and black stripes, cor- 

 responding to this interval ; although, upon closer inspection, the distinct 

 effects of an infinite number of stripes of different breadths appear to be com- 

 pounded together, so as to produce a beautiful diversity of tints, passing by 

 degrees into each other. The central whiteness is first changed to a yellow- 

 ish, and then to a tawny colour, succeeded by crimson, and by violet and 

 blue, which together appear, when seen at a distance, as a dark stripe; after 

 this a green light appears, and the dark space beyond it has a crimson hue; 

 the subsequent lights are all more or less green, the dark spaces purple and 

 reddish; and the red light appears so far to predominate in all these effects, 

 that the red or purple stripes occupy nearly the same place in the mixed 

 fringes as if their light were received separately. 



The comparison of the results of this theory Avith experiments fully esta- 

 blishes their general coincidence ; it indicates, however, a slight correction 

 in some of the measures, on account of some unknown cause, perhaps con- 

 nected with the intimate nature of diffraction, which uniformly occasions the 

 portions of light, proceeding in a direction very nearly rectilinear, to be divided 

 into stripes or fringes a little wider than the external stripes, formed by the 

 light which is more bent (Plate XXX. Fig. 442, 443.) 



When the parallel slits are enlarged, and leave only the intervening sub- 

 stance to cast its shadow, the divergence from its opposite margins still con- 

 VOL. I. 3 o 



