206 THE RAINBOW 



regard to the slopes of a watershed. A series of paira 

 of points of the same elevation can be found upon the 

 two sides of the ridge ; and, in the case of the rainbow, 

 on the two sides of the maximum deviation we have a 

 succession of pairs of rays having the same deflection. 

 Such rays travel along the same line, and add their 

 forces together after they quit the drop. But light, 

 thus reinforced by the coalescence of non-divergent 

 rays, ought to reach the eye. It does so ; and were 

 light what it was once supposed to be a flight of 

 minute particles sent by luminous bodies through space 

 then these pairs of equally-deflected rays would 

 diffuse brightness over a large portion of the area within 

 the primary bow. But inasmuch as light consists of 

 waves, and not of particles, the principle of interfer- 

 ence comes into play, in virtue of which waves alter- 

 nately reinforce and destroy each other. Were the 

 distance passed over by the two corresponding rays 

 within the drop the same, they would emerge as they 

 entered. But in no case are the distances the same. 

 The consequence is that when the rays emerge from 

 the drop they are in a condition either to support or 

 to destroy each other. By such alternate reinforce- 

 ment and destruction, which occur at different places 

 for different colours, the coloured zones are produced 

 within the primary bow. They are called ' super- 

 numerary bows,' and are seen, not only within the 

 primary, but sometimes also outside the secondary bow. 

 The condition requisite for their production is, that 

 the drops which constitute the shower shall all be of 

 nearly the same size. When the drops are of different 

 sizes, we have a confused superposition of the different 

 colours, an approximation to white light being the con- 

 sequence. This second step in the explanation of the 

 rainbow was taken by a man the quality of whose 



