INTERFERENCE OF LIGHT. 83 



more or less near to each other. We may imagine them 

 to coincide exactly that is, that the crest of the wave of 

 one shall coincide exactly with the crest of the wave of the 

 other, and the hollow Avith the hollow and that by this 

 coincidence the height of the wave would be increased, and 

 also the depth of the hollow, so that the two undulations 

 combined should form a united one of double intensity. 

 We can also, on the other hand, imagine that the two sets 

 of undulations may be separated from each other by half 

 the breadth of the tcave, so that the hollow of one should 

 just correspond with the swell of the other, and thus that 

 each should counteract the effect of the other, and the water 

 consequently remain smooth." 



"Oh, Lawrence," said John, "it would not be possible to 

 fit the two lines of the waves so exactly as that." 



" True," replied Lawrence ; " but is it impossible to con- 

 ceive of it ?" 



" Xo," rejoined John, " I don't think it is impossible to 

 conceive of it." 



"You are right, probably," added Lawrence, "in saying 

 that it would be impossible to perform this experiment 

 with any waves and by means of ducks, but it can be rep- 

 resented perfectly by artificial waves." 



" Artificial waves ?" repeated John. 



" Yes," replied Lawrence ; " there is an article of appa- 

 ratus by which the action and appearance of waves can be 

 produced by means of bars of wood rising and falling, so 

 as to illustrate the laws of their motion. You turn a 

 crank, and a motion representing a wave runs along the 

 machine. With this they can show very plainly what I 

 have been explaining to you. There are two sets of mov- 

 ing bars, or two systems, either of which alone makes a line 

 of waves. When they are combined in a way to make the 

 elevations and depressions of both systems correspond^ the 



