84 INTERMINGLING OF UNDULATIONS. 



waves are of double height ; but when they are combined 

 so as to make the elevations of one correspond with the 

 depressions of the other, then there are no waves at all. 

 The mechanism moves, but the surface of the water, or 

 that which represents the surface of the water, remains 

 smooth." 



" That's curious," said John. 



"And so with light," added Lawrence. "There is a way 

 of contriving to make the two sets of luminous undulations 

 unite in such a manner as to produce darkness. I can not 

 explain to you now how it is done ; only remember that in 

 the books it is called interference. It is only interference, 

 however, in one sense. Strictly speaking, neither interferes 

 with or hinders the real and proper action of the other, but 

 the two actions combined produce a remarkable result. 



"And now," added Lawrence, rising, "your five minutes 

 are out, and more too, and it is time for us to go home. 

 Only you must try to remember exactly what is meant 

 philosophically by interference, as the word is used, in the 

 science of light." 



As Lawrence and John walked over the suspension 

 bridge on their return home, they stopped to watch the 

 motions of the ducks and swans which were swimming 

 about in one place near the shore. John looked attentive- 

 ly to see whether he could detect any thing like the phe- 

 nomenon of interference in the optical sense, but he could 

 not. 



" At any rate," said he, " I should like to have one of 

 those wave machines that you described." 



"You can buy one, perhaps, when you get to Paris," 

 said Lawrence, " with the money that you save by going 

 second class." 



John's father was a very wealthy man, and was perfect- 



