84 INTERFERENCE OF LIGHT. 



as the first of one with the first of the other, the second of 

 one with the second of the other, &c. This line is ne- 

 cessarily straight. On either side of this there will be 

 a line, BB, B X B X , consisting of those points where the first 

 wave from one origin encounters the second from the other, 

 the second from one the third from the other, of all those 

 points, in short, whose distances from the two centres differ 

 by the length of a single wave. The next pair of lines, CO, 

 C'C', consist of those points whose distances from the two centres 

 differ by the length of two waves ; and so on. All these lines 

 are hyperbolas, and on all of them the disturbance is doubled, 

 and an elevated ridge raised on the surface. But there are 

 likewise intermediate lines, composed of those points whose 

 distances from the two centres differ by half a wave, by a icave 

 and half, by two waves and half, &c. On all these lines, the 

 crest of the wave from one origin meets the hollow of a wave 

 from the other ; and these, therefore, are the lines of no dis- 

 turbance. They are evidently hyperbolas like the former. 



All that has been now said applies strictly to the phe- 

 nomena of light, in the aspect under which they are pre- 

 sented by the wave-theory. In the same medium the waves 

 of any given length are propagated with a constant velocity. 

 When therefore two series of waves of equal length diverge 

 at the same time from two centres, they will arrive at the 

 same point in the same phase, provided that the lengths of 

 the paths traversed are equal, or differ by any whole number 

 of undulations. They meet in opposite phases, on the other 

 hand, when the lengths of their paths differ by half a wave, 

 or by any odd multiple of half a wave. The central bright 

 band, then, is formed at those points where the distances tra- 

 versed are equal. The next bright band on either side is pro- 

 duced where the distances traversed differ by the length of 

 one entire wave ; the succeeding pair where the distances differ 

 by two whole waves ; and so on. In the same manner, the 



