Astronomical and Nautical Collections, 443 



sufficient distance from the middle, where the two routes are 

 equal, the dark and light stripes of the different species of 

 rays of light employed will efface each other mutually by 

 their mixture, and will exhibit a uniform tint. 



The more homogeneous the light has been rendered, the 

 more distant from the middle will this point of perfect com- 

 pensation be, and consequently the greater number of 

 fringes will be perceptible. When we employ white light, 

 which is the most compound, the number of fringes visible 

 is also the least possible, and we scarcely discover more than 

 seven on each side of the centre. They exhibit the tints of 

 the coloured rings, and the reason of their appearing co- 

 loured is exactly the same. If the length d were equal for 

 the rays of different colours, the breadth of their fringes, 

 that is to say, the interval between the middle of two con- 

 secutive bright or dark stripes, being also the same, there 

 would be a perfect coincidence in their darkest as well as 

 their brightest points ; and the different rays which compose 

 white light being found every where in similar proportions, 

 would produce a series of black and white stripes without 

 any distinction of colour. But this is by no means the 

 case : as the length d varies very considerably for rays of 

 different colours, that is, nearly in the proportion of two to 

 one, from one extremity of the solar spectrum to the other, 

 the breadth of the fringes varies in the same proportion, so 

 that their dark and bright stripes cannot fall on each other, 

 but differ so much the more in their situations as they are 

 further removed from the middle line. It must therefore 

 happen, that the bright stripe of the rays of a certain colour 

 corresponds with the dark stripe of the rays of another kind, 

 whence arises the predominance of the former, and the ex- 

 clusion of the latter. Thus the fringes will exhibit a suc- 

 cession of tints, varying according to the unequal proportions, 

 in which the different rays contained in white light will be 

 mixed. 



The middle line of the whole system is always white, be- 

 cause, since the difference of the routes there vanishes, it 

 affords the maximum of brightness for all kinds of rays, 

 whatever the length of d may be. On each side of thig 



2G2 



