126 Astronomical and Nautical Collections, 



a half, it is obvious that the semiundulations, superinduced 

 on each other, will now possess opposite qualities, as in the 

 case of the half interval first supposed : and that all the 

 undulations must in this manner be neutralised, except 

 the extreme three semiundulations on each side, which will 

 be free from interference. Thus almost the whole of the 

 motion will again be destroyed, and the combination of the 

 two pencils of light must produce darkness, as in the case first 

 considered. 



Continuing to increase the supposed difference by the 

 length of a semiundulation at each step, we shall have alter- 

 nately complete darkness and a maximum of light, accord- 

 ingly as the difference amounts to an odd or an even number 

 of semiundulations : that is, supposing always that the sys- 

 tems of undulations are of equal intensity : for if the on6 

 series were less vivid than the other, they would be inca- 

 pable of destroying them altogether : the velocities of the 

 one series would be subtracted from those of the other, since 

 they would tend to move the particles of the ether in con- 

 trary directions, but the remainders would still constitute 

 light, though feebler than that of the strongest single pencil. 

 Thus the second pencil would still occasion a diminution of 

 the light : but the diminution would be the less sensible as 

 the pencil is supposed to be weaker. 



Such are the consequences of the principle of the inter- 

 ference of undulations, which agree perfectly, as we have 

 seen, with the law of the mutual influence of the luminous 

 rays which is deduced from experiment : for the results are 

 expressed precisely in the same words, if we give the name 

 of length of undulation to the difference of routes which had 

 been represented by the symbol d. Admitting, therefore, 

 as there is every reason to believe, that light consists in the 

 undulations of a subtile fluid, the period d, after which the 

 same effects of interference are repeated, must be the length 

 of an undulation. 



It appears from the table already given for the seven 

 principal kinds of coloured rays, that this period d, or the 

 length of the undulation, varies greatly, according to the 



