ON THE NATURE OF LIGHT AND COLOURS. 365 



light is diffracted in every direction. In this case, when the two newly 

 formed beams are received on a surface placed so as to intercept them, their 

 light is divided by dark stripes into portions nearly equal, but becoming- 

 wider as the surface is more remote from the apertures, so as to subtend 

 very nearly equal angles from the apertures at all distances, and wider also 

 in the same proportion as the apertures are closer to each other. The 

 middle of the two portions is always light, and the bright stripes on each 

 side are at such distances, that the light coming to them from one of the 

 apertures, must have passed through a longer space than that which comes 

 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 supposed undulation, of one and 

 a half, of two ctnd 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 dimensions 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, corresponding to this interval : although, upon closer inspec- 

 tion, the distinct effects of an infinite number of stripes of different 

 breadths appear to be compounded together, so as to produce a beautiful 

 diversity of tints, passing by degrees into each other. The central white- 

 ness is first changed to a yellowish, 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 with 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 

 substance to cast its shadow, the divergence from its opposite margins still 

 continues to produce the same fringes as before, but they are not easily 

 visible, except within the extent of its shadow, being overpowered in other 

 parts by a stronger light ; but if the light thus diffracted be allowed to fall 

 on the eye, either within the shadow or in ite neighbourhood, the stripes 



