THE PHYSICAL CAUSE OF ANIIMAL COLOURS 7 



motion of an object in still water, and then another 

 set is started from another object moved, so that the 

 ripples succeed each other at exactly the same rate 

 as the preceding set, and if the second set is begun 

 when the first has advanced half a complete ripple 

 (viz. a movement up and down), it is clear that the 

 upward movement of the second will correspond to 

 the downward movement of the first and vice versa, 

 so that, if the objects are so placed that the two sets 

 of ripples are traversing the same sheet of water, 

 they will neutralise and destroy each other. 



If we compare a number of sheets of glass which 

 are successively thinner and thinner, interference will 

 first occur among the longest undulations of light, 

 because half an undulation will of course require a 

 greater distance (or thickness) than when the undula- 

 tions are shorter. As thinner and thinner sheets are 

 examined interference will gradually pass through 

 the whole spectrum from red to violet, destroying 

 sets of waves with shorter and shorter undulations. 

 The colour seen in each case will be due to the other 

 sets of waves which are not destroyed. 



The amount of reflection, and therefore of inter- 

 ference and of colour produced, depends upon the 

 difference between the refractive power of the thin 

 sheet and the substance on each side of it. 



Such interference colom'S are seen in a soap- 

 bubble, and the colours change as the bubble be- 

 comes larger and the film thinner : they differ, too, 



